Mechanical properties and failure behavior of unidirectional porous ceramics

We show that the honeycomb out-of-plane model derived by Gibson and Ashby can be applied to describe the compressive behavior of unidirectional porous materials. Ice-templating allowed us to process samples with accurate control over pore volume, size, and morphology. These samples allowed us to evaluate the effect of this microstructural variations on the compressive strength in a porosity range of 45-80%. The maximum strength of 286 MPa was achieved in the least porous ice-templated sample (P(%) = 49.9), with the smallest pore size (3 μm). We found that the out-of-plane model only holds when buckling is the dominant failure mode, as should be expected. Furthermore, we controlled total pore volume by adjusting solids loading and sintering temperature. This strategy allows us to independently control macroporosity and densification of walls, and the compressive strength of icetemplated materials is exclusively dependent on total pore volume.

Macroporous ceramics are widely used in applications such as filtration, thermal insulation, scaffolds for tissue engineering, oxygen transport membranes, and solid oxide fuel cells [START_REF] Green | Cellular Ceramics: Intriguing Structures, Novel Properties, and Innovative Applications[END_REF][START_REF] Colombo | In Praise of Pores[END_REF] . They must combine mechanical stability with at least one other functional property such as high permeability, low thermal conductivity, or biocompatibility.

Strength is also an important parameter. It is usually increased by decreasing the total pore volume, even though it may degrade other functional properties. Morphological parameters such as pore size, morphology, or tortuosity can become crucial to maximize the performance while maintaining high strength. For example, a significant improvement in strength can be achieved by engineering anisotropic structures to mechanically reinforce the direction of the principle stress. However, most of the techniques conventionally used to produce macroporous ceramics with aligned porosity do not offer this level of flexibility.

Ice-templating is a processing technique suitable for producing anisotropic macroporous materials. It is based on freezing of colloidal suspensions and segregation of particles by the solidification front. After solidification, the frozen solvent is removed, leaving pores whose morphologies are a replica of the sublimated crystals [START_REF] Deville | Freeze-Casting of Porous Ceramics: A Review of Current Achievements and Issues[END_REF] . Finally, the green body is sintered to consolidate the microstructure. This process provides control of the pore architecture (pore volume, size, and morphology) through initial solids loading, cooling rate, or additives. Therefore, a good understanding of these parameters is essential to understanding the relationships between processing, microstructure, and the mechanical properties of anisotropically porous materials to extend their use to the aforementioned applications.

As predicted by Gibson and Ashby [START_REF] Ashby | The Mechanical Properties of Cellular Solids[END_REF][START_REF] Gibson | Modelling the Mechanical Behavior of Cellular Materials[END_REF][START_REF] Zhang | The out-of-plane properties of honeycombs[END_REF] , pore architecture greatly determines the mechanical response of porous materials. Identifying the failure mechanisms that govern fracture can lead to more optimized architectures and better performance. However, ice-templated materials have more complex structures than the models proposed in references [START_REF] Ashby | The Mechanical Properties of Cellular Solids[END_REF][START_REF] Gibson | Modelling the Mechanical Behavior of Cellular Materials[END_REF][START_REF] Zhang | The out-of-plane properties of honeycombs[END_REF] . Recently, different groups studied the mechanical properties of ice-templated materials based on their pore structure. For example, Ojuva et al. [START_REF] Ojuva | Mechanical performance and CO 2 uptake of ion-exchanged zeolite A structured by freeze-casting[END_REF] investigated the effect of solids loading and freezing temperature on zeolite materials and related the compressive strength to the pore aspect ratio. Porter et al. [START_REF] Porter | Bioinspired Scaffolds with Varying Pore Architectures and Mechanical Properties[END_REF] found a similar result in TiO 2 by controlling the pore size through the polyethylene glycol (PEG) viscosity, pH, and isopropanol alcohol (IPA) concentration. Hunger et al. [START_REF] Hunger | Structure-property-processing correlations in freeze-cast composite scaffolds[END_REF] described the mechanical behavior of a chitosan-hydroxiapatite composite using an hybrid model accounting for the unidirectional pores and the porosity in the walls. However, these studies are mostly limited to a highly porous materials (> 80%) and a more detailed work describing the microstructural effects on compressive strength of unidirectional porous ceramics in a broader porosity range is still lacking.

Our objective is to characterize the compressive strength of ice-templated ceramics in a broad porosity range and link their mechanical behavior to the pore architecture. We will discuss the effects of pore volume, size, and morphology on the failure mechanism. This discussion will allow us to assess the validity of the Gibson and Ashby models for the prediction of the compressive strength of unidirectional porous materials.

Experimental Procedure

Sample preparation. Ice-templated specimens were produced following the same procedure reported elsewhere [START_REF] Seuba | The effect of wall thickness distribution on mechanical reliability and strength in unidirectional porous ceramics[END_REF] . Basically, suspensions were prepared by mixing distilled water with 3 mol% yttria-stabilized zirconia (TZ-3YS, Tosoh, Tokyo, Japan) at different weight ratios (from 45% to 70%), 0.75 wt.% of dispersant (Prox B03, Synthron, Levallois-Paris, France), and 3 wt.% of organic binder PVA (PVA2810, Wacker, Burghausen, Germany). In some suspensions, zirconium acetate (20 g/L) was added to the slurry to modify the pore morphology. Afterwards, the slurry was magnetically stirred to ensure a good dispersion and ball milled for a minimum of 18 h to break up the agglomerates. It was then deaired for at least 10 min.

The ice templating process consisted of pouring 10 ml of slurry into a PTFE mould (20 mm diameter and 25 mm height) placed on a copper plate and freezing from the bottom to the top. The top of the samples was exposed to air and kept at room temperature. The freezing temperature was controlled by circulating silicone oil regulated by a cryothermostat (Model CC 905, Hubert, Offenburg, Germany). The cooling rate was set at 2 °C/min. A faster cooling rate was achieved dipping a copper rod with the mould on top in liquid nitrogen. The cooling rate was monitored with a thermocouple and it was determined to be 25 °C/min on average. After solidification, samples were demoulded and sublimated for at least 48 h in a commercial freeze-dryer (Free Zone 2.5 Plus, Labconco, Kansas City, Missouri, USA).

Binder was removed from the green bodies by heating to 500 °C at 3 °C/min with a 5 h hold. Then, samples were sintered between 1300 °C and 1400 °C at 5 °C/min and held 3 h. The cooling rate was kept constant at 5 °C/min until room temperature.

Samples with a non-oriented porosity were also prepared. The same zirconia powder was mixed with a commercially available pore former polypropylene (Propyltex 140S, Micro Powders Inc, Tarrytown, USA) at different ratios (50 wt.%, 60 wt.%, and 70 wt.%) and mixed with distilled water. The slurry was magnetically stirred and ball milled for a minimum of 24 h to break up the agglomerates. Afterwards, the slurry was frozen by dipping the container in liquid nitrogen and freeze-dried to obtain an homogeneous mix. Then, 8 g of the powder was pressed at 0.8 MPa in a mould of 20 mm diameter. The sintering temperature and dwell time were the same as those used for the ice-templated samples. To ensure a proper burn-out and avoid the formation of cracks an extra hold was added at 900 °C.

Morphological characterization.

The overall porosity, P(%), was calculated based on the mass (m) and volume (V) of the samples with respect to that of fully dense TZ-3YS (ρ ysz = 5.8 g/cm 3 ), as:

ρ ρ ρ ρ = = - mV ( 1
)
rel ysz y sz 1 ρ = - × P (%) (1 ) 100% (2)
rel

The results were confirmed in at least two samples per condition by the Archimedes method (ASTM B962-13).

