Srirat Chuayboon

Stéphane Abanades
email: stephane.abanades@promes.cnrs.fr

Dr Sylvain Rodat

Stepwise solar methane reforming and water-splitting via lattice oxygen transfer in iron and cerium oxides

Keywords: (concentrated solar energy, chemical looping, ceria, iron oxide, syngas)

Chemical looping reforming of methane (CLRM) involves lattice oxygen transfer in metal oxides. This study aims to compare iron (Fe 2 O 3 ) and cerium (CeO 2 ) oxides as oxygen carrier materials for isothermal solar-driven stepwise CH 4 reforming and H 2 O splitting.

Experiments were conducted in a directly-irradiated lab-scale solar reactor heated by concentrated sunlight in the temperature range 950-1150 °C. Using solar energy for process heat reduces the dependence on fossil energy resources and avoids CO 2 emissions, while converting solar energy into chemical fuels. The performance of the oxygen carrier materials was compared and evaluated by determining the averaged amount of oxygen transferred, methane conversion, syngas yield, and thermochemical cycling stability. As a result, iron oxide reduction with methane strongly depended on temperature and displayed relatively lower reaction rate than CeO 2 . The reduced iron (Fe) was not completely re-oxidized to iron oxide (Fe 3 O 4 ) after water-splitting step because of its low thermal stability resulting in strong sintering and agglomeration, thereby decreasing syngas yield and eventually leading to material deactivation. In contrast, cerium oxide exhibited faster reaction rate and stable syngas yield with H 2 /CO molar ratio approaching two over repeated cycles. Stable patterns in the averaged oxygen nonstoichiometry (=0.35-0.38), methane conversion (X CH₄ =46.9-60.9%), and total syngas yield (5.67-6.80 mmol/g CeO₂ for reduction and 2.38-2.57 mmol/g CeO₂ for oxidation) over twelve successive cycles for ceria reticulated foam demonstrated excellent thermal cycling stability. Thus, employing Fe 2 O 3 as oxygen carrier was not suitable for solar CLRM, but iron oxide reduction with methane could be a promising option for solar metallurgy aiming at producing both metallic iron and syngas.

Introduction

Synthesis gas (syngas) is widely considered as one of the most important valuable chemical energy carriers as it is a key intermediate resource to produce hydrogen, ammonia, methanol, or synthetic hydrocarbon fuels. The syngas can be produced from a variety of sources, including either renewable (biomass) or fossil sources (natural gas, coal, or any hydrocarbon feedstock), by partial oxidation reactions with steam or oxygen. Methane reforming is the conventional method for producing syngas. Steam [1] and dry reforming [2][3][4][5] are two possible routes for methane reforming: Steam reforming: CH 4 +H 2 O→3H 2 +CO ∆H 0 =+206 kJ/mol (1) Dry reforming: CH 4 +CO 2 →2H 2 +2CO ∆H 0 =+247 kJ/mol (2) Both reactions are highly endothermic, and the required energy to drive these reactions is generally supplied by the combustion of fossil fuels, resulting in greenhouse gas emissions, especially CO 2 responsible for climate change and global warming. Using solar energy in place of fossil fuels for supplying process heat thus represents a suitable option to alleviate these issues.

Alternatively, a promising modern pathway to produce clean syngas is the utilization of metal oxide redox pairs for partial oxidation of methane (namely, chemical looping reforming of methane, CLRM) using concentrated solar energy to drive endothermic reactions. [6][7][8][9] Such a process usually encompasses two steps: (1) partial oxidation of methane along with metal oxide reduction, and (2) re-oxidation of the oxide with steam (or CO 2 ) to produce pure H 2 (or CO). The advantages of solar CLRM over metal oxides when compared to the conventional process are: (1) the discharge of pollutants is totally avoided, as the required heat is provided by solar energy [10][11][12][13] , (2) the use of catalysts is bypassed, eliminating deactivation issues, (3) the supply of gaseous oxygen associated with the need for upstream air separation for oxygen production are eliminated, (4) isothermal cycle operation is made possible (~1000°C), avoiding sensible heat losses and reactor materials issues [14] , (5) syngas is produced with H 2 /CO ratio of two, suitable for methanol synthesis, and (6) carbon deposition on redox materials is inherently eliminated during oxidation step (carbon gasification with H 2 O or CO 2 ), thus avoiding oxygen carriers deactivation.

The possibility of utilizing metal oxides as oxygen carriers for CH 4 partial oxidation has been experimentally reported. The materials include either volatile metal oxides such as ZnO [10,15] and MgO [16] , or non-volatile oxides such as WO 3 [12] , ceria (CeO 2 ) [17,18] , cerium-based oxides [19][20][21][22][23][24] , and iron oxide [25][26][27] . Cerium and iron oxides are particularly attractive because of their different physical and chemical properties. [28] Ceria keeps a stable cubic fluorite structure during large changes in oxygen non-stoichiometries (reduction extents), and exhibits rapid oxygen storage/release through lattice transfer. [29][30][31][32] On the other hand, iron oxide (Fe 2 O 3 ) is abundant and low cost, and exhibits large oxygen releasing ability compared to CeO 2 and other candidates, but it may encounter sintering issues, leading to low cycling stability. [33,34] The main motivation of using iron oxide for CLRM is thus related to process cost reduction because of large material availability, compared to cerium oxide. Moreover, the sintering issue of iron oxide may be counterbalanced by its superior fuel production capability due to reactions involving multivalent iron species (Fe, FeO, Fe 3 O 4 ), thus potentially leading to higher amounts of oxygen transferred during redox reactions. Conversely, the exchanged oxygen and fuel productivity from ceria are determined by the reduction extent corresponding to the achieved non-stoichiometry (amount of oxygen vacancies in CeO 2- , which is always lower than  max =0.5 mol O /mol CeO₂ ). Thus, evaluation and comparison of the materials reactivity for fuel production upon cycling under real solar irradiation conditions is necessary to identify the most suitable one.

Otsuka et al. first investigated the partial oxidation of CH 4 over ceria in a fixed bed quartz tubular reactor (non-solar). [18] They showed that the use of ceria enables the direct conversion of CH 4 into syngas, and the reaction provides an appropriate syngas composition (H 2 /CO=2).

Besides, the ceria redox cycle was coupled with the utilization of concentrated solar energy for producing H 2 from two-step thermochemical water-splitting, as first proposed by Abanades and Flamant. [35] Thermodynamic and experimental analysis of solar-driven methane reforming over ceria were also investigated. [6,30,36] Maximum predicted solar-to-fuel efficiency of 40% was reported for partial oxidation of methane with ceria during isothermal cycling at 950 °C [37] , and the maximum projected solar-to-fuel thermal efficiency of 27% for cycling at 1000 °C was later identified [8] . Solar CLRM in a prototype reactor operated in a high-flux solar simulator was performed with solar-to-fuel efficiency of 7% and thermal efficiency of 25%. [9] Concerning iron oxides, Steinfeld et al. examined the Fe 3 O 4 +4CH 4 system and reported that the chemical equilibrium species consist of metallic iron in the solid phase and a mixture of 67% H 2 and 33% CO in the gaseous phase (at 1 bar and 1027 °C), while showing experimentally that the reduction of Fe 3 O 4 with CH 4 is strongly dependent on temperature and residence time. [27] Lu et al. investigated the reactivity of magnetite (Fe 3 O 4 )

with respect to CH 4 reforming and H 2 O splitting in a continuous prototype. [33] The reduction kinetics was also studied with both fresh and recycled magnetite. The hydrogen yields from the original and calcinated magnetite after successive cycling were 4.94 and 5.25 mmol/g, respectively, and the activation energy for the reduction was 93 kJ/mol. In addition, Bleeker et al. studied the deactivation of iron oxide used in the steam-iron process to produce hydrogen and reported that the main drawback of using iron oxide is the inherent structural changes that take place during oxygen loading and unloading, leading to severe deactivation due to loss of specific surface area. [34] The performance comparison of the oxygen carrier materials in a solar reactor using representative operating conditions has not been performed so far. In the present work, the CLRM process is performed in a directly-irradiated lab-scale solar reactor using iron and cerium oxides with different structures (packed-bed powder and reticulated porous foam).

High-temperature concentrated solar heat is used to provide the reaction enthalpy, and therefore the feedstock is entirely dedicated to produce H 2 and CO (syngas). Solar energy is thus stored into a high-quality syngas and can easily be transported for later use as a convertible and dispatchable chemical form. Solar CLRM promotes methane valorization and offers an efficient means of storing intermittent solar energy into renewable solar fuels. The relevant performance metrics of the process are quantified including lattice oxygen transfer (n O ) and average oxygen non-stoichiometry (𝛿), methane conversion (X CH₄ ), syngas yield, and thermochemical cycling stability in order to compare the influence of oxygen carrier materials on solar reactor performance. In addition, the demonstration of the whole two-step solar process in a solar reactor operated with a real solar concentrating system is presented with determination of the fuel production capacity during both steps.

Thermodynamics

Iron oxide

The endothermic reduction reaction of iron oxide (Fe 2 O 3 ) with CH 4 is composed of successive steps:

Fe 2 O 3 +1/3CH 4 ⟶2/3Fe 3 O 4 +2/3H 2 +1/3CO (3) 2/3Fe 3 O 4 +2/3CH 4 ⟶2FeO+4/3H 2 +2/3CO (4) 2FeO+2CH 4 ⟶2Fe+4H 2 +2CO (5)
The summation of the above reactions corresponds to: (11) The thermodynamic feasibility of chemical reactions involving iron oxide species reduction with methane can be assessed from the Gibbs free enthalpy change (∆G°) (Figure 1). The ∆G° values decrease with temperature (∆G° equals zero at 400 °C for 3Fe 2 O 3 +CH 4 , and at above 600 °C for the other reactions), which means that all the reduction reactions are thermodynamically favorable when increasing the temperature. The thermodynamic equilibrium composition was calculated with HSC Chemistry to appraise the species distribution and system composition as a function of temperature. It provides a realistic overview of the chemical product species and the most favorable conditions for carrying out the reduction reaction with CH 4 and reaching completion. The main assumptions of the method must be reminded, including closed system approach without any gas flow and available thermodynamic properties for any chemical species to represent the system as closely as possible.

Fe 2 O 3 +3CH
The equilibrium species composition of Fe

Cerium oxide

Solar CLRM based on non-stoichiometric cerium oxide is represented as:

Reduction step: CeO 2 + 𝛿CH 4 ⟶ CeO 2-𝛿 + 𝛿CO + 2𝛿H 2 (12)
Oxidation step:

CeO 2-𝛿 + 𝛿H 2 O ⟶ CeO 2 + 𝛿H 2 (13)
The thermodynamic equilibrium composition for the reduction step (Figure 5) shows that CH 4 first starts thermally decomposing into both solid carbon and H 2 (g), while the reduction of CeO 2 proceeds with the formation of CeO

Experimental set up and methods

The solar reactor concept is schematically showed in Figure 6. [7] The solar reactor consists of a cylindrical metallic cavity receiver with a conical part (60° angle) at its bottom (volume: A given amount of cerium or iron oxide is loaded within the cavity receiver directly subjected to concentrated sunlight. The materials preparation is described in Supporting Information (materials synthesis). Two types of materials structures for each oxide (powders and porous foams) were assessed to compare their thermochemical reactivity. Since the objective is to conduct the thermochemical reactions in a solar reactor, efficient solar radiation absorption and high heat and mass transfer rates are necessary to warrant homogeneous material heating and reaction in the whole structure for utilization of the entire reactant mass. Therefore, using reticulated porous foam may be an attractive option for volumetric solar radiative absorption while offering a large available interface for gas species access and solid/gas reactions.