The deviation between the overall porosity obtained by Archimedes and by Eq. 2 ranged 1-2%. The determination of pore size and wall thickness distributions were performed by image analysis using the "Local thickness" plug-in of the Fiji software [START_REF] Schindelin | Fiji: an open-source platform for biological-image analysis[END_REF] . In order to obtain a representative value of pore size and wall thickness, a minimum of 5 images per sample were analyzed. All the images were taken at different locations in a cross section perpendicular to the freezing direction (7 mm from the bottom of the sample) with a scanning electron microscope (Nova NanoSEM 230, FEI, Hillsboro, USA)) at 10-15 kV. The results were confirmed by mercury intrusion porosimetry (AutoPore IV 9500, Micromeritics), 2 samples per condition, with an applied pressure up to 0.31 bar.

Mechanical characterization.

The mechanical properties of specimens with isotropic (pore formers)

and anisotropic (ice-templated) porosity were measured by a compression test (LR15K Plus, Lloyd Instruments, Meerbusch, Germany) at a crosshead speed of 0.5 mm/min. The bottom and the top of the samples were removed with a slow speed saw leaving the final dimensions around 15 mm diameter and 17 mm height. Samples were tested with a cardboard pad on both sides to minimize the effect of superficial defects and misalignment. In all the tests, the maximum load at the end of the elastic stage was used to calculate the compressive strength.

Results and Discussion

We investigated the effects of pore volume, size, morphology, and directionality on the compressive behavior of porous materials. Figure 1 shows the representative microstructures of the studied samples.

Microstructural control.

The typical lamellar microstructure obtained in ice-templated samples when water is used as a solvent it is shown in Fig. 1a. Total pore volume P(%) control was achieved by adjustment of the solids loading, as reported in Table 1. Increasing the solids loading from 50% to 65% caused a decrease in P(%) from 69.5% to 52.5%. This effect is independent of the freezing rate used. Thus, samples ice-templated at different freezing rates but with the same initial solids loading have a comparable total pore volume. For example, specimens in Fig. 1a,b exhibited a pore volume of 52.5% and 51.5% although they were frozen at different rates, 2 °C/min and 25 °C/min respectively. These results are consistent previous results for the same materials and similar processing conditions [START_REF] Han | Highly porous ZrO2 ceramics fabricated by a camphene-based freeze-casting route: Microstructure and properties[END_REF][START_REF] Hong | Ultra-high-porosity zirconia ceramics fabricated by novel room-temperature freezecasting[END_REF][START_REF] Hu | Effect of sintering temperature on compressive strength of porous yttria-stabilized zirconia ceramics[END_REF][START_REF] Zuo | Properties of Microstructure-Controllable Porous Yttria-Stabilized Ziroconia Ceramics Fabricated by Freeze Casting[END_REF][START_REF] Liu | The effect of particle size in freeze casting of porous alumina-zirconia composite[END_REF] .

For ice-templated materials, the freezing rate is the most important feature controlling pore size. Figure 2 shows the pore size distributions of the samples described in Fig. 1 and Table 1. Sample frozen at 25 °C/min (Fig. 1b) exhibits a smaller mean pore size (3.1 ± 1.2 μm) and a narrower distribution compared with the sample frozen at 2 °C/min (13.7 ± 4.8 μm), Fig. 1a. The magnitude of supercooling ahead of the ice front increases with freezing rate, while the tip radius of the ice crystals decreases, thus creating a finer microstructure [START_REF] Deville | Freezing as a path to build complex composites[END_REF] . We measured pore size distribution using two different techniques: image analysis (d pIA ) and mercury porosimetry (d pHg ), Table 1. Both gave comparable results in all the experimental conditions with the exception of the specimen frozen at 2 °C/min and 65 wt.% solids loading. Most likely, increasing the particle concentration in the solution increases the probability of creating closed pores or bottlenecks that affect the measurement by mercury porosimetry but does not necessarily affect the image analysis.

The freezing rate is not the only parameter affecting the mean pore size. The comparison of samples with different solids loading and frozen at constant cooling rate, either 2 °C/min or 25 °C/min, shows a decrease in pore size when the solids loading increases, Table 1. However, this pore size reduction is of secondary importance compared to the effects of the cooling rate. The main influence of solids loading on pore size variation is increasing the number of particles rejected by the solidification front. When the solids loading increases, it becomes more difficult for the advancing solidification front to repel the particles, thus hindering the ice growth and eventually reducing the final pore size.

Pore morphology is primarily defined by the nature of the solvent [START_REF] Deville | Freeze-Casting of Porous Biomaterials: Structure, Properties and Opportunities[END_REF] . Many solvents have been used before: water [START_REF] Deville | Freezing as a path to build complex composites[END_REF][START_REF] Fukasawa | Pore structure of porous ceramics synthesized from water-based slurry by freeze-dry process[END_REF] , camphene [START_REF] Han | Highly porous ZrO2 ceramics fabricated by a camphene-based freeze-casting route: Microstructure and properties[END_REF][START_REF] Hong | Ultra-high-porosity zirconia ceramics fabricated by novel room-temperature freezecasting[END_REF][START_REF] Koh | In situ Fabrication of a Dense/Porous Bi-layered Ceramic Composite using Freeze Casting of a Ceramic-Camphene Slurry[END_REF][START_REF] Araki | New freeze-casting technique for ceramics with sublimable vehicles[END_REF] , TBA [START_REF] Hu | Control of pore channel size during freeze casting of porous YSZ ceramics with unidirectionally aligned channels using different freezing temperatures[END_REF][START_REF] Liu | A novel way to fabricate tubular porous mullite membrane supports by TBA-based freezing casting method[END_REF][START_REF] Chen | Ceramics with special porous structures fabricated by freeze-gelcasting: Using tert-butyl alcohol as a template[END_REF] , cyclohexane [START_REF] Naviroj | Directionally aligned macroporous SiOC via freeze casting of preceramic polymers[END_REF] . In the case of water, pores usually exhibit a lamellar morphology, (Fig. 1a). However, the addition of components such as glycerol [START_REF] Munch | Architectural Control of Freeze-Cast Ceramics Through Additives and Templating[END_REF][START_REF] Sofie | Fabrication of Functionally Graded and Aligned Porosity in Thin Ceramic Substrates With the Novel Freeze Tape-Casting Process[END_REF] , polystyrene [START_REF] Koh | In situ Fabrication of a Dense/Porous Bi-layered Ceramic Composite using Freeze Casting of a Ceramic-Camphene Slurry[END_REF] , or sucrose [START_REF] Munch | Architectural Control of Freeze-Cast Ceramics Through Additives and Templating[END_REF] can modify the pore size and shape.

Recently, it has been reported that the addition of zirconium acetate (ZRA) into the initial slurry turns the pore morphology into a honeycomb-like structure with smooth surfaces [START_REF] Deville | Ice-structuring mechanism for zirconium acetate[END_REF] . ZRA modifies the ice-crystal morphology by affecting incorporation of water molecules into the growing ice crystal. Even though the pore morphology changes (Fig. 1c), the fundamental mechanism for pore formation remains unaltered, and therefore, it is possible to control the pore morphology independently of the total pore volume. However, the addition of ZRA modifies the pore size distribution, even if the solids loading and the freezing rate used are identical to samples processed without ZRA, Fig. 2.

A set of samples with isotropic porosity (Fig. 1d) was also prepared by organic burn-out (pore formers). Since the macroporosity obtained by this technique is a replica of the organic part originally mixed with the ceramic powder, the total pore volume is controlled by the organic-ceramic ratio. Pores are homogeneously dispersed with no preferential orientation, leaving an isotropic material. The processing parameters (volume and size of organic, and sintering temperature) were adjusted to obtained a total pore volume of 49.6% and a mean pore size 19.8 ± 11.9 μm, similar to the samples obtained by ice-templating. However, Fig. 2 shows that the pore size distribution is remarkably broader than the ice-templated samples.

Prediction by mechanical models. The most widely used models to predict the mechanical response of cellular materials were developed by Gibson and Ashby [START_REF] Ashby | The Mechanical Properties of Cellular Solids[END_REF][START_REF] Zhang | The out-of-plane properties of honeycombs[END_REF] . These models differentiate between three morphologies: open-cell, closed-cell, and honeycomb (Fig. 3). Although these models have been applied to different mechanical properties such as fracture toughness, creep, or Young modulus, we discuss only the compressive strength described by: where C 6 = 0.65 and C " 6 = 1 are geometric constants determined empirically, φ represents the solid fraction in the edges, ρ s and ρ * are the apparent density of the dense and cellular material respectively, and σ p the modulus of rupture of the solid material within the walls.

• Closed-cell (brittle crushing) σ σ φ ρ ρ φ ρ ρ =                     + -        ⁎ ⁎ C C s " (1 ) (3)
• Open-cell (brittle crushing)

σ σ ρ ρ =              ⁎ C (4) 0 4 0 3/2
with C 4 = 0.2, and where σ p , ρ s , and ρ * have the same meaning as in the closed-cell model. • Honeycomb (out-of-plane)

σ ρ ρ =              ⁎ E 6 ( 5
) s 0 3
where E s is the Young's modulus of the corresponding dense material, and ρ s and ρ * are again the apparent density of the dense and cellular material respectively. Figure 3 shows the normalized compressive strength of samples processed in this work along with those obtained by other processing methods (replica, foaming, and pore formers) extracted from [START_REF] Studart | Processing Routes to Macroporous Ceramics: A Review[END_REF] . In all cases, the strength was normalized by the flexural strength of the bulk material because bending has been identified as the main failure mechanism in cellular materials within the porosity range studied here [START_REF] Meille | Mechanical properties of porous ceramics in compression: On the transition between elastic, brittle, and cellular behavior[END_REF] . This normalization allows us to isolate the effect of pore microstructure as we assess the mechanical models.

Figure 3A shows that the strength of samples obtained by the replica method always have a lower strength value than that predicted by Eq. 4. This behavior has been extensively reported in the literature [START_REF] Dam | Compressive behavior and deformation-mode map of an open cell alumina[END_REF][START_REF] Zhu | Preparation of silicon carbide reticulated porous ceramics[END_REF][START_REF] Oliveira | Behaviour of open-cell cordierite foams under compression[END_REF][START_REF] Acchar | Characterization of cellular ceramics for high-temperature applications[END_REF] and it is caused by the presence of microstructural flaws in the struts created during the burn-out of the organic template.

On the other hand, Fig. 3B,C show that the strength of specimens processed by either foaming or pore formers fell in the broad range predicted by the closed (Eq. 3) and open cell models (Eq. 4). The macroporosity obtained at low relative density by both techniques better resemble the open-cell structure, and thus Eq. 4 becomes the best descriptor of the compressive strength. Nevertheless, it is important to highlight the high variability of the data in this range, probably due to microstructural defects such as: cracks originated during burn-out or drying, presence of closed pores, inclusions, or dispersion in pore size distribution. When the relative density increases, the proportion of closed pores increases accordingly and the specimens exhibited a compressive strength near the upper limit determined by Eq. 3. Interestingly, the specimens obtained by pore formers fell in the prediction made by the honeycomb-out-of-plane model (Eq. 5). This effect can be related with the variation in load distribution through the structure when the porosity decreases. When the number of closed pores increases the structure tends to fail more by buckling, as predicted by honeycomb model [START_REF] Zhang | The out-of-plane properties of honeycombs[END_REF] , instead of bending of the struts, like in the open-cell model [START_REF] Ashby | The Mechanical Properties of Cellular Solids[END_REF] .

Figure 3D shows that ice-templated architectures are better described by the honeycomb out-of-plane model (Eq. 5). Figure 4 shows a broken sample that failed perpendicular to the external load at the middle of the specimen, where the stresses reach the maximum value, and indicative of a buckling fracture. The combination of anisotropic porosity with perpendicular struts connecting the main walls, characteristic of the ice-templated materials, interlock their lateral motion and prevent the shear stresses. Similarly as it was previously explained for porous materials obtained by pore formers, the ice-templated structure becomes more prone to collapse by buckling of the walls instead of localized bending as it was identified in foams [START_REF] Ashby | The Mechanical Properties of Cellular Solids[END_REF][START_REF] Zhu | Preparation of silicon carbide reticulated porous ceramics[END_REF][START_REF] Brezny | Factors Controlling the Fracture Resistance of Brittle Cellular Materials[END_REF] . Buckling also has been reported to be the main failure mechanism in different ice-templated materials [START_REF] Ojuva | Mechanical performance and CO 2 uptake of ion-exchanged zeolite A structured by freeze-casting[END_REF][START_REF] Hunger | Structure-property-processing correlations in freeze-cast composite scaffolds[END_REF] and in other unidirectional porous materials processed by wood pyrolisis [START_REF] Rambo | Microcellular Al 2 O 3 Ceramics from Wood for Filter Applications[END_REF] , extrusion [START_REF] Isobe | Gas permeability and mechanical properties of porous alumina ceramics with unidirectionally aligned pores[END_REF] , or 3D printing [START_REF] Bauer | High-strength cellular ceramic composites with 3D microarchitecture[END_REF] .

Figure 5 shows the compressive strength obtained experimentally versus the strength predicted by Eq. 5 for three sets of ice templated samples: lamellar frozen at 2 °C/min (Fig. 1a), lamellar frozen at 25 °C/min (Fig. 1b), and honeycomb frozen at 2 °C/min (Fig. 1c). The model describes the compressive behavior remarkably well for both pore morphologies. Although the addition of zirconium acetate modifies the pore morphology towards a honeycomb structure, the ceramic walls are still continuous and oriented parallel to the direction of the load, and therefore the predominant failure mechanism is still buckling. As we showed previously [START_REF] Deville | A meta-analysis of the mechanical properties of ice-templated ceramics and metals[END_REF] , when the continuity of the walls decreases, like in the dendritic porosity created when camphene is used as a solvent, Eq. 5 is not a good descriptor of the strength. The validity of the model is thus mainly determined by the directionality and continuity of the walls rather than their cross sections. Nevertheless, the Gibson and Ashby models were initially derived from the response of an idealized unit cell to a stress and do not consider microstructural effects such as particle size, microstructure of the struts, interaction between pores, or pore size. The effects of microstructure can be observed in Fig. 5 where Eq. 5 underestimates the strength of ice-templated samples with smaller pores (3-5 μm). This effect is particularly remarkable at high relative density (i.e. high strength) where the amount of energy stored during compression is higher and materials tend to be more brittle. In contrast, at low values of relative density, the total pore volume and the presence of microstructural defects increase and favor the progressive damage by buckling of the struts and the fit by Eq. 5 becomes more accurate.

Compression tests.