Reactive gases (CH 4 or H 2 O) and Ar carrier gas are fed via the single inlet port at the bottom of the cavity receiver. Prior to gas analysis, product gases flow through a gas scrubber composed of both bubbler and gas filtering unit (two micro filters with 0.1 µm pore diameter)

to remove moisture and entrained solid carbon particles. The gas composition is then analyzed by an on-line syngas analyzer (GEIT 3100, uncertainty <±0.1% of full scale), and all the measured data are recorded by an automated data acquisition system (BECKHOFF).

The reactor cavity was initially flushed by Ar and sucked by a venturi pump to eliminate residual air from the system. Ar protective gas (2.0 Nl/min) was also provided to the window area to prevent the hot gas contact with the transparent window. Subsequently, the reactor was gradually heated by concentrated sunlight to the desired temperature (referred by T 3 ) under Ar.

The solar power input was regulated by means of shutter opening to stabilize T 3 . Once T The pressure was maintained at ~0.9 bar (P atm = ~0.85 bar at site elevation 1,500 m above sea level) for both steps.

During partial oxidation of methane with metal oxides, the formation of both H 2 O and CO 2 is also possible according to Equation 14:

4M x O y +𝛿CH 4 ⟶4M x O y-δ +𝛿CO 2 +2𝛿H 2 O (14)
Therefore, oxygen released from the oxide structure is recovered in the forms of CO, CO 2 , and H 2 O (twice the amount of CO 2 ). The global amount of oxygen released from the oxide structure (n O,red ) is therefore determined by:

𝑛 𝑂,𝑟𝑒𝑑 = 𝑛 𝐶𝑂 + 2𝑛 𝐶𝑂 2 + 𝑛 𝐻 2 𝑂 (15)
Where n i are the mole amounts of species i.

𝛿 𝑟𝑒𝑑 = 𝑛 𝑂,𝑟𝑒𝑑 𝑛 𝐶𝑒𝑂 2 (17)
The replenished oxygen (δ ox ) during ceria oxidation with H 2 O (Equation 13) can be calculated as follows:

𝛿 𝑜𝑥 = 𝑛 𝑂,𝑜𝑥 𝑛 𝐶𝑒𝑂 2 (18)
For reduction, the CH 4 conversion is defined by:

𝑋 𝐶𝐻 4 = 1 - 𝑚̇𝑢 𝑛𝑟𝑒𝑎𝑐𝑡𝑒𝑑 𝐶𝐻 4 𝑚̇𝐶 𝐻 4 (19)
Where 𝑚̇𝑢 𝑛𝑟𝑒𝑎𝑐𝑡𝑒𝑑 𝐶𝐻 4 is the mass flow rate of unreacted CH 4 , and 𝑚̇𝐶 𝐻 4 is the mass flow rate of injected CH 4 .

Moreover, the quality of products is assessed from the selectivity of H 2 (S H ₂) and CO (S CO ) towards syngas products:

𝑆 𝐻 2 = 𝑛 𝐻 2 𝑛 𝐻 2 +𝑛 𝐻 2 𝑂 ( 20
)
𝑆 𝐶𝑂 = 𝑛 𝐶𝑂 𝑛 𝐶𝑂 +𝑛 𝐶𝑂 2 (21)
Where 𝑛 𝐻 2 and 𝑛 𝐶𝑂 are the total moles of produced H 2 and CO during oxidation and reduction steps, 𝑛 𝐻 2 𝑂 and 𝑛 𝐶𝑂 2 the moles of produced H 2 O (equal to twice the mole of CO 2 ) and CO 2 during reduction step.

All the operating conditions and experimental results for 29 cycles performed isothermally with iron and cerium oxides in the solar reactor are listed in Supporting Information (Table S1). The transient syngas production rates along with nominal reactor temperature (T 3 ) during iron oxide powder reduction with methane and during reduced iron oxidation with H 2 O for four consecutive cycles are plotted for the respective temperatures of 1000, 1100, 950, and 1000 °C (Figure S4). Note that the outlet flow rate of each gas specie (F i ) was calculated from their measured mole fraction (y i ) and the known total inlet flow rate of Ar (F Ar ):

Results and discussion

Methane reforming over iron oxide

(𝐹 𝑖 = 𝐹 𝐴𝑟 • 𝑦 𝑖 𝑦 𝐴𝑟 ⁄ ). During reduction step, the maximum CO 2 production rate (0.09 Nl/min) was noticed at the initial stage of the reaction at cycle #1 (Figure S4a); moreover, its amount was higher than those of CO and H 2 . Actually, H 2 O was also formed simultaneously in accordance with thermodynamic analysis; however, it cannot be detected from gas analysis.

The CO 2 production rate was observed again at cycle #2 (but it was lower than cycle #1)

when increasing the temperature to 1100 °C (Figure S4c), and after initial stage its trend remained stable along with H 2 and CO production rates. The peak H 2 and CO production rates (0.07 and 0.04 Nl/min) were found at the highest reduction temperature (1100 °C, Figure S4c).

After cycle #2, the syngas (H 2 , CO, and CO 2 ) production rates during reduction were found in negligible amounts (Figure S4e andS4g). During oxidation step, the peak of H 2 production rate (0.077 Nl/min) was also found at the same temperature (1100 °C, Figure S4d), followed by 1000 °C (0.052 Nl/min, Figure S4b) and 950 °C (0.035 Nl/min, Figure S4f), while extremely small evolution of CO and CO 2 was noticed for any oxidation temperatures. Thus, the negligible impact of carbon deposition from methane cracking was demonstrated for the CLRM over iron oxide. Noticeably, the methane reforming over iron oxide required long duration to reach completion (because of strong oxide sintering during reduction and low available specific surface area) and strongly depended on the reduction temperature as evidenced in Figure S4c (the required reduction temperature was higher than for CH 4 reforming over ceria (see next section)). First, the reduction of fresh 9) [38] , as evidenced by n O,ox much lower than n O,red (Figure 7). The material was deactivated after cycle #2 because operation at 1100 °C resulted in strong sintering despite achieving high reduction rate, as reflected by increased CO and H 2 production rates and maximum X CH₄ (33.8%). Figure 8 shows the syngas yields quantified from the integration of the measured syngas production rates (Figure S4) during reduction and oxidation steps. As expected, the highest CO 2 yield (1.06 mmol/g Fe₂O₃ ) was found in cycle #1, followed by cycle #2 (0.5 mmol/ g Fe₂O₃ ) while the maximum total syngas yield (4.0 mmol/g Fe₂O₃ ) was produced in cycle #2 (Figure .   8a), and no effect of CH 4 cracking reaction (Equation 7) was observed as evidenced by a zero amount of H 2 (CH 4 cracking). Note that the amount of H 2 (CH 4 cracking) was quantified by the total H 2 yield measured by gas analysis minus the H 2 yield produced by the main reaction of iron oxide with methane (Equation 6), which is equivalent to twice the quantity of produced CO. In addition, no syngas production was observed in cycle #3 (the reduction reaction was negligible as evidenced by X CH₄ =0%, Figure 7), and a negligible syngas yield was found in cycle #4. These are because of a dramatic decrease in available surface area resulting from high sintering/coarsening and densification of the new-formed metallic iron particles after being reduced at high temperature (1100 °C, cycle #2), thereby leading to material deactivation and negligible re-oxidation. For these reasons, its powder structure cannot be backed to the initial stage despite passing re-oxidation step, thus adversely influencing the CLRM process in cycles #3 and #4 and thereby leading to negligible amounts of syngas produced. This was confirmed by the high sintering of iron powder structure observed after 4 cycles (Figure S5).

Figure 8b shows that most of the produced syngas yields during oxidation step was enticing from H 2 associated with the main reaction (Equation 9), and the maximum total syngas yield 10), while the H 2 (C+2H 2 O) yield is equal to twice the CO 2 yield measured by gas analysis (Equation 11)). In addition, the H 2 (Equation 20) and CO (Equation 21) selectivity (Table S1) rose sharply during cycling (e.g. from 28.6 and 34.8% at cycle #1 to 59.2 and 96.6% at cycle #3, respectively), because of the drop of H 2 O and CO 2 yields. The reduced iron oxide after reduction step in cycle #4 was not re-oxidized, as it was kept for X-ray diffraction (XRD) analysis to characterize its phase composition (Figure S6). Phase identification shows the presence of mainly FeO, followed by traces of Fe 3 O 4 and Fe. Thus, most of the iron oxide powder did not completely reduce to Fe despite already passing the reduction step with methane, thereby confirming material deactivation. In order to confirm experimental repeatability and compare the results, another iron oxide structure (reticulated porous foam, total Fe 2 O 3 mass: 13.91 g) was prepared (Figure S3b) via a replication technique [7] and subsequently employed as an oxygen carrier material for the During reduction step, the peak rates of CO 2 production decreased, while the peak of H 2 and CO remained stable with cycles repetition from cycle #1 to cycle #3 at 1000 °C (Figure S7).

(
For example, the peak rates of CO 3).

Similar to previous tests with iron oxide powder, the kinetics of methane reforming with iron oxide porous foam were too slow at 1000 °C to allow reaching completion (CH 4 injection was therefore stopped before H 2 and CO approached zero (cycles #1-3, Figure S7a, S7c and S7e)).

The reaction rates were enhanced when increasing the temperature to 1050 °C according to Figure S7g and S7h. For example, the peak rates of H 2 and CO production were 0.09 and 0.05 Nl/min during reduction step (Figure S7g), while the peak rate of H 2 production was 0.11

Nl/min during oxidation step at 1050 °C, and negligible CO and CO 2 production rates were observed (Figure S7h), confirming the absence of carbon formation during reduction step.

The temperature was increased to 1150 °C in cycle #5 (Figures S7i andS7j) to further hasten the kinetic rate of reduction. As a result, the syngas evolution profile (Figure S7i) increased considerably; however, the effect of methane cracking reaction was detected as reflected by the peak of H 2 after 18 min with the maximum X CH₄ (68.3%, Figure 9) while CO approached zero (thus indicating the end of iron oxide reduction). Note that the formed carbon can also act as reducing agent (Fe 3 O 4 reduction) according to thermodynamics (Figure 4 and S2), and the cracking reaction may also be catalyzed by the newly formed metallic iron. [27] During oxidation with H 2 O, both a sharp growth in CO (consistent with the H 2 evolution profile) and a significant increase in CO 2 were noticed (Figure S7j), arising from the reaction of deposited C with H 2 O and confirming that 1150 °C is favorable for CH 4 cracking reaction in this cycle.