Compressive strength is the most reported mechanical property for ice-templated materials due to its simplicity in testing and sample preparation. Nonetheless, it is difficult to compare with the results in the literature because structures are notably different in terms of size, morphology, directionality, and even microstructure of the walls. However, it is possible to point out several trends common in the majority of samples.

Effect of solids loading, pore size, and pore morphology. Representative stress-strain curves recorded during the compression test are shown in Fig. 6 for three ice templated samples with different pore volumes, average pore size of 10-20 μm, and lamellar morphology. All specimens were tested with the pores aligned in the direction of the applied load. The P(%) = 45% and 62% samples exhibited an initial linear stage up to a sudden rupture, typical of brittle behavior. In contrast, samples with higher porosity (P(%) = 71%) presented a cellular-like rupture commonly observed in ceramic foams: after an initial elastic phase, the stress reaches a steady value caused by the progressive collapse of the struts [START_REF] Gibson | Cellular solids[END_REF] .

Figure 7 shows the effect of total pore volume (from 45% to 80%), size (Fig. 7A vs. Fig. 7C), morphology (Fig. 7A vs. Fig. 7B), and directionality (Fig. 7A vs. Fig. 7D) on compressive strength. As expected, when the total pore volume decreases the compressive strength increases independent of the pore microstructure and architecture.

Although samples with isotropic porosity (pore formers) processed in this work have lower strength than the ice-templated ones (Fig. 7A,D), we must be careful to evaluate the particular role of pore directionality. As Fig. 3 shows, the structures that obey the honeycomb out-of-plane model tend to collapse at higher strength than those that fail by localized bending or shear (open-cell) at relatively high relative density. The parallel alignment of the walls with the load allows a more optimal distribution of the stresses and prevents the bending of the struts at lower loads. Therefore, we can hypothesize than the optimized microstructure of ice-templated materials explains this strength increment.

However, there are other microstructural factors that also affect the mechanical performance of the isotropic samples such as the presence of large agglomerates (Fig. 1d) or the broad pore distribution (Fig. 2). Both features certainly might impact the strength through the existence of weaker points and stress concentrators.

Samples with lamellar and honeycomb structure (Fig. 7A,B) followed a similar trend, and therefore compressive strength seems to be unaffected by the pore morphology. In contrast, samples with smaller pore size (Fig. 7C) have a higher strength than their counterparts with larger pores and similar pore volume. This effect becomes less important when the pore volume increases, even reaching a point (around 75%) where the strength seems unaffected by the pore size. Although the influence of pore size has been extensively reported in different types of porous ceramics [START_REF] Porter | Bioinspired Scaffolds with Varying Pore Architectures and Mechanical Properties[END_REF][START_REF] Zuo | Properties of Microstructure-Controllable Porous Yttria-Stabilized Ziroconia Ceramics Fabricated by Freeze Casting[END_REF][START_REF] Fu | Toward Strong and Tough Glass and Ceramic Scaffolds for Bone Repair[END_REF][START_REF] Colombo | Macro-and micro-cellular porous ceramics from preceramic polymers[END_REF][START_REF] Genet | A two-scale Weibull approach to the failure of porous ceramic structures made by robocasting: Possibilities and limits[END_REF] , the main cause of this behavior is unclear and it might result from a combination of factors. One of the main contributions could be related with reduction of the volume of the struts. For a constant pore volume, a decrease in pore size will leads to a wall thickness reduction, affecting the probability of finding a catastrophic defect and the strength [START_REF] Seuba | The effect of wall thickness distribution on mechanical reliability and strength in unidirectional porous ceramics[END_REF][START_REF] Brezny | The effect of cell size on the mechanical behavior of cellular materials[END_REF] . The strength of ceramics is strongly related with the volume of the material solicited, it is therefore more convenient to describe their mechanical properties based on the distribution of the solid parts (struts) rather than the organization of the voids (pores). For this reason it is more accurate to refer this strengthening effect as a wall thickness effect instead of pore size.

Interestingly, when the pore volume increases the experimental strength values tend to those predicted by the honeycomb out-of-plane model, regardless of the pore structure. This behavior might be related with the transition between failure mechanisms (brittle and cellular-like fracture) [START_REF] Meille | Mechanical properties of porous ceramics in compression: On the transition between elastic, brittle, and cellular behavior[END_REF] . When the pore volume increases above a certain value (typically above 60%), samples tend to fail progressively after the local buckling of the struts and pore morphology and size become secondary parameters to optimize to increase the strength. When the pore volume decreases, the material fails in a brittle manner with cracks propagating parallel to the load, and microstructural parameters like wall thickness distribution are of primary importance to increase the strength with no further reduction on the pore volume. Effect of sintering temperature. Total pore volume can also be tailored by the sintering temperature. Figure 8A shows the effect of solids loading and sintering temperature on total pore volume P(%). The solids loading investigated ranged between 50 wt.% and 65 wt.% for three different sintering temperatures (1300 °C, 1350 °C, and 1400 °C). Although, both parameters can modify P(%), the nature of the variation is different in each case. The spaces between the walls (interlamellar porosity) are determined for the most part by the ice templating process and it is a direct consequence of the solids loading and the sublimation of the ice crystals. Alternatively, porosity in the ceramic walls (intralamellar porosity) is mainly determined by the densification within the walls and therefore controlled by the sintering temperature. The SEM micrographs in Fig. 8A show the microstructural evolution in the walls when the sintering temperature increases from 1300 °C to 1400 °C for a given solids loading (65 wt.%). At 1300 °C (Fig. 8A) the residual porosity was estimated by image analysis to be around 24%, significantly higher compared to the 12% of the samples sintered at 1400 °C (Fig. 8C).

The higher densification of the walls at 1400 °C increases the observed shrinkage, Fig. 9. Samples sintered at 1300 °C exhibited a radial shrinkage of 17%, significantly lower than the 21% obtained at 1400 °C. At higher sintering temperatures, the densification of the walls drives the shrinkage of macropores and therefore the pore size distribution becomes narrower. This behavior can be observed in the image analysis performed at samples sintered at 1400 °C and 1300 °C and shown in Fig. 9.

Interestingly, the relative reduction of pore volume caused by the sintering temperature is remarkably different depending on the initial wall thickness, and hence the particle concentration used. For example, samples in Fig. 8A with 65 wt.% exhibit a porosity reduction around 20% when the sintering temperature increases from 1300 °C to 1400 °C. In contrast, at 50 wt.% solids loading the reduction of the total porosity is around 8% for the same variation in temperature.

However, no additional increment in the strength has been observed caused by the reduction on the residual porosity in the walls at a given total pore volume, (Fig. 8B). All the samples, independently of the sintering temperature, followed the same relationship between strength and porosity as predicted by Eq. 5. This suggests that the compressive strength of ice-templated materials is determined mainly by the total pore volume, rather than the strength of the individual struts. This result is contrary with Li et al. [START_REF] Li | Sintering of bi-porous titanium dioxide scaffolds: Experimentation, modeling and simulation[END_REF] , that reports an exclusive effect of interlamellar porosity on the strength of ice-templated materials. Nevertheless, further work should be made in that regard to assess the particular role of inter-and intralamellar porosity on mechanical properties.

Conclusions

This work reveals that the compressive behavior of unidirectional porous ceramics can be predicted by the honeycomb out-of-plane model in a wide porosity range, from 45% to 80%. However, the use of the model is limited to structures with a continuous array of unidirectional walls and where buckling is the main failure mode. Further improvements of the model considering microstructural effects on the strength like the pore size reduction could lead to more accurate results.