When the rate of oxide reduction is lower than the rate of methane decomposition, chemisorbed carbon may accumulate at the surface. This occurs when the rate of bulk lattice oxygen diffusion to the surface becomes lower than the CH 4 supply rate. In other words, when a lack of oxygen at the surface occurs, carbon deposition is fastened, which is increasingly favored as oxygen is being depleted during the iron oxide reduction progress.

The temperature was then decreased by 50 °C in cycle #6 to alleviate the adverse impact of CH 4 cracking reaction; nevertheless, the resulting syngas associated with CH 4 decomposition remained high as reflected by a stable H 2 evolution profile in the reduction step even if CO approached zero (Figure S7k) and a steep increase of the CO along with H 2 in the oxidation step (Figure S7l). This may possibly be also due to the carbon accumulation from the previous cycle.

The cycling experiment was completed with the reduction step performed at 1000 °C (cycle #7, Figure S7m). It was found that no CO 2 production was evolved; in contrast, the peak H 2 production rate was higher (0.06 Nl/min) while the peak CO production rate remained the same (0.02 Nl/min) as compared to those obtained from cycles #1-3. This suggests that the amount of excess available oxygen at the iron oxide surface was restrained in this last cycle, explaining the preferential formation of CO rather than CO Once again, the time integration of the measured syngas production rates in Figure S7 during reduction and oxidation steps was used to calculate the syngas yields according to Figure 10.

Note that the reduced iron after reduction in the last cycle (cycle #7) was not re-oxidized, as it was kept for analyzing its phase composition via XRD. Therefore, no syngas yield during oxidation at cycle #7 was presented.

During reduction step (Figure 10a), as expected a large amount of CO 2 was observed during the first reduction (1.00 mmol/g Fe₂O₃ ) arising from Fe 2 O 3 reduction to Fe 3 O 4 (0.059 mole of oxygen released, Figure 9), and total syngas yield decreased with cycles repetition (cycles #1-3). The syngas yields (especially H 2 and CO) grew with increasing reduction temperature (cycles #4-6), thereby confirming that the partial oxidation of CH 4 over iron oxide strongly depends on the reduction temperature, and temperatures higher than 1000 °C were required to favor reactions. However, a remarkable amount of H 2 associated with CH 4 cracking (3.01-5.57 mmol/g Fe₂O₃ ) was inherently measured at cycles #5-6 because of higher reduction temperatures (1100-1150 °C). Noticeably, CO 2 yield decreased with cycles repetition (except in cycle #5 at which the amount of CO 2 was higher than that obtained from cycle #1 as a result of the exacerbated temperature effect at 1150 °C), and it was not observed during the last cycle (cycle #7). This is because a large amount of the oxygen contained in the iron oxide lattice structure was released during the previous cycles favoring CO 2 formation. In fact, Fe 3 O 4 reduction takes place into two stages: first the reduction from Fe 3 O 4 to FeO (Equation 4) and then FeO to Fe (Equation 5). The first stage is faster than the second one presumably as a result of produced H 2 that reacts with Fe 3 O 4 (Figure 2) but not with FeO. [27] The slower second reaction stage is caused by the material surface reduction resulting from sintering. [27] These variations may explain the decrease of CO 2 with cycles repetition (as evidenced in cycles #1-7) since the iron oxide structure does not completely return to the initial state despite passing the oxidation step (as proved by n O,ox values always lower than n O,red , Figure 9), thereby leading to lower available oxygen for the subsequent reduction reaction.

During oxidation step (Figure 10b), most of the produced syngas yield at cycles #1-4 was ascribed to the production of H 2 formed by Equation 9. The CO (C+H 2 O), CO 2 (C+2H 2 O), H 2 (C+H 2 O), and H 2 (C+2H 2 O) yields formed by side reactions (Equation 10 and 11) were negligible, implying negligible amount of carbon deposition. In contrast, they were found in significant amounts in cycles #5-6, suggesting important impact of carbon deposition on syngas yields at 1100-1150 °C. Likewise, the reduced iron oxide composition after reduction in cycle #7 was characterized by XRD. Its phase identification is presented in Figure S8. The material is mainly composed of Fe, followed by traces of FeO, in agreement with the hard structure of iron in Figure S9b. Most of the iron oxide foam was converted to metallic Fe. It can be assumed that the iron oxide porous foam structure might be lastingly converted to metallic iron coarsened structure after being reduced at 1150 °C (cycle #5), and that the process is not reversible due to extremely high sintering entailing low re-oxidation ability, as observed in Figure S9b. This assumption can be evidenced by a sharp drop of H 2 (Fe+H 2 O) yield associated with the oxidation reaction in cycles #5-6 (ranging between 0.80-0.92 mmol/g Fe₂O₃ , Figure 10b) with extremely low oxygen uptake (0.013 mole for cycle #5 and 0.011 mole for cycle #6, Figure 9). In conclusion, the CLRM over both iron oxide powder and foam dramatically suffers from morphological instability (high sintering), slow reaction kinetics, and unfavorable oxidation because of strong material coarsening and densification, thus strongly altering the oxygen exchange reversibility. Then, the oxygen uptake during oxidation is always lower than the oxygen released during reduction because of low material thermal stability. For these reasons, iron oxide is not suitable for CLRM process and would require stabilization for hightemperature processing (e.g. using inert ceramic support for improving material stability).

Alternatively, iron oxide reduction using CH 4 is suitable for producing both metallic iron and syngas in a solar metallurgical process.

Methane reforming over cerium oxide

In order to compare the performance of oxygen carrier materials for CLRM, nonstoichiometric ceria was experimentally investigated. Pure ceria powder (total mass: 17.01 g, Figure S3c) was employed during six consecutive cycles at a constant temperature of 1000 °C. 12) and demonstrating complete reduced ceria re-oxidation during cycling. These results thus confirmed the performance stability of ceria for solar CLRM. In order to confirm the experimental repeatability and assess the effect of material shaping, a ceria reticulated porous foam (total mass: 18.37 g, Figure S3d) was prepared and then for CO, Table S1). The H 2 (CH 4 cracking) yield at 1000 °C fluctuated slightly (in the range 0.35-1.27 mmol/g CeO₂ ) because of a small difference in the CH 4 injection duration, as mentioned before. When either increasing or decreasing the temperature (1050 °C at cycle #5

or 950 °C at cycle #6), the total syngas yield varied significantly because of a change in the reaction kinetics. The maximum total syngas yield (7.48 mmol/g CeO₂ ) was consequently found at the maximum temperature (1050 °C), demonstrating kinetic rate improvement (Figure 15).

Moreover, the H 2 (CH 4 cracking) yield was negligible at 900 °C, in agreement with the lowest X CH ₄ (20%); in contrast, it was maximal (2.0 mmol/g CeO₂ ) at the highest temperature (1050 °C) in agreement with the highest X CH₄ (65.9%), thereby indicating that the extent of CH 4 cracking reaction is strongly dependent on the temperature. [9] For these reasons, a temperature trade-off at 1000 °C is advocated to hasten the kinetic rate of ceria reduction while alleviating the side reaction associated with CH 4 cracking.

In order to examine the reaction kinetics for ceria during methane-induced reduction, the Arrhenius expression was applied to evaluate the effect of temperature on ceria reduction rates.

𝑘 = 𝐴 • exp(-𝐸 𝑎 /𝑅𝑇) (22)
Where k is the reaction rate constant, A is the pre-exponential factor, E a is the activation energy, R is the gas constant and T is absolute temperature.

The reaction rate constants (k) were quantified from the peak production rates of H 2 and CO at 950 °C (cycle #6), 1000 °C (cycle #4), and 1050 °C (cycle #5). The logarithm evolution of the reaction rates versus inverse temperature (Equation 22) was subsequently plotted (Figure 15) to determine the activation energy (E a ) of the ceria reduction process. As a result, the slope of ln k for both H 2 and CO production rates increased linearly with the inverse temperature. The E a values were 114.2 kJ/mol for H 2 and 93.4 kJ/mol for CO. The E a value related to H 2 production rates was slightly higher compared to that of CO as a result of the side reaction effect attributed to CH 4 cracking. This side reaction produces additional H 2 and thus modifies the global H 2 production rate arising only from the reaction with ceria. In contrast, CO is only produced from methane reforming (Equation 12) and better represents the kinetics of ceria reduction reaction. In addition, the E a for reduction was consistent with previous study [30].

Regarding oxidation (Figure 13b), the H 2 (CeO 14). Thus, the cycling stability of ceria was validated with respect to stable patterns in produced syngas, δ, and X CH₄ , thereby outperforming iron oxide (whether in the form of powder or foam structures) and exhibiting superior thermochemical stability over cycles. The main advantages of CeO 2 when compared with iron oxide have thus been highlighted, including faster reaction rates, reversibility and cycling stability. Besides, combining both cerium and iron oxides may promote the oxygen carrier performance by taking advantage of their mutual benefits, while alleviating the drawbacks of both iron oxide (sintering and poor reaction reversibility) and cerium oxide (higher cost and moderate oxygen exchange capacity).

Conclusion

Solar chemical looping reforming of methane (CLRM) combined with isothermal H 2 O splitting over iron and cerium oxides was thermodynamically and experimentally investigated in a directly-irradiated lab-scale solar reactor using concentrated solar heat for supplying process energy. The thermodynamic analysis provided insights into the theoretically possible chemical reactions and equilibrium species distribution for comparison with experimental results.

Iron oxide (Fe 2 O 3 ) in the form of powder or porous foam showed relatively low reaction rate when reacting with methane at 1000 °C, and the reduction extent strongly depended on temperature. However, increasing the temperature (≥1100 °C) resulted in strong sintering (dense structure), lowered syngas yield, and material deactivation. In contrast, cerium oxide (CeO 2 ) showed faster reaction rates than iron oxide when reacting with methane at 1000 °C, and increasing the temperature between 900-1050 °C promoted syngas yield and ceria reduction kinetics (with activation energy for ceria foam reduction in the range 93.4-114.2 kJ/mol). Stable patterns in the ceria reduction and oxidation extents, CH 4 conversion, and syngas yields during successive cycles for both powder and reticulated porous foam demonstrated remarkable thermochemical cycling stability.