In addition, ice-templating exhibits a high versatility to control almost independently the main pore descriptors (volume, size, and morphology). The possibility to aim for a specific porosity, and hence strength, (through the solids loading) and tailor the densification of the walls (through the sintering temperature) is a powerful tool to adjust the specific surface area of the walls while keeping a constant pore volume. This flexibility can be useful in applications like SOFC cathodes or tissue engineering scaffolds, where a sufficient amount of macroporosity (interlamellar porosity) is required to guide the flow of the fluid, coupled with intralamellar porosity to improve conversion efficiency [START_REF] Greene | Mass transfer in graded microstructure solid oxide fuel cell electrodes[END_REF] or cell proliferation [START_REF] Wagoner Johnson | A review of the mechanical behavior of CaP and CaP/polymer composites for applications in bone replacement and repair[END_REF] .

Seuba 1 ,

 1 Sylvain Deville 1 , Christian Guizard 2 & Adam J. Stevenson 1

Figure 1 .

 1 Figure 1. SEM cross-sections of ice-templated samples (a) frozen at 2 °C/min, (b) frozen at 25 °C/min, (c) frozen at 2 °C/min with zirconium acetate (ZRA), and (d) made by pore formers. In all cases the total pore volume was around 51%.

Figure 2 .

 2 Figure 2. Pore size distribution obtained by image analysis of samples shown in Fig. 1. Ice-templated samples (a) frozen at 2 °C/min, (b) frozen at 25 °C/min, (c) frozen at 2 °C/min with zirconium acetate (ZRA), and (d) made by pore formers.

Figure 3 .

 3 Figure 3. Each plot in the panel highlights the compressive strength data versus relative density for four different macroporous ceramics processed by: (A) Replica, (B) Foaming, (C) Pore formers, and (D) Icetemplating. The experimental points of the other techniques are also shown in grey for comparison. The dotted lines represent the models proposed by Gibson and Ashby: Closed-cell (Eq. 3), Open-cell (Eq. 4), and Honeycomb out-of-plane (Eq. 5). Blue points in (D) and purple in (C) correspond to experimental points obtained in this work and the rest have been extracted from 29 .

Figure 4 .

 4 Figure 4. Detail of a buckling fracture in an ice-templated sample.

Figure 5 .

 5 Figure 5. Parity plot comparing the experimental and the theoretical strength predicted by Eq. 5. The three sets of samples have been ice-templated at: 2 °C/min with lamellar morphology (blue), 2 °C/min with honeycomb morphology (green), and 25 °C/min with lamellar morphology (red).

Figure 6 .

 6 Figure 6. Effect of total pore volume on the compressive behavior.

Figure 7 .

 7 Figure 7. Compressive strength dependence on porosity in ice templated samples frozen at 2 °C/min (A), 25 °C/min (C), and with a honeycomb morphology (B). Isotropic samples made in this work by sacrificial method are also represented (D). The dashed line corresponds to the Honeycomb out-of-plane model shown in Eq. 5. The experimental points of the other conditions are represented in grey for comparison.

Figure 8 .

 8 Figure 8. (A) Effect of solid loading and sintering temperature on % porosity and evolution of wall densification. (B) Relation between compressive strength and total pore volume for ice-templated samples frozen at 2 °C/min, with a pore size in the range of 10-20 μm, and sintered at different temperatures (1300-1400 °C). Dashed line represents the honeycomb out-of-plane model, Eq. 5.

Figure 9 .

 9 Figure 9. Image analysis showing the pore size population of ice templated samples frozen at 2 °C/min, 65 wt.% solids loading and sintering temperature (A) 1400 °C (B) 1300 °C.

Solid loading(wt.%) Freezing rate (°C/min) Porosity(%) Mean d pHg (μm) Mean d PIA (μm) Mean wall thickness(μm)

	50%	2	69.5%	19.1	20.0 ± 8.5	11.2 ± 4.5
	55%	2	64.2%	17.5	17.7 ± 6.7	14.7 ± 6.2
	60%	2	60.4%	14.2	13.9 ± 5.5	15.8 ± 6.8
	65%	2	52.5%	8.3	13.7 ± 4.8	19.1 ± 8.2
	50%	25	70.4%	5.4	4.7 ± 1.9	3.6 ± 1.5
	55%	25	65.2%	4.3	4.3 ± 1.7	3.9 ± 1.6
	60%	25	60.7%	3.9	4.1 ± 1.7	5.2 ± 2.2
	65%	25	51.5%	2.9	3.1 ± 1.2	3.0 ± 1.3

Table 1 . Summary of the most relevant pore descriptors and their respective ice-templating conditions

 1 . d pHg and d pIA correspond to the mean pore size obtained by mercury porosimetry and image analysis respectively.

Scientific RepoRts | 6:24326 | DOI: 10.1038/srep24326 www.nature.com/scientificreports

Scientific RepoRts | 6:24326 | DOI: 10.1038/srep24326

Acknowledgements

The research leading to these results has been supported by the CNRS and Saint-Gobain under BDI grant agreement 084877 for the Institut National de Chimie (INC) and the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement 278004.

Author Contributions

Experiments were conducted by J.S., and conceived by J.S. and A.S. The results and their implications were discussed by J.S., S.D., C.G. and A.S. All authors reviewed the manuscript.

Additional Information

Competing financial interests: The authors declare no competing financial interests.


Julie Chenot
email: chenot.julie@gmail.com

Laurence Affre
email: laurence.affre@imbe.fr

Raphaël Gros

Laura Dubois
email: duboislaura@laposte.net

Sarah Malecki
email: sarahmalecki@hotmail.fr

Aurélie Passetti
email: aurelie.passetti@gmail.com

Annie Aboucaya
email: annie.aboucaya@portcros-parcnational.fr

Elise Buisson
email: elise.buisson@univ-avignon.fr

Raphael Gros
email: raphael.gros@imbe.fr

Eradication of invasive Carpobrotus sp.: effects on soil and vegetation

Keywords: Bagaud Island, biological invasion, Carpobrotus affine acinaciformis, Carpobrotus edulis, invasive species control, pest management, small Mediterranean island, strict nature

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Biological invasions are one of the main threats to native biodiversity [START_REF] Richardson | Naturalization and invasion of alien plants: concepts and definitions[END_REF][START_REF] Milbau | Factors Associated with Alien Plants Transitioning from Casual, to Naturalized, to Invasive[END_REF]. Human activities, including national and international commercial exchanges, are responsible for increased species introductions and invasions [START_REF] Vitousek | Biological Invasions as Global Environmental Change[END_REF][START_REF] Mckinney | Biotic homogenization: a few winners replacing many losers in the next mass extinction[END_REF][START_REF] Mack | Biotic Invasions: Causes, Epidemiology, Global Consequences, and Control[END_REF][START_REF] Mooney | The evolutionary impact of invasive species[END_REF][START_REF] Vilà | Ecological impacts of invasive alien plants: a meta-analysis of their effects on species, communities and ecosystems[END_REF][START_REF] Van Kleunen | Global exchange and accumulation of non-native plants[END_REF]. Invasive species lead to drastic changes in native ecosystem composition and functioning (D 'Antonio & Haubensak 1998), and to native species regression or local extinction [START_REF] Vitousek | Biological Invasions as Global Environmental Change[END_REF][START_REF] Mack | Biotic Invasions: Causes, Epidemiology, Global Consequences, and Control[END_REF][START_REF] Vilà | Ecological impacts of invasive alien plants: a meta-analysis of their effects on species, communities and ecosystems[END_REF]. Moreover, they have important impacts on agriculture, fisheries, public health, and recreation, and consequently lead to costly actions of local ecological remediation [START_REF] Born | Economic evaluation of biological invasions -a survey[END_REF][START_REF] Chenot | Consequences of iceplant (Carpobrotus) invasion on the vegetation and seed bank structure on a Mediterranean island: response elements for their local eradication[END_REF].