Thus, utilizing iron oxide (Fe 2 O 3 ) as oxygen carrier is not suitable for solar CLRM with respect to poor material re-oxidation capability; instead, iron oxide reduction with CH 4 could be an attractive route for producing both metallic iron and syngas via CO 2 -free solar metallurgical process. For these reasons, the use of cerium oxide as oxygen carrier for solar CLRM is more appropriate than iron oxide. The advantages of using ceria as oxygen carrier have thus been demonstrated, such as faster reaction rates and superior thermochemical cycling stability (oxygen can be released and recovered reversibly in the oxide crystal lattice), which are key assets for the two-step solar chemical-looping process. Combining concentrated solar energy and CLRM was shown to be a promising and sustainable pathway toward clean solar fuels. Further work should be focused on taking the benefits of both iron and cerium oxides by considering their mixed oxides to promote the oxygen carrier performance. Syngas is produced in a solar chemical reactor using reversible cycling of solid oxides for isothermal alternating methane reforming and H 2 O splitting. Solar chemical looping reforming using oxygen carrier materials promotes methane valorization and offers an efficient means of storing intermittent solar energy into renewable clean fuels.

Solar fuels

S. Chuayboon, S. Abanades * , S. Rodat

Stepwise solar methane reforming and water-splitting via lattice oxygen transfer in iron and cerium oxides

Figure 1 .

 1 Figure 1. ∆G° variations for iron oxides reduction with methane as a function of temperature.

Figure 2 .

 2 Figure 2. ∆G° variations for the reactions of Fe 3 O 4 with H 2 , CO, and C as a function of

Figure 3 .

 3 Figure 3. Thermodynamic equilibrium composition of methane reforming over Fe 2 O 3 as a

Figure 4 .

 4 Figure 4. Thermodynamic equilibrium composition of Fe 2 O 3 carbothermal reduction as a

Figure 5 .

 5 Figure 5. Thermodynamic equilibrium composition of methane reforming over ceria as a

  0.299 L and total height: 115 mm), wrapped by a 30 mm-thick alumino-silicate insulation layer. The insulated cavity receiver is vertically placed in a water-cooled cylindrical stainlesssteel shell and then closed by an alumina cap with a 20 mm-diameter aperture for the access of concentrated sunlight. A protective graphite plate (2 mm-thick) with a 15 mm-diameter aperture is positioned on top of the alumina cap to protect it. A hemispherical transparent glass window equipped with a small fluorine window is lastly attached to the front flange edge of the reactor shell. A 2-m diameter parabolic mirror with a solar concentration ratio above 10000 suns (peak flux density of ~10.5 MW/m 2 at the focal plane for a DNI of 1 kW/m 2 ) is employed to concentrate sunlight to the focal point where the reactor aperture is positioned.

Figure 6 .

 6 Figure 6. Schematic of the 1.5 kW th directly-irradiated solar reactor and external components

  Pure raw Fe 2 O 3 powder (total mass: 20.52 g) (FigureS3a) was employed as an oxygen carrier material for the solar CLRM test. Experiments were conducted with 4 cycles at 950-1100 °C. Both CH 4 and Ar carrier gas flow-rates were injected at 0.2 Nl/min for reduction step (50% inlet CH 4 mole fraction), while H 2 O flow-rate was fed at 200 mg/min along with Ar carrier gas (0.2 Nl/min) for subsequent oxidation step, yielding 55% steam mole fraction at inlet.

  Fe 2 O 3 to Fe 3 O 4 in cycle #1 resulted in a large amount of oxygen released (n O,red =0.095 vs. n O,ox =0.012, Figure 7) in the form of CO 2 . Then, during the oxidation step of cycle #1, re-oxidation to Fe 2 O 3 is not possible (only Fe 3 O 4 can be formed, Equation

Figure 7 .

 7 Figure 7. Comparison of n O,red and n O,ox along with CH 4 conversion during four consecutive

Figure 8 .

 8 Figure 8. Syngas yields for (a) iron oxide powder reduction with CH 4 and (b) oxidation with

  solar-driven CLRM. Experiments were conducted with seven consecutive cycles at different reduction temperatures in the range 1000-1150 °C, while the flow rates of CH 4 , H 2 O, and Ar were the same as the previous tests with iron oxide powder (CH 4 : 0.2 Nl/min, H 2 O: 200 mg/min, and total Ar: 2.2 Nl/min).

Figure 9 .

 9 Figure 9. Comparison of n O,red and n O,ox along with CH 4 conversion during seven consecutive

Figure 10 .

 10 Figure 10. Syngas yields for (a) iron oxide reticulated porous foam reduction with CH 4 and

Figure 11 .

 11 Figure 11. Syngas yields for both reduction and re-oxidation of ceria powder during 6

Figure 12 .

 12 Figure 12. Comparison of δ red and δ ox in ceria powder along with CH 4 conversion during 6

Figure 13 .

 13 Figure 13. Syngas yields for both reduction and re-oxidation of ceria porous foam during 12

Figure 14 .

 14 Figure 14. Comparison of δ red and δ ox in ceria foam along with CH 4 conversion during 12

Figure 15 .

 15 Figure 15. Arrhenius plot for H 2 and CO production rates in the range 950-1050 °C during

Figure 1 .

 1 Figure 1. ∆G° variations for iron oxides reduction with methane as a function of temperature.

Figure 2 .

 2 Figure 2. ∆G° variations for the reactions of Fe 3 O 4 with H 2 , CO, and C as a function of

Figure 3 .

 3 Figure 3. Thermodynamic equilibrium composition of methane reforming over Fe 2 O 3 as a

Figure 4 .

 4 Figure 4. Thermodynamic equilibrium composition of Fe 2 O 3 carbothermal reduction as a

Figure 5 .

 5 Figure 5. Thermodynamic equilibrium composition of methane reforming over ceria as a

Figure 6 .

 6 Figure 6. Schematic of the 1.5 kW th directly-irradiated solar reactor and external components

Figure 7 .

 7 Figure 7. Comparison of n O,red and n O,ox along with CH 4 conversion during four consecutive

Figure 8 .

 8 Figure 8. Syngas yields for (a) iron oxide powder reduction with CH 4 and (b) oxidation with

Figure 9 .

 9 Figure 9. Comparison of n O,red and n O,ox along with CH 4 conversion during seven consecutive

Figure 10 .

 10 Figure 10. Syngas yields for (a) iron oxide reticulated porous foam reduction with CH 4 and

Figure 11 .

 11 Figure 11. Syngas yields for both reduction and re-oxidation of ceria powder during 6

Figure 12 .

 12 Figure 12. Comparison of δ red and δ ox in ceria powder along with CH 4 conversion during 6

Figure 13 .

 13 Figure 13. Syngas yields for both reduction and re-oxidation of ceria porous foam during 12

Figure 14 .

 14 Figure 14. Comparison of δ red and δ ox in ceria foam along with CH 4 conversion during 12

Figure 15 .

 15 Figure 15. Arrhenius plot for H 2 and CO production rates in the range 950-1050 °C during

  Fe, along with CO and CO 2 formation. Complete reduction to Fe is reached above 700 °C, while C, H 2 O, and CO 2 decrease constantly. Finally, the main products are both H 2 and CO with H 2 /CO ratio approaching two, and Fe above 1000 °C.

	starts reducing into
	2 O 3 +3CH 4 at 1 bar as a function of temperature is
	shown in Figure 3 (and in Figure S1 for Fe 3 O 4 +4CH 4 ). Fe 2 O 3 is first reduced to Fe 3 O 4 while
	methane is decomposed to C and H 2 when the temperature exceeds 400 °C. Then, Fe 3 O 4 is
	reduced to FeO, along with intermediate non-stoichiometric species (Fe 0.945 O, Fe 0.947 O, and
	FeO 1.056 ) while H 2 , C, and H 2 O(g) become the main product components. Above 400 °C, FeO

  1.81 and CeO 1.78 . Meanwhile, small amounts of both H 2 O(g) and CO 2 (g) are formed at the beginning of reaction. Subsequently, C deposition starts reacting with oxygen released from ceria resulting in CO formation at above 500 °C.

Non-stoichiometric compounds (CeO 1.72 and CeO 1.83 mainly, and CeO 1.67 in negligible amount) are formed as intermediate species, which provide a good representation of the overall reduction mechanism during Ce(IV) reduction into Ce(III) species. The reaction reaches completion at above 1000 °C, yielding Ce 2 O 3 and H 2 /CO mixture (with H 2 /CO ratio approaching two).

  2 (and H 2 O). Indeed, CO 2 and H 2 O formation is favored when an excess amount of surface oxygen is available, thus occurring during the first cycle because the pristine oxide is Fe 2 O 3 . In the subsequent cycles, the participating oxide during reduction is either FeO or Fe 3 O 4 (because re-oxidation to Fe 2 O 3 is not thermodynamically possible when using H 2 O as oxidant) with less available oxygen for the reduction step. Moreover, selectivity increased from 23.2% to 99.7% (H 2 ) and 33.5% to 99.9% (CO), in agreement with the decline in CO 2 and H 2 O formation during cycling.

  The same CH 4 /Ar and H 2 O/Ar gas mixtures were alternately delivered to carry out the reduction and oxidation reactions (CH 4 : 0.2 Nl/min, H 2 O: 200 mg/min, and Ar: 0.2 Nl/min). CeO₂ for H 2 (C+H 2 O), 0.11-0.22 mmol/g CeO ₂ for H 2 (C+2H 2 O), 0.08-0.22 mmol/g CeO₂ for CO(C+H 2 O), and 0.06-0.11 mmol/g CeO₂ for CO 2 (C+2H 2 O)). Moreover, the δ ox values were in the range 0.33-0.35, thus matching well the δ red values (Figure

	0.22 mmol/g
	Figure 11 shows syngas yields quantified from the integration of the measured syngas
	production rates for six reduction and oxidation cycles. According to Figure 11a, the H 2
	(CeO 2 +CH 4 ), CO, and CO 2 yields remained quite constant over the entire cycling (in the
	range 3.39-3.55 mmol/g CeO ₂ for H 2 , 1.69-1.77 mmol/g CeO ₂ for CO, and 0.05 mmol/g CeO ₂ for
	CO 2 ). The total syngas yield was thus stable (in the range 5.77-6.53 mmol/g CeO ₂), in turn
	leading to stable patterns in δ red (0.32-0.36), X CH₄ (40.8-44.3%, Figure 12), and selectivity
	(97.1-97.5% for H 2 and 98.5-98.6% for CO, Table S1) and demonstrating ceria cycling
	stability. The H 2 (CH 4 cracking) yield fluctuated slightly in the range 0.64-1.16 mmol/g CeO ₂
	because of variation in the required CH 4 injection duration to reach ceria reduction
	completion in each cycle. This points out that increasing CH 4 injection duration favors the H 2
	yield associated with CH 4 cracking.
	Similarly, a stable pattern in the H 2 (CeO 2-δ +H 2 O) yield, produced during ceria oxidation
	(Equation 13) was observed (1.94-2.05 mmol/g CeO2 , Figure 11b). The amounts of gases
	produced by steam gasification of carbon deposition were fairly constant (in the range 0.08-

  2-δ +H 2 O) yield was stable at 1000 °C (in the range 2.04-2.17 mmol/g CeO₂ ), except for cycles #5 and #6 caused by the temperature impact during the reduction step. Likewise, the H 2 (C+H 2 O), H 2 (C+2H 2 O), CO(C+H 2 O), and CO 2 (C+H 2 O) yields at 1000 °C were fairly constant in negligible amounts (0.11-0.18, 0.04-0.10, 0.11-0.18, and 0.02-0.05 mmol/g CeO₂ , respectively). The total syngas yield was in the range 2.38-2.57 mmol/g CeO₂ , and δ ox was in the range 0.35-0.37 (consistent with δ red ) (Figure

Acknowledgements

The King Mongkut's Institute of Technology Ladkrabang (KMITL), Thailand and the Franco-Thai scholarship program are gratefully acknowledged for fellowship granting.

Received: ((will be filled in by the editorial staff))

Revised: ((will be filled in by the editorial staff)) Published online: ((will be filled in by the editorial staff))

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.


Manuele Bazzichetto
email: manuele.bazzichetto@univ-rennes1.fr

François Massol

Marta Carboni
email: marta.carboni@uniroma3.it

Jonathan Lenoir
email: jonathan.lenoir@u-picardie.fr

Jonas J Lembrechts
email: jonas.lembrechts@uantwerpen.be

Rémi Joly

David Renault

Bazzichetto

F Carboni

R & Renault

Massol François
email: francois.massol@univ-lille.fr

Johan

Joly Rémi
email: remi.joly88@hotmail.fr

Renault David
email: david.renault@univ-rennes1.fr

Once upon a time in the south: local

Keywords: Poaceae, Caryophyllaceae, Juncaceae) alien plants, anthropogenic propagule pressure, invasiveness, plant invasion, species distribution models, sub-Antarctic islands, topoclimate

published or not. The documents may come   L'archive ouverte pluridisciplinaire

Introduction

Sub-Antarctic islands are extremely remote territories which harbour a unique biodiversity with a high degree of endemism [START_REF] Shaw | Southern Ocean Islands invaded: conserving biodiversity in the world's last wilderness[END_REF]. As a consequence of their relatively recent discovery and environmental harshness, these islands have long remained pristine and largely free of human disturbances. Yet, due to the gradual relaxation of these natural barriers, sub-Antarctic islands are now counted among the most threatened environments on Earth. In particular, invasion by alien plants, boosted by ongoing climate changes and increasing human disturbances [START_REF] Duffy | Ice-free area expansion compounds the non-native species threat to Antarctic terrestrial biodiversity[END_REF][START_REF] Hughes | Human-mediated dispersal of terrestrial species between Antarctic biogeographic regions: A preliminary risk assessment[END_REF], has become one of the main threats to the endemic biodiversity of these territories, and is bound to rise in the next decades [START_REF] Lebouvier | The significance of the sub-Antarctic Kerguelen Islands for the assessment of the vulnerability of native communities to climate change, alien insect invasions and plant viruses[END_REF][START_REF] Hughes | Biological invasions in terrestrial Antarctica: what is the current status and can we respond?[END_REF]. Over the past century, alien plants have been increasingly introduced in the sub-Antarctic region [START_REF] Frenot | Biological invasions in the Antarctic: extent, impacts and implications[END_REF][START_REF] Huiskes | Aliens in Antarctica: assessing transfer of plant propagules by human visitors to reduce invasion risk[END_REF]. European whalers and scientific activities, respectively in the 19 th and 20 th century, determined the first main introduction events [START_REF] Convey | Environmental change and human impacts on terrestrial ecosystems of the sub-Antarctic islands between their discovery and the mid-twentieth century[END_REF][START_REF] Shaw | Southern Ocean Islands invaded: conserving biodiversity in the world's last wilderness[END_REF], while, since the mid-twentieth century, climate modifications have progressively made sub-Antarctic islands more suitable to cold-tolerant alien species [START_REF] Shaw | Southern Ocean Islands invaded: conserving biodiversity in the world's last wilderness[END_REF][START_REF] Pertierra | Global thermal niche models of two European grasses show high invasion risks in Antarctica[END_REF][START_REF] Duffy | Ice-free area expansion compounds the non-native species threat to Antarctic terrestrial biodiversity[END_REF]. Nevertheless, despite their demonstrated impacts on native biodiversity, little attention has been given to plant compared to animal invasions in these islands [START_REF] Molina-Montenegro | Occurrence of the non-native annual bluegrass on the Antarctic mainland and its negative effects on native plants[END_REF][START_REF] Le Roux | Human activities, propagule pressure and alien plants in the sub-Antarctic: Tests of generalities and evidence in support of management[END_REF], leaving a knowledge gap in the mechanisms underpinning plant invasion processes in these unique environments [START_REF] Greve | Terrestrial invasions on sub-Antarctic Marion and Prince Edward Islands[END_REF].

The outcome of any biological invasion is jointly determined by propagule pressure (i.e. frequency of propagules introduction), abiotic conditions (i.e. physico-chemical features of the invaded environment) and biotic features (i.e. alien species characteristics and interactions with the recipient community), with anthropogenic disturbances affecting all three [START_REF] Richardson | Plant invasions: merging the concepts of species invasiveness and community invasibility[END_REF][START_REF] Catford | Reducing redundancy in invasion ecology by integrating hypotheses into a single theoretical framework[END_REF][START_REF] Lembrechts | Disturbance is the key to plant invasions in cold environments[END_REF]. The relative importance of these factors is, however, context-dependent and species-specific [START_REF] Catford | Reducing redundancy in invasion ecology by integrating hypotheses into a single theoretical framework[END_REF]. In sub-Antarctic islands, due to the high specialization but low diversity of the native flora, biotic interactions are thought to play a minor role [START_REF] Le Roux | Human activities, propagule pressure and alien plants in the sub-Antarctic: Tests of generalities and evidence in support of management[END_REF][START_REF] Duffy | Barriers to globally invasive species are weakening across the Antarctic[END_REF][START_REF] Moser | Remoteness promotes biological invasions on islands worldwide[END_REF], so it is mainly the first two factors that determine the distribution and spread of alien plants. First, invasions depend on humaninduced propagule pressure: the frequency of propagule introduction correlates with the number of ship landings and is highest in the vicinity of human facilities [START_REF] Huiskes | Aliens in Antarctica: assessing transfer of plant propagules by human visitors to reduce invasion risk[END_REF]. Second, local abiotic conditions are strongly limiting, and particularly the climatic mismatch between the alien species' native range and the sub-Antarctic can strongly constrain invasions [START_REF] Frenot | Biological invasions in the Antarctic: extent, impacts and implications[END_REF]. Some alien plants are more limited during the introduction phase, while others quickly become relatively independent of human-related propagule-pressure and are only climatically limited. Once, established, the species which are least dependent on continuous introductions are the most likely to spread widely and become invasive [START_REF] Richardson | Plant invasions: merging the concepts of species invasiveness and community invasibility[END_REF][START_REF] Catford | Reducing redundancy in invasion ecology by integrating hypotheses into a single theoretical framework[END_REF]. Therefore, quantifying the degree of alien species dependence on propagule pressure might aid in identifying potentially invasive species.

A lower dependence on human-related propagule pressure is potentially related to certain species features which are more generally known to affect invasiveness. First of all, alien species with longer residence times are more likely to become independent of anthropogenic propagule pressure [START_REF] Wilson | Residence time and potential range: crucial considerations in modelling plant invasions[END_REF][START_REF] Pyšek | Naturalization of central European plants in North America: species traits, habitats, propagule pressure, residence time[END_REF]. Second, certain plant traits are considered key for profiling successful invaders [START_REF] Pyšek | Traits associated with invasiveness in alien plants: where do we stand?[END_REF]: invasive alien plants across most environments are growing faster and taller than non-invasive alien species, and typically produce resource-acquisitive leaves and many small seeds [START_REF] Van Kleunen | A meta-analysis of trait differences between invasive and non-invasive plant species[END_REF][START_REF] Van Kleunen | Characteristics of successful alien plants[END_REF]. More specifically, [START_REF] Mathakutha | Invasive species differ in key functional traits from native and non-invasive alien plant species[END_REF] performed a first functional comparison between invasive and non-invasive alien species colonizing the sub-Antarctic Marion Island, reporting that species generally considered invasive had lower plant height, smaller leaf area, lower frost tolerance and higher specific leaf area than other alien species. Nevertheless, it is still unclear which traits can actually make some alien plants less dependent on human-related propagule pressure, and thus more likely to become invasive, especially in the sub-Antarctic islands. This knowledge could facilitate the early screening of highly invasive alien species in these environments [START_REF] Frenot | Biological invasions in the Antarctic: extent, impacts and implications[END_REF][START_REF] Mathakutha | Invasive species differ in key functional traits from native and non-invasive alien plant species[END_REF].

Correlative species distribution models (SDMs) are statistical tools that model the speciesenvironment relationship relying on geo-referenced occurrence data and spatial environmental layers [START_REF] Guisan | Habitat suitability and distribution models: with applications in R[END_REF]. SDMs already proved to be valuable tools for analysing alien plant invasion in Antarctica and the sub-Antarctic regions. For instance, [START_REF] Pertierra | Global thermal niche models of two European grasses show high invasion risks in Antarctica[END_REF] modelled the distribution of Poa annua and Poa pratensis in the Antarctic peninsula as a function of bioclimatic variables, while [START_REF] Duffy | Barriers to globally invasive species are weakening across the Antarctic[END_REF] generated future scenarios of invasion across Antarctica and the sub-Antarctic regions using climate-based SDMs. Whilst these previous SDM applications have revealed large-scale determinants of alien plant invasion in the Antarctic biogeographic region, they have up till now failed to account for how environmental and anthropogenic factors regulate alien plant distributions at the fine spatial resolutions meaningful for local management. This is chiefly due to the lack of high-resolution environmental (e.g. climatic, topographic) and human-related data layers, which limits the implementation of SDMs at fine spatial resolutions in remote areas [START_REF] Gutt | Correlative and dynamic species distribution modelling for ecological predictions in the Antarctic: a cross-disciplinary concept[END_REF].

The sub-Antarctic Possession Island constitutes an ideal arena to analyse alien plant invasions in the sub-Antarctic region. The availability of historical vegetation observations allows retracing the invasion history of most alien species. Moreover, this island witnessed past human colonization and climate changes comparable to the other sub-Antarctic islands, allowing inference on the mechanisms underpinning alien plant invasion in these unique areas. Previous work showed that there is considerable variation in the spread of alien plants established on Possession Island, with some species clustering close to their introduction locations and others spreading widely and far from the initial introduction sites [START_REF] Frenot | Biological invasions in the Antarctic: extent, impacts and implications[END_REF], which allows testing for differences in the dependence on human introductions. In the present study, we model the distribution of the most relevant alien plant species colonizing Possession Island using a combination of environmental and human-related spatial data derived at an unprecedented high spatial resolution (i.e. 30-m) for these latitudes, and long-term monitoring observations of plant occurrences. Our aim is to test the local effect of environmental and anthropogenic factors on alien plant invasion in sub-Antarctic ecosystems. We hypothesize that both abiotic and human-related factors jointly define the local occurrence of alien plant species, but that these two factors will not be equally important among species. Furthermore, to identify plant characteristics conferring high invasiveness in sub-Antarctic ecosystems, we investigate how plant functional traits affect species dependence on anthropogenic propagule pressures. In this regard, our working hypothesis is that the more invasive species share specific functional characteristics allowing them to become independent of human-assisted introductions and spread widely once established.