Island ecosystems are particularly vulnerable to biological invasions [START_REF] Sax | Species Invasions Exceed Extinctions on Islands Worldwide: A Comparative Study of Plants and Birds[END_REF][START_REF] Sax | Species invasions and extinction: The future of native biodiversity on islands[END_REF]) because of their geographical isolation, ecological characteristics, low species richness, and disharmonic and simple food webs with high rates of endemism [START_REF] Nunn | Oceanic Islands[END_REF][START_REF] Cronk | Islands: stability, diversity, conservation[END_REF][START_REF] Denslow | The ecology of insular biotas[END_REF][START_REF] Drake | The biology of insularity: an introduction[END_REF][START_REF] Whittaker | Island Biogeography: Ecology, Evolution, and Conservation[END_REF].

Invasive species introduction has led to many deleterious ecological impacts on native ecosystems that are disproportionate when compared to the analogous effects in continental areas [START_REF] Courchamp | Mammal invaders on islands: impact, control and control impact[END_REF][START_REF] Trevino | Threats to Avifauna on Oceanic Islands[END_REF][START_REF] Berglund | Endemism Predicts Intrinsic Vulnerability to Nonindigenous Species on Islands[END_REF][START_REF] Simberloff | Impacts of biological invasions: what's what and the way forward[END_REF].

Because of this, island ecosystems are frequently marked by local extinctions of native species [START_REF] Ricketts | Pinpointing and preventing imminent extinctions[END_REF].

Invasive species eradication via the complete elimination of all individuals in a population [START_REF] Simberloff | Impacts of biological invasions: what's what and the way forward[END_REF]) can be an effective tool for the management and restoration of island native plant communities. Some examples of successful eradication operations can be found in the literature [START_REF] Loope | L'éradication: une mesure de gestion des populations allochtones[END_REF]; particularly where island environments are concerned [START_REF] Mack | Biotic Invasions: Causes, Epidemiology, Global Consequences, and Control[END_REF][START_REF] Cacho | Evaluating the Feasibility of Eradicating an Invasion[END_REF][START_REF] Mack | Eradicating invasive plants: hard-won lessons for islands[END_REF]. Such environments present the advantage to be isolated by water and thus to have limited risks of reinvasion [START_REF] Saunders | The Cooperative Islands Initiative: "turning back the tide of invasions[END_REF]. The eradication of invasive species can have beneficial environmental results, from the recovery of native plant species communities [START_REF] Andreu | Vegetation Response after Removal of the Invasive Carpobrotus Hybrid Complex in Andalucía, Spain[END_REF] to the restoration of soil properties [START_REF] Vilà | Local and regional assessments of the impacts of plant invaders on vegetation structure and soil properties of Mediterranean islands[END_REF][START_REF] Marchante | Short-and long-term impacts of Acacia longifolia invasion on the belowground processes of a Mediterranean coastal dune ecosystem[END_REF][START_REF] Santoro | Assessing the effects of Carpobrotus invasion on coastal dune soils. Does the nature of the invaded habitat matter?[END_REF]) and the restoration of ecological processes at the ecosystem level [START_REF] Gratton | Arthropod Food Web Restoration Following Removal Of An Invasive Wetland Plant[END_REF][START_REF] Marchante | Short-and long-term impacts of Acacia longifolia invasion on the belowground processes of a Mediterranean coastal dune ecosystem[END_REF]). However, undesirable ecological effects can occur (e.g. drastic increased in height and coverage of another non-native plant species, diet change of phytophagous species towards native species, etc.; [START_REF] Courchamp | Eradication of alien invasive species: surprise effects and conservation successes[END_REF] and must be foreseen through thorough knowledge on the species [START_REF] Zavaleta | Viewing invasive species removal in a wholeecosystem context[END_REF].

The Carpobrotus species found in this study are Carpobrotus edulis and Carpobrotus affine acinaciformis (Aizoaceae). Originating from South Africa and introduced in the Mediterranean, they are considered major invasive species [START_REF] Hulme | Islands, invasions and impacts: a Mediterranean perspective[END_REF]. They form monospecific carpets [START_REF] Sintes | Clonal growth dynamics of the invasive Carpobrotus affine acinaciformis in Mediterranean coastal systems: A non-linear model[END_REF] and spread over open areas, such as rocky coastlines, matorrals and dunes [START_REF] Au | Carpobrotus edulis in Coastal California Plant Communities[END_REF]Suehs et al. 2004a;[START_REF] Vilà | Local and regional assessments of the impacts of plant invaders on vegetation structure and soil properties of Mediterranean islands[END_REF][START_REF] Traveset | Spreading of the invasive Carpobrotus aff. acinaciformis in Mediterranean ecosystems: The advantage of performing in different light environments[END_REF], attaining near-dominance through their fast clonal growth, high genetic diversity, multiple reproduction strategies and competitiveness (Suehs et al. 2004b;Suehs et al. 2004a;Roiloa et al. 2009;[START_REF] Roiloa | Effect of physiological integration in self/non-self genotype recognition on the clonal invader Carpobrotus edulis[END_REF].