Materials and methods

Study area

The study was carried out on Possession Island in the Crozet archipelago, which is included in the Réserve Naturelle Terres Australes Françaises (RN-TAF) and listed as UNESCO World Heritage site since 2019. Possession Island (Longitude: 51.7469, Latitude: -46.4046; EPSG: 4326) is characterized by a complex topography, with an altitudinal gradient ranging from 0 to 934 m above the sea level (Pic du Mascarin) over a relatively short spatial extent (147 km 2 ). The island is characterized by a typical sub-Antarctic climate, with mean annual temperature of 5.6 °C and annual precipitation of 2300 mm (Météo France, data: 1960-2019). Frequent and strong western winds occur throughout most of the year.

The first human settlements date back to the 19 th century, when whalers and sealers established on the north-east side of the island during the hunting season, facilitating a first series of alien species introductions. In 1963, a permanent research station (Alfred Faure, hereafter the 'research station') was built on the easternmost area of Possession Island, fostering a new invasion front. Beyond the research station, other shelters (inhabited for short periods) are currently present on each side of the island: north (Pointe Basse); south-west (Baie du La Pérouse); and north-east (Baie Américaine).

Among these, the research station is by far the biggest human settlement and main hub of propagule introduction. The vegetation at Possession Island has experienced relatively low grazing pressure from large herbivores in the past [START_REF] Convey | Environmental change and human impacts on terrestrial ecosystems of the sub-Antarctic islands between their discovery and the mid-twentieth century[END_REF], in comparison to other sub-Antarctic islands where introduced large herbivores still strongly affect the distribution of alien plants [START_REF] Shaw | Southern Ocean Islands invaded: conserving biodiversity in the world's last wilderness[END_REF].

Study species

Despite the 68 alien species recorded on Possession Island (RN-TAF management plan 2018-2027: https://taaf.fr//content/uploads/sites/2/2019/09/180607-Volet-A_pour-CNPN.pdf), only few have established persistent populations [START_REF] Frenot | Human activities, ecosystem disturbance and plant invasions in subantarctic Crozet, Kerguelen and Amsterdam Islands[END_REF]. In this study, we restricted our analysis to those alien plants that are either known to be generally widespread on sub-Antarctic islands or are particularly widespread on Possession Island, and for which sufficient occurrence data were available (total number of presences > 100). Specifically, we selected the following species: Poa annua and Poa pratensis (Poaceae); Cerastium fontanum, Sagina procumbens and Stellaria alsine (Caryophyllaceae); and Juncus bufonius (Juncaceae). The two grasses, P. annua and P. pratensis, have colonized most of the sub-Antarctic islands [START_REF] Shaw | Southern Ocean Islands invaded: conserving biodiversity in the world's last wilderness[END_REF], and are the longestestablished alien plants in the Antarctic Peninsula [START_REF] Pertierra | Global thermal niche models of two European grasses show high invasion risks in Antarctica[END_REF]. Cerastium fontanum and S. procumbens are currently widely distributed in this environment [START_REF] Frenot | Biological invasions in the Antarctic: extent, impacts and implications[END_REF][START_REF] Shaw | Southern Ocean Islands invaded: conserving biodiversity in the world's last wilderness[END_REF] with, in particular, S. procumbens exhibiting the highest rate of spread among the alien plants of Marion and Prince Edward Islands (le Roux et al., 2013). Finally, both J. bufonius and S. alsine currently occur at significant distances from the research station on Possession Island (Frenot el al., 2001). While the former has been recently observed up to the Maritime Antarctica latitudes (Cuba-Díaz, Fuentes, & Rondanelli-Reyes, 2015), the latter has been singled out by some authors as the potentially most problematic future invasive plant species on Possession Island [START_REF] Frenot | Human activities, ecosystem disturbance and plant invasions in subantarctic Crozet, Kerguelen and Amsterdam Islands[END_REF][START_REF] Convey | The establishment of a new ecological guild of pollinating insects on sub-Antarctic South Georgia[END_REF].

Species distribution data

We analysed the invasion patterns of the six selected alien plant species relying on georeferenced occurrence (presence/absence) data collected within the context of a yearly vegetation monitoring survey carried out by the RN-TAF since 2010. The vegetation sampling is implemented within a system of 675 squared cells 100x100-m, where floristic data (presence and abundance of vascular plant species) are collected along with habitat characteristics through phytosociological relevés [START_REF] Dengler | Phytosociology. International Encyclopedia of Geography: People, the Earth, Environment and Technology: People, the Earth[END_REF]. In this study, we used data collected from 2010 to 2017 (3354 occurrences for the selected species across 1572 sampled plot).

Species features and functional trait data

To inform species features potentially related to invasiveness, we collected data on plants residence time and functional traits. Residence time positively interacts with propagule pressure in determining plant invasion success [START_REF] Richardson | Plant invasions: merging the concepts of species invasiveness and community invasibility[END_REF][START_REF] Lockwood | The role of propagule pressure in explaining species invasions[END_REF][START_REF] Pyšek | Naturalization of central European plants in North America: species traits, habitats, propagule pressure, residence time[END_REF], and this relationship was also observed on sub-Antarctic islands [START_REF] Le Roux | Human activities, propagule pressure and alien plants in the sub-Antarctic: Tests of generalities and evidence in support of management[END_REF][START_REF] Shaw | Southern Ocean Islands invaded: conserving biodiversity in the world's last wilderness[END_REF][START_REF] Mathakutha | Invasive species differ in key functional traits from native and non-invasive alien plant species[END_REF]. To test how residence time influences alien species' dependence on propagule pressure, we considered the introduction date of the selected plants on Possession Island [START_REF] Frenot | Human activities, ecosystem disturbance and plant invasions in subantarctic Crozet, Kerguelen and Amsterdam Islands[END_REF] and used this information to assign them to two groups: old vs new resident species (Appendix S1, Table S1.1). In particular, we considered as old resident species those which were firstly observed on Possession Island before the research station was built (1963), while referring to the others as new resident species.

We then collated data on seven plant traits commonly used to synthesize species strategies known to be related to invasiveness [START_REF] Van Kleunen | A meta-analysis of trait differences between invasive and non-invasive plant species[END_REF][START_REF] Van Kleunen | Characteristics of successful alien plants[END_REF]: (1) life history (annual vs perennial); (2) plant height; (3) leaf area; (4) specific leaf area (SLA); (5) vegetative reproduction (present vs absent, i.e. sexual and vegetative vs only sexual reproduction); (6) seed dry mass; and (7) number of seeds/plant. We excluded traits related to flowering since pollinating insects are absent from almost all sub-Antarctic islands [START_REF] Convey | The establishment of a new ecological guild of pollinating insects on sub-Antarctic South Georgia[END_REF]. Life history, plant height and leaf area relate to plant persistence and tolerance to environmental stress [START_REF] Cornelissen | A handbook of protocols for standardised and easy measurement of plant functional traits worldwide[END_REF][START_REF] Pérez-Harguindeguy | New handbook for stand-ardised measurement of plant functional traits worldwide[END_REF]. In addition, life history is used to assess maximum lifespan and plant height is associated with competitiveness for light and whole plant fecundity [START_REF] Pérez-Harguindeguy | New handbook for stand-ardised measurement of plant functional traits worldwide[END_REF]. Specific leaf area is the one-sided leaf area per leaf mass and is associated with resource acquisition and photosynthetic rate [START_REF] Pérez-Harguindeguy | New handbook for stand-ardised measurement of plant functional traits worldwide[END_REF]. Reproduction strategy, seed dry mass and number of seeds per plant do not only relate with species persistence, but also with dispersal capacity [START_REF] Ottaviani | Linking Plant Functional Ecology to Island Biogeography[END_REF]. In particular, alien species reproducing predominantly sexually may benefit from lower dispersal limitation and greater genetic diversity [START_REF] Van Kleunen | Characteristics of successful alien plants[END_REF]. At the same time, while small and light seeds are better dispersed at longer distances, largeseeded plants may benefit from more stored resources [START_REF] Van Kleunen | Characteristics of successful alien plants[END_REF].

Functional trait data collected in areas environmentally analogous to sub-Antarctic islands were compiled from the literature [START_REF] Frenot | Biological invasions in the Antarctic: extent, impacts and implications[END_REF][START_REF] Mathakutha | Invasive species differ in key functional traits from native and non-invasive alien plant species[END_REF]. Whenever we could not find information collected in comparable environments, we relied on functional trait data included in the TRY database [START_REF] Kattge | TRY plant trait database-enhanced coverage and open access[END_REF]. For each alien species, the dominant reproduction strategy in the study area was assessed relying on expert-based knowledge (personal communication, Lebouvier, M., & Bittebiere, A.K.).

Topoclimatic layers

To model the species-environment relationship at fine spatial resolution, we first downloaded coarsegrained temperature (BIO1, BIO5 and BIO6 -annual mean temperature, max temperature warmest month and min temperature coldest month) and annual precipitation (BIO12) grid layers at 1-km resolution (at the equator) from the CHELSA database [START_REF] Karger | Climatologies at high resolution for the earth's land surface areas[END_REF] and then disaggregated their spatial resolution using physiographically informed models fitted through geographically weighted regression (GWR;Fotheringham & Rogerson, 2008). This downscaling technique allows statistically predicting the local value of the coarse-grain CHELSA climatic variables as a function of environmental grid layers available at finer spatial resolution (30-m) and known to drive microclimate heterogeneity [START_REF] Lenoir | Climatic microrefugia under anthropogenic climate change: implications for species redistribution[END_REF][START_REF] Lembrechts | Comparing temperature data sources for use in species distribution models: From in-situ logging to remote sensing[END_REF]. GWR-derived topoclimatic layers, beyond allowing to model the species-environment relationship at a more meaningful spatial resolution, have already proved to account better for the complex interactions between macroclimate and topography [START_REF] Lenoir | Climatic microrefugia under anthropogenic climate change: implications for species redistribution[END_REF][START_REF] Lembrechts | Comparing temperature data sources for use in species distribution models: From in-situ logging to remote sensing[END_REF].

As using BIO5 (max temperature warmest month) and BIO6 (min temperature coldest month) in place of BIO1 did not improve species distribution models, we ultimately used BIO1 (hereafter mean temperature) and BIO12 as topoclimatic predictors. A full description of the downscaling procedure is reported in Appendix S2 along with the results of the GWR models.

Human-related layers

As human disturbances are known to favour the establishment of alien plants through propagule introduction and alteration of habitat conditions, we generated a 30-m resolution layer reporting the distance between each human settlement (the research station, Baie du La Pérouse, Pointe Basse and Baie Américaine) and any location on the island. Specifically, assuming that human disturbance is stronger in most accessible areas, we derived a least cost distance grid layer providing a measure of accessibility. Terrain slope changes between both orthogonally and diagonally neighbouring raster cells were used to compute the cost of reaching any location on Possession Island starting from any human settlement and following all potential paths of raster cells (function "accCost", "gdistance" R package; [START_REF] Etten | gdistance: Distances and Routes on Geographical Grids[END_REF]. High costs were thus associated with locations not easily reachable from human settlements due to high topographic roughness (Appendix S3, Figure S3.2).

A network of hiking paths has been designed by the RN-TAF to restrict human movements for wildlife conservation purposes, and walking these paths currently constitutes the only authorized way to move across the island. As humans are a critical vector of propagule introduction and dispersal on sub-Antarctic islands, we derived a 30-m resolution raster layer reporting the distance between any location on Possession Island and the closest hiking path using the function "distance" from the "raster" R package [START_REF] Hijmans | raster: Geographic Data Analysis and Modeling. R package version 3[END_REF] (Appendix S3, Figure S3.2).

Alien species distribution modelling

The occurrence probability of the six alien plant species was separately modelled as a function of the topoclimatic (mean temperature and annual precipitation) and human-related variables (path distance and least cost) by means of logit binary generalized linear models (GLM). The single-species distribution models (single-SDMs) were trained and tested on datasets obtained through a re-sampling procedure of the presence/absence data performed in the environmental space to reflect all available environmental conditions on Possession Island (see Appendix S4). All four topoclimatic and humanrelated predictors were retained to fit the single-SDMs as the relative variance inflation factor (function "vif", R package "car"; Fox & Weisberg, 2019) was always below a threshold of 3. Secondorder polynomial terms were included in the model to allow for intermediate niche optima of the species or in case lack-of-fit tests detected consistent departure from linearity in the profile of Pearson residuals (function "residualPlots", R package "car"; [START_REF] Fox | An R Companion to Applied Regression (Third)[END_REF]. The statistical significance of each predictor was tested using type II analysis of deviance (function "Anova", R package "car"; [START_REF] Fox | An R Companion to Applied Regression (Third)[END_REF]. We then computed the likelihood profile-based 95% confidence intervals of the regression parameters.

Single-SDMs predictive performance was measured using the true skill statistic (TSS, equal to sensitivity + specificity -1; function "ecospat.max.tss", R package "ecospat"; Broennimann, Di Cola, & Guisan, 2018) computed on the testing datasets obtained through the environmental matching described in Appendix S4. We used the TSS as it has desirable properties of other accuracy measures (e.g. Kappa and AUC), while being unaffected by prevalence [START_REF] Allouche | Assessing the accuracy of species distribution models: prevalence, kappa and the true skill statistic (TSS)[END_REF]. Also, we computed the deviance-based R² value as a measure of goodness-of-fit of each single-SDM.

The occurrence probability estimated by the full single-SDMs (including both topoclimatic and human-related predictors) for each alien plant species was mapped on a 30-m raster grid layer to visualize their predicted distribution across Possession Island.

Relationship between plant traits and alien species dependence on propagule pressure

As preliminary analyses, we measured the relative importance of human-related variables in determining alien species occurrence in the single-SDMs. To this aim, we used the sum of Akaike weights (w), which provides an easily interpretable measure of variable importance (it ranges from 0 to 1, with a high value for a given variable indicating its high importance relative to the others; [START_REF] Burnham | Model Selection and Multimodel Inference: a Practical Information-Theoretic Approach[END_REF]. Then we graphically related the species-specific values of the functional traits to the sum of weights to look for relationships between plant traits and the importance of human-related variables (see Appendix S7).

Secondly, we investigated how the interaction between human-related variables and plant traits affected alien species occurrence in a multi-species distribution model (multi-SDM), focusing on those functional traits that showed some relationship with the dependence on human-related variables in the single-SDMs. To this aim, we modelled the occurrence of all alien species together as a function of topoclimatic and human-related variables by means of a logit binary GLM, including the interaction between species identity and topoclimatic variables on the one hand and the interaction between species functional traits and human-related variables on the other hand. This allowed exploring how the effect of human-related variables on alien species occurrence varied according to plant traits, while controlling for species-specific responses to topoclimate. To select the most parsimonious model, we fitted all possible sub-models including different combinations of the functional traits-anthropogenic variables interaction terms (function "dredge", R package "MuMIn"; [START_REF] Barton | MuMIn: Multi-Model Inference[END_REF], always retaining the species-topoclimate interaction terms and the main effect of path distance and least cost in each candidate sub-model. Then, we computed the sum of Akaike weights for each model term and used the evidence ratio as a measure of the relative importance of variables [START_REF] Massol | The influence of trophic status and largescale climatic change on the structure of fish communities in Perialpine lakes[END_REF][START_REF] Burnham | Model Selection and Multimodel Inference: a Practical Information-Theoretic Approach[END_REF]. Specifically, we computed the evidence ratio of the i-th variable (ER i ) as the odds of its sum of Akaike weights:

!"# = &# 1 -&#
The evidence ratio was then compared with its expected value (ER null ) under the "null hypothesis" that the variable explained as much deviance as a randomly generated explanatory variable, and would thus be as likely as not to be incorporated in the best models. As all the variables were tested in a balanced design, ER null = 1 in all tested cases. Following [START_REF] Massol | The influence of trophic status and largescale climatic change on the structure of fish communities in Perialpine lakes[END_REF], the effect of a variable i was deemed unlikely if ER i < 0.37 ER null , implausible when 0.37 ER null < ER i < ER null , plausible when ER null < ER i < 2.72 ER null , and likely when ER i > 2.72 ER null . These thresholds correspond to differences in Akaike information criterion equal to +2 or -2, which are commonly admitted as a good rule-of-thumb gap to compare model performance.

Results

Effect of topoclimatic and human-related variables on single species occurrence

Predictive performances of the single-SDMs varied greatly across species (Table 1): high values of TSS were observed for P. pratensis, S. alsine and J. bufonius (0.80-0.82), while low values were obtained for the remaining species (from 0.09 to 0.29). The R 2 values showed a similar trend, with the highest value obtained for J. bufonius (0.48) and the lowest for C. fontanum (0.02).

Overall, the occurrence of P. pratensis, S. alsine and J. bufonius appeared to be strongly conditioned by both topoclimatic and human-related variables, while C. fontanum, P. annua and S. procumbens were less affected by human-related variables (Table 1, Figure 1, 2).

All alien species, except S. alsine, exhibited a significant positive or humped-shaped relationship with mean temperature (Table 1), meaning that their occurrence probability increased with increasing temperature (Figure 1, Appendix S5). More specifically, the occurrence probability of J. bufonius, S.

procumbens and C. fontanum peaked at mean temperature values around 4.5 °C, while the presence of P. pratensis and P. annua "linearly" increased with temperature.

Annual precipitation significantly affected the presence of P. pratensis, S. alsine and J. bufonius, while it had a minor influence on the occurrence of the other species (Table 1). In particular, the odds of finding P. pratensis and J. bufonius decreased approximately by 90% for each 500 mm increment in annual precipitation, while the occurrence probability of S. alsine sharply decreased for annual precipitation values above 1500 mm (Figure 1).

All species except C. fontanum exhibited a significant negative relationship with path distance (i.e.

the occurrence probability of the species decreased at increasing distances from the hiking paths), though its influence varied among species (Table 1, Figure 1, Appendix S5). In this regard, the odds of finding P. pratensis, S. alsine and J. bufonius decreased respectively by 20%, 16% and 19% moving 100 m away from the paths, while the odds of finding P. annua and S. procumbens decreased by 4% and 6%, respectively.

Least cost distance to settlements appeared to influence the occurrence of all analysed species except S. procumbens (Table 1). In particular, the odds of finding P. pratensis, S. alsine and J. bufonius decreased, respectively, by 17%, 13% and 44% for each increment of 5000 units of cost of travelling a given path from a human settlement (Figure 1, Appendix S5). On the contrary, C. fontanum and P.

annua showed a positive relationship with least cost, with their odds of occurring increasing respectively by 9% and 5% for each increment of 5000 units of cost of travelling a given path from a human settlement (Figure 1 and Appendix S5). (which was not certified by peer review) is the author/funder. All rights reserved. No reuse allowed without permission.

The copyright holder for this preprint this version posted July 21, 2020.

; https://doi.org/10.1101/2020.07.19.210880 doi: bioRxiv preprint

Plant traits and species dependence on propagule pressure

In the preliminary analyses, residence time, life history, vegetative reproduction and plant height showed some relationship with the sum of weights of the human-related variables in the single-SDMs (Appendix S7, Figure S7.6 and S7.8), while seed-and leaf-related traits clearly showed no relationship (Appendix S7, Figure S7.7 and S7.9).

Then, the multi-SDM confirmed significant interactions of residence time, life history and plant height with the human-related variables (Appendix S7, Figure S7.10). Residence time and plant height appeared to interact with both human-related variables, while life history seemed to interact only with least cost in determining alien species occurrence. In particular, the effect of human-related variables on alien species occurrence varied with plant height, and while the occurrence probability of taller plants sharply decreased moving far from human facilities, a weaker and sometimes opposite trend was observed for shorter species (Figure 3a

Discussion

As hypothesized, both environmental and human-related variables locally affected alien species occurrence on Possession Island, though with differences among species. Overall, results confirmed the key role of human-related propagule pressure in favouring alien species establishment and spread on sub-Antarctic islands [START_REF] Frenot | Biological invasions in the Antarctic: extent, impacts and implications[END_REF][START_REF] Le Roux | Human activities, propagule pressure and alien plants in the sub-Antarctic: Tests of generalities and evidence in support of management[END_REF][START_REF] Shaw | Southern Ocean Islands invaded: conserving biodiversity in the world's last wilderness[END_REF], though we also observed a significant effect of abiotic conditions. Indeed, climate barriers seemed to prevent alien species occurrence in the most environmentally stressful areas of the island [START_REF] Lembrechts | Disturbance is the key to plant invasions in cold environments[END_REF].

In particular, our results suggested the existence of two main invasion patterns arising from the species-specific dependence on human-related propagule pressure [START_REF] Frenot | Biological invasions in the Antarctic: extent, impacts and implications[END_REF][START_REF] Shaw | Southern Ocean Islands invaded: conserving biodiversity in the world's last wilderness[END_REF]: low-spread species (P. pratensis, S. alsine and J. bufonius) strongly relying on human-assisted dispersal along hiking trails and in the vicinity of human settlements; and high-spread species (C.

fontanum, P. annua and S. procumbens) mostly limited by harsh climatic conditions at high altitudes.