Numerous studies have shown major effects of these plant species on the native plant and animal species, and on soil characteristics (D 'Antonio & Mahall 1991;[START_REF] Antonio | Mechanisms Controlling Invasion of Coastal Plant Communities by the Alien Succulent Carpobrotus edulis[END_REF][START_REF] Suehs | Ecological and genetic features of the invasion by the alien Carpobrotus plants in Mediterranean island habitats[END_REF][START_REF] Palmer | Correlational patterns between invertebrate species composition and the presence of an invasive plant[END_REF][START_REF] Moragues | Effect of Carpobrotus spp. on the pollination success of native plant species of the Balearic Islands[END_REF][START_REF] Vilà | Local and regional assessments of the impacts of plant invaders on vegetation structure and soil properties of Mediterranean islands[END_REF][START_REF] Orgeas | Conséquences écologiques de l'envahissement des griffes de sorcière (Carpobrotus spp.) sur les communautés d'insectes d'un îlot du Parc national de Port-Cros (Var)[END_REF][START_REF] Galán | Efecto de la planta invasora Carpobrotus edulis sobre la densidad del eslizón tridáctilo (Chalcides striatus) en una localidad costera de Galicia[END_REF][START_REF] Conser | Assessing the residual effects of Carpobrotus edulis invasion, implications for restoration[END_REF][START_REF] Peña E De La | Plantsoil feedback as a mechanism of invasion by Carpobrotus edulis[END_REF][START_REF] Zedda | Impacts of alien plants and man-made disturbance on soil-growing bryophyte and lichen diversity in coastal areas of Sardinia (Italy)[END_REF][START_REF] Affre | Ecological and Evolutionary Insights from Plant Invasions: The Case of Carpobrotus spp. (Aizoaceae) in the Mediterranean Basin[END_REF][START_REF] Santoro | Assessing the effects of Carpobrotus invasion on coastal dune soils. Does the nature of the invaded habitat matter?[END_REF][START_REF] Novoa | Constraints to native plant species establishment in coastal dune communities invaded by Carpobrotus edulis: Implications for restoration[END_REF][START_REF] Novoa | Impacts of Carpobrotus edulis (L.) N.E.Br. on the Germination, Establishment and Survival of Native Plants: A Clue for Assessing Its Competitive Strength[END_REF]. Carpobrotus management (eradication and control) could therefore be used to promote native communities, particularly in protected areas [START_REF] Andreu | Vegetation Response after Removal of the Invasive Carpobrotus Hybrid Complex in Andalucía, Spain[END_REF][START_REF] Ruffino | Eradications as scientific experiments: progress in simultaneous eradications of two major invasive taxa from a Mediterranean island[END_REF]. Removing living biomass will stop vegetative growth and open gaps for other plants to colonize [START_REF] Andreu | Vegetation Response after Removal of the Invasive Carpobrotus Hybrid Complex in Andalucía, Spain[END_REF]. Carpobrotus branches and leaves decompose slowly and form a thick litter [START_REF] Conser | Assessing the residual effects of Carpobrotus edulis invasion, implications for restoration[END_REF]) that can be left on site or removed. If the litter is left on site, germination will occur from the seeds contained in it; although the litter retains the seeds of native species, as much as 77.6% of the seeds are Carpobrotus [START_REF] Chenot | Consequences of iceplant (Carpobrotus) invasion on the vegetation and seed bank structure on a Mediterranean island: response elements for their local eradication[END_REF]. Moreover, the litter releases allelopathic compounds thus leaving it on site would hamper the germination of native species [START_REF] Novoa | Effects of Soil Characteristics, Allelopathy and Frugivory on Establishment of the Invasive Plant Carpobrotus edulis and a Co-Occuring Native, Malcolmia littorea[END_REF]. If both living Carpobrotus and its litter are removed, germination will occur from the soil seed bank. However, with no litter remaining, there is a good chance that these seeds would be lost to soil erosion, mainly occurring with autumn heavy rains. Because Carpobrotus was often planted to reduce soil erosion in the first place, the suppression of its dense canopy can increase the impact of rain drops on an unstable soil structure then favoring the displacement of soil particles and seeds. The risks and impacts of extensive soil loss should always be evaluated prior to the eradication of both live-mats and litter in any restoration scenario.

The aim of the present study is to compare different treatments in order to guide the selection of future management modalities for Carpobrotus species invasion in a strict nature reserve.

Different removal methods were compared, based on the following three objectives: i) limiting soil erosion, ii) limiting Carpobrotus recolonization and iii) favoring native vegetation recolonization. Four treatments were thus tested: [START_REF] Cacho | Evaluating the Feasibility of Eradicating an Invasion[END_REF]

Study area

Bagaud Island, a strict nature reserve since 2007, is part of Port-Cros National Park (southeastern France) in the Mediterranean Sea (43°00'42 N; 6°21'45 E). This 58 ha island is located 7.5 km from the mainland shore and exhibits a low relief (maximum elevation at 57 m). The climate on Bagaud Island is characterized by mild winters and a high relative air humidity (81% / year) that persists into the summer. The annual average temperature is about 15°C with daily maximum summer often exceeding 30°C. The average temperature of the coldest month is above 9°C [START_REF] Krebs | Résilience de la flore indigène après éradication des griffes de sorcière (Carpobrotus sp.) sur une île méditerranéenne[END_REF]. The native vegetation of the island is dominated by matorral, i.e. shrublands found in regions with a Mediterranean climate. Other vegetation types include oak woodlands, halo-resistant coastal plant communities on the coast, and more or less halonitrophilous grassland patches with a mix of annual and perennial plant species [START_REF] Krebs | Résilience de la flore indigène après éradication des griffes de sorcière (Carpobrotus sp.) sur une île méditerranéenne[END_REF].

Carpobrotus invasion

Carpobrotus edulis (L.) N. E. Br. and Carpobrotus affine acinaciformis (Suehs et al. 2004a(Suehs et al. ,b, 2006) ) were introduced to Bagaud Island intentionally in the mid-19th century to stabilize the embankments that were created during military fort construction. Both species are grouped under the term Carpobrotus thereafter. All are succulent chamaephytes with a slightly ligneous base. They grow prostrated on the ground, rooting at nodes and forming large, dense mats. Their invasiveness depends on characteristics typical of invasive plant species, such as (i) a lack of efficient pathogens in their new geographical range (i.e. Enemy Release Hypothesis: [START_REF] Williamson | [END_REF][START_REF] Keane | Exotic plant invasions and the enemy release hypothesis[END_REF], (ii) a high clonal and genetic diversity, (iii) vigorous and fast clonal growth, (iv) several sexual reproduction strategies, (v) high and recurrent frequencies of introgression and hybridization, or (vi) seed and fruit dispersion by endozoochory [START_REF] Suehs | Ecological and genetic features of the invasion by the alien Carpobrotus plants in Mediterranean island habitats[END_REF](Suehs et al. , 2004a[START_REF] Suehs | Unexpected insularity effects in invasive plant mating systems: the case of Carpobrotus (Aizoaceae) taxa in the Mediterranean Basin[END_REF][START_REF] Suehs | The evolutionary potential of invasive Carpobrotus (Aizoaceae) taxa: are pollen-mediated gene flow potential and hybrid vigor levels connected?[END_REF][START_REF] Bourgeois | Invasional meltdown potential: Facilitation between introduced plants and mammals on French Mediterranean islands[END_REF]. Like elsewhere in the world, they have locally caused (i) a decrease in the species richness and diversity of rare and endemic plants; (ii) changes in soil chemistry; and (iii) a decrease in pollinator visits to native plants (Suehs et al. 2004b;[START_REF] Affre | Ecological and Evolutionary Insights from Plant Invasions: The Case of Carpobrotus spp. (Aizoaceae) in the Mediterranean Basin[END_REF].

Experimental eradication design and sampling

The experiment was set up on Bagaud Island in July 2010 and monitored in November 2010 and April 2011. The four sites were located between 10 and 600m from the coast, and between 10 and 40m of altitude (the two sites the most distant to each other were 1 km away while the two closest were less than 100 m away). Each of the study sites is composed of a native vegetation area, bordering a patch of Carpobrotus. On each site, three replicate blocks were established, each consisting of four treatments implemented in one 2m × 2m quadrat each (4 quadrats / block and 3 replicate blocks / site; Figure 1a). In each replicate block, a first quadrat serving as reference state was placed in native vegetation. Within the patch of Carpobrotus, three quadrats with the following treatments were defined: (1) living Carpobrotus manual removal, consisting in pulling living Carpobrotus stems with their roots; (2) living Carpobrotus and litter removal; (3) Carpobrotus presence (control) and (4) native vegetation (reference).

Removal took place at the end of June 2010 (Figure 1a). Such manual removal of Carpobrotus is feasible at larger scales: it requires 52 man-days to remove 1 ha if uprooted Carpobrotus material is left and composted in piles on site [START_REF] Ruffino | Eradications as scientific experiments: progress in simultaneous eradications of two major invasive taxa from a Mediterranean island[END_REF].