Due to their dependence on human-related variables, low-spread species were predicted to occur mainly close to hiking paths and human settlements, pointing to the importance of anthropogenic activities as key drivers of continuous propagule pressure favouring species establishment [START_REF] Whinam | Subantarctic hitchhikers: expeditioners as vectors for the introduction of alien organisms[END_REF][START_REF] Pickering | Do tourists disperse weed seed? A global review of unintentional human-mediated terrestrial seed dispersal on clothing, vehicles and horses[END_REF]. Once introduced through ship-to-shore transport, propagules are then likely to be dispersed on hiking paths through trampling [START_REF] Whinam | Subantarctic hitchhikers: expeditioners as vectors for the introduction of alien organisms[END_REF]. However, the harsher environmental conditions characterizing the west side of Possession Island also limit the occurrence of low-spread species to the east. In particular, the west-east gradient of annual precipitation (Appendix S2, Figure S2.1) clearly overlaps with the low-spread species distribution (Figure 2 and Appendix S6), suggesting that their establishment might be prevented in areas with abundant precipitation. Nevertheless, the precipitation gradient is also connected to human presence, so that in the west side of the island (less inhabited and more preserved) anthropogenic propagule pressure is weaker. In any case, our results evidenced that low-spread species may lack important adaptations to successfully colonize less disturbed areas with limiting abiotic conditions, while remaining relegated to areas of high human presence [START_REF] Lembrechts | Microclimate variability in alpine ecosystems as stepping stones for non-native plant establishment above their current elevational limit[END_REF].

On the contrary, high-spread species appeared weakly (yet positively) influenced by human-related variables, suggesting that, in spite of the undisputed importance of anthropogenic activities in promoting alien plants establishment [START_REF] Whinam | Subantarctic hitchhikers: expeditioners as vectors for the introduction of alien organisms[END_REF][START_REF] Huiskes | Aliens in Antarctica: assessing transfer of plant propagules by human visitors to reduce invasion risk[END_REF], high-spread species may possess key traits releasing them from direct dependence on anthropogenic propagule pressure.

Consequently, these species appeared to be mostly limited by the extreme climatic conditions of the high and cold inner sectors of Possession Island. Furthermore, the low predictive performance of high-spread SDMs indicate that the occurrence of these species can only be partly explained by the influence of topoclimatic and human-related variables, so that other factors not considered here (e.g. soil properties, plant-soil microbiota interactions, snow cover) may also play an important role in driving their distribution at even finer spatial resolutions.

Critically, although we managed to obtain relatively high-resolution topoclimatic data, it is important to realize that the CHELSA climate for the island 1) might lack the accuracy it has at temperature latitudes, being based on extrapolations from a single weather station only, and 2) represents air temperature only, while short plants as those analysed here relate more strongly to soil and nearsurface temperatures [START_REF] Lembrechts | Comparing temperature data sources for use in species distribution models: From in-situ logging to remote sensing[END_REF]. This highlights the need for in-situ soil-and nearsurface temperature measurements in remote locations to get more ecologically meaningful climate data.

Although the small set of analysed alien plant species calls for caution in interpretation, we confirmed here that certain plant traits confer greater invasiveness in sub-Antarctic environments. By relating plant traits to species responses to human-related variables and analysing the effect of their interaction on alien species occurrence, we found evidence that low-stature was a key feature that discriminated invasive from non-invasive alien species on Possession Island. However, residence time and life history also appeared to affect species invasiveness.

High-spread species were shorter than low-spread species [START_REF] Mathakutha | Invasive species differ in key functional traits from native and non-invasive alien plant species[END_REF]. Consistently, we observed a sharper decrease in the occurrence probability of taller plants moving away from both hiking paths and human settlements. As plant height is generally associated with species adaptations to harsh environments [START_REF] Cornelissen | A handbook of protocols for standardised and easy measurement of plant functional traits worldwide[END_REF] and, specifically, low-stature has been attributed to frost avoidance mechanisms in high mountains [START_REF] Márquez | Freezing tolerance in grasses along an altitudinal gradient in the Venezuelan Andes[END_REF][START_REF] Ladinig | How endangered is sexual reproduction of highmountain plants by summer frosts? Frost resistance, frequency of frost events and risk assessment[END_REF], shorter species may be reasonably favoured in windy and cold sub-Antarctic environments [START_REF] Mathakutha | Invasive species differ in key functional traits from native and non-invasive alien plant species[END_REF] and therefore be more easily released of human dependence. Indeed, the importance of functional traits providing tolerance to abiotic stress increases with environmental harshness, even under strong anthropogenic disturbance [START_REF] Zefferman | Plant communities in harsh sites are less invaded: a summary of observations and proposed explanations[END_REF].

Further, our results supported the hypothesis that residence time positively affects invasiveness [START_REF] Lockwood | The role of propagule pressure in explaining species invasions[END_REF][START_REF] Pyšek | Naturalization of central European plants in North America: species traits, habitats, propagule pressure, residence time[END_REF], though with some exceptions. Generally, old residents (e.g. C. fontanum and P. annua) were less dependent on human-related propagule pressure and more widely spread than new resident species. Nevertheless, among the old residents, P. pratensis was strongly dependent on human-related variables and was still mostly restricted to the original introduction sites. On the other hand, S. procumbens, a new resident, has been able to spread extensively and quicker than the other (old residents) high-spread species. However, this might be due to the difference in plant height of the two species: while P. pratensis is among the tallest analysed species, S. procumbens is the shortest. The multi-SDM showed that perennials were slightly less dependent on human presence than annuals (Figure 3c). Although annuals might benefit from high dispersal abilities (e.g. abundant light seeds) and usually spread quicker and wider [START_REF] Pertierra | Global thermal niche models of two European grasses show high invasion risks in Antarctica[END_REF], perennials can sustain short growing seasons [START_REF] Frenot | Human activities, ecosystem disturbance and plant invasions in subantarctic Crozet, Kerguelen and Amsterdam Islands[END_REF][START_REF] Shaw | Southern Ocean Islands invaded: conserving biodiversity in the world's last wilderness[END_REF] and potentially colonize harsher environments [START_REF] Dietz | Recognition that causal processes change during plant invasion helps explain conflicts in evidence[END_REF]. In our case, short perennials (e.g. C.

fontanum) might be favoured over tall annuals (e.g. J. bufonius) due to the interaction between stresstolerant traits, such as plant height, and high abiotic tolerance.

Though the interaction between vegetative reproduction and human-related variables was not included in the most parsimonious multi-SDM, alien species may still benefit from sexual reproduction, as suggested from the lower importance of human-related variables for the occurrence of alien plants reproducing sexually in the single-SDMs (Appendix S7). As discussed above, the high dispersal potential of sexually reproducing alien species may foster their extensive spread as, for instance, observed for Poa annua in the Antarctic Peninsula [START_REF] Pertierra | Global thermal niche models of two European grasses show high invasion risks in Antarctica[END_REF]. Nonetheless, by reproducing vegetatively, perennials might outcompete annuals in maintaining viable and persistent populations during unfavourable seasons. Finally, in spite of their acknowledged importance in conferring invasiveness in sub-Antarctic islands [START_REF] Mathakutha | Invasive species differ in key functional traits from native and non-invasive alien plant species[END_REF], we found no evidence of the role of seed and leaf traits in affecting species dependence on human-related variables. This is possibly due to the small set of analysed alien species, which might have prevented the emergence of further functional traits-anthropogenic variables relationships.

Overall, our approach allowed identifying fine-scale drivers of alien species distribution on sub-Antarctic islands, along with the most likely features that favour their spread beyond sources of continuous human-assisted introductions. Despite some limitations (e.g. limited number of species), our study opens avenues for a more targeted management of alien plant invasions in these unique environments, potentially allowing to anticipate future problematic invasions.

Figure 1 -

 1 Figure 1 -Response curves of the analysed alien species in the single-SDMs.

Figure 2 -

 2 Figure 2 -Predicted occurrence of Sagina procumbens and Juncus bufonius. Pocc: occurrence probability. Dashed lines represent hiking paths, while black dots represent human settlements. Occurrence maps of the other alien species are reported in Appendix S6 (Figure S6.4 and S6.5).

  ,b and Appendix S7, FigureS7.11). In addition, old residents were on average less affected by the human-related variables than new residents (Figure3a,b and Appendix S7, FigureS7.11a). Finally, perennials appeared to be on average less negatively affected by least cost distance to human settlements than annuals (Figure3cand Appendix S7, FigureS7.11b,c).

Figure 3 -

 3 Figure 3 -Effect of the interaction between human-related variables and plant features on alien species occurrence probability (logit scale). Panel (a): effect of the path distance-plant height interaction on old and new resident occurrence probability. Panel (b): effect of the least cost-plant height interaction on old and new (annual) resident occurrence probability. Panel (c): effect of least cost distance to human settlements on annual and perennial (100 mm height) alien species occurrence (for both old and new residents). All plots are reported on a logit scale.

Table 1 -

 1 Single-SDM type II analysis of deviance tables and performance measures (R 2 and TSS). LR: Likelihood Ratio statistic; Df: degrees of freedom; p-val: p-value (*** p < 0.001; ** p < 0.01; * p < 0.05).(which was not certified by peer review) is the author/funder. All rights reserved. No reuse allowed without permission.

	The copyright holder for this preprint this version posted July 21, 2020.	Poa pratensis LR Df p-val 5.083 1 Annual precipitation 106.443 Predictors Mean temperature 1 *** 73.647 Juncus bufonius LR Df p-val * 32.516 2 *** 1 *** 68.406 Stellaria alsine LR Df p-val 0.312 1 = 0.576 55.538 Poa annua LR Df p-val 1 *** 62.554 Sagina procumbens Cerastium fontanum LR Df p-val LR Df p-val 2 *** 18.325 2 *** 2 *** 4.759 1 * 3.681 1 = 0.055 0.041 1 = 0.840 Least cost 10.837 1 *** 40.250 1 *** 4.420 1 * 5.778 1 * 1.118 1 = 0.290 12.483 1 *** Path distance 15.623 1 *** 8.740 1 ** 7.524 1 ** 3.927 1 * 6.782 1 ** 2.877 1 = 0.089 R 2 0.46 0.48 0.36 0.06 0.10 0.02 TSS 0.80 0.81 0.82 0.19 0.29 0.09
	; https://doi.org/10.1101/2020.07.19.210880
	doi:
	bioRxiv preprint

Acknowledgements

We would like to thank Marc Lebouvier (Ing. CNRS) for his suggestions which strongly improved the manuscript. We acknowledge the contribution of Lise Chambrin (Ing. RN-TAF) and all civil volunteers who collected data used in this research. Funding: Project Granted by the French Polar Institute Paul-Emile Victor 'IPEV 136 Subanteco', Institut Universitaire de France (IUF) 'ENVIE', Stratégie d'attractivité durable (SAD) Région Bretagne 'INFLICT'. JJL is funded by the Research Foundation Flanders (FWO, project OZ7828), MC by a Rita Levi-Montalcini grant.

Data availability statement

Data and R code available on Zenodo: https://doi.org/10.5281/zenodo.3951465

Authors' contribution

DR, MB, FM and JL conceived the idea; MB analyzed the data with FM and MC; MB led the writing of the manuscript. All authors contributed critically to the drafts and gave final approval for publication.

Conflict of interest disclosure

The authors of this article declare that they have no financial conflict of interest with the content of this article.