To measure soil erosion after applying the various treatments, monitoring was established by implanting plastic boxes downhill from the quadrats in order to collect eroded soil over a period of nine months. This could not be done in native vegetation, because this would have required the destruction of small patches of native vegetation, and this is forbidden in a strict nature reserve. Boxes were thus only placed in Carpobrotus patches where soil depth was equal to or greater than that of the plastic boxes (19.1 cm). In July 2010, two plastic boxes (L 78 × W 25 × H 19.1 cm) were positioned in trenches dug downhill from each quadrat. For each treatment, one plastic box was placed directly below the quadrat, and another was placed behind a 50 cm-wide Carpobrotus strip to see if the strip would limit soil erosion (Figure 1b). The plastic boxes were collected in November 2010 and April 2011 and soil samples were dried and weighed. ). The total number of boxes was 35: 11 in living Carpobrotus removal (6 upper boxes and 5 lower boxes), 12 in living Carpobrotus and litter removal (6 each in upper and lower boxes), 12 in Carpobrotus presence (6 each in upper and lower boxes).

The composition of plant communities was studied before (April 2010) and after (April 2011) treatment application on all quadrats. In each 2m × 2m treatment quadrat, the percent cover of each species was visually assessed in a centered 1 m² quadrat in order to avoid border effects. We also visually estimated the total percent cover of vegetation, the percent cover of native species and Carpobrotus.

Data analysis

Analyses were performed using the program R (R Core Team 2014) and made use of the following R packages: "coin" [START_REF] Zeileis | Implementing a Class of Permutation Tests: The coin Package[END_REF], and "vegan" [START_REF] Oksanen | Package 'vegan[END_REF].

The effects of the treatments on soil erosion were analyzed using a permutation test [START_REF] Zeileis | Implementing a Class of Permutation Tests: The coin Package[END_REF]. Permutation tests are a modern and powerful type of statistical significance test in which the population distribution is obtained by calculating the sample statistics under every possible permutation of the observed data points, and such tests are appropriate for small sample sizes. The p-values for the multiple comparison tests were recalculated with the BH adjustment [START_REF] Benjamini | Controlling the False Discovery Rate: A Practical and Powerful Approach to Multiple Testing[END_REF]. This test was run on the weight of soil depending on the treatments (Carpobrotus removal, Carpobrotus and litter removal, Carpobrotus presence) and on the position of the box (upper box, lower box).

To study the effects of the treatments on plant recolonization, permutation tests were run on i) total plant cover, ii) Carpobrotus plant cover, iii) native plant cover and iv) species richness, depending on the treatments. Treatments were compared to the Carpobrotus presence and the native vegetation treatments.

In order to study the composition of the vegetation before and after applying the treatments, a NMDS (Non-Metric Multidimensional Scaling) analysis [START_REF] Borcard | Numerical Ecology with R[END_REF]) was run on plant species percent covers of spring 2010 and spring 2011 (81 samples × 47 species; N=12 for most treatment/year combinations except for Carpobrotus removal in 2011 N=11 and Carpobrotus and litter removal in 2011 N=10). This analysis was followed by a Multiple Response Permutation Procedure analysis (MRPP) to determine the statistical significance of the differences between years and between treatments.

Results

Treatment effects on soil erosion

The removal of Carpobrotus and its litter led to a soil erosion 4.5 to 23 times higher (665.5 g on average / upper box or 3.33 tons/ha) than other combinations of treatments: removing Carpobrotus alone (53.0 g or 0.27 ton/ha) or leaving Carpobrotus in place (control, 69.8 g or 0.35 ton/ha) (maxT = 4.07, p-value = 0.001; Figure 2). Leaving a 50-cm strip of Carpobrotus downhill from where Carpobrotus and its litter were removed significantly prevented eroded soil from leaving the area by a factor of 4.5 (148.7 g on average / lower box for this treatment vs. 665.5 g on average / upper box; Figure 2).

Treatment effects on plant percent cover and species richness

Total vegetation cover 10 months after the treatments were applied was significantly lower where Carpobrotus had been removed, regardless of whether the litter was also removed or not, intermediate on the Carpobrotus presence treatment and highest in the native vegetation (maxT = 4.85, p-value < 0.001; Figure 3). Native vegetation cover was significantly higher in the native vegetation quadrats and lower for the other treatments (maxT = 6.42, p-value < 0.001; Figure 3). Carpobrotus cover was significantly higher in the control and lower in the other treatments (maxT = 6.55, p-value < 0.001; Figure 3). Finally, species richness was highest in the native vegetation, intermediate where Carpobrotus had been removed without removing the litter and lowest in the Carpobrotus presence treatment and where Carpobrotus and its litter were removed (maxT = 3.16, p-value < 0.009; Figure 3).

Treatment effects on plant communities

The NMDS analysis on the vegetation (stress: 0.12) and the Multiple Response Permutation Procedure analysis (MRPP) showed a significant difference between some combinations of treatments and years (observed delta: 0.34; expected delta: 0.68; p-value<0.001). As shown in

Discussion

Interestingly, our experimental eradication study shows that, when comparing the two removal methods (i.e. living Carpobrotus removal or living Carpobrotus and litter removal) leaving Carpobrotus litter on site limits soil erosion and leads to higher native plant species recolonization but also to higher reinvasion potential.

Ten months after applying the treatments, Carpobrotus removal (whether the litter is removed or not) obviously leads to a large decrease in Carpobrotus cover compared to quadrats where it is left in place, although some germinations of Carpobrotus can be observed.

Recolonization of native vegetation is scarce, although a slightly higher species richness of native vegetation can be observed on these quadrats relative to the invaded ones or even to the same quadrats prior to removal. Colonization of these open areas begins with resprouting species, such as Aetheoriza bulbosa, and Arisarum vulgare. This is followed by the species that either have large seed banks (Frankenia hirsuta and Lotus cytisoides) or employ longdistance dispersals (Sonchus asper asper and Sonchus asper glaucescens) [START_REF] Krebs | Résilience de la flore indigène après éradication des griffes de sorcière (Carpobrotus sp.) sur une île méditerranéenne[END_REF].

Most of the colonizing species are coastal (e.g. Atriplex prostrata, Frankenia hirsuta, Lotus cytisoides, Senecio leucanthemifolius and Sonchus asper glaucescens) and early matorral succession (e.g. Aetheoriza bulbosa, Arisarum vulgare, Senecio cineraria) species; very few opportunistic or weedy species are recorded, and when found, their percent covers are low (e.g.

Sonchus asper asper and Solanum nigrum).

The recovery rate of native plants following Carpobrotus removal will depend on the life and growth forms, and dispersal abilities of native plants: on Bagaud Island, pioneer species colonize quickly, because the invaded areas are relatively small (consisting of only about 1 ha in each case) and are surrounded by native vegetation that can serve as an source for seeds:

Aetheoriza bulbosa, Crithmum maritimum, Euphorbia pithyusa, Frankenia hirsuta, Lotus cytisoides, Sonchus asper glaucescens, are therefore found early on [START_REF] Krebs | Résilience de la flore indigène après éradication des griffes de sorcière (Carpobrotus sp.) sur une île méditerranéenne[END_REF]. In addition, the increase in soil temperature and light following invasive plant removal favor the annual species germination (D' Antonio and Meyerson 2002;[START_REF] Andreu | Vegetation Response after Removal of the Invasive Carpobrotus Hybrid Complex in Andalucía, Spain[END_REF], such as Atriplex prostrata, Bromus sp., Catapodium marinum, Poa annua, Polycarpon tetraphyllum, Rostraria cristata, Spergularia sp. [START_REF] Krebs | Résilience de la flore indigène après éradication des griffes de sorcière (Carpobrotus sp.) sur une île méditerranéenne[END_REF]. In time, the full development of a low matorral plant community will become dominated by relatively slow-growth woody species (D 'Antonio & Meyerson 2002), such as Pistacia lentiscus, etc. [START_REF] Krebs | Résilience de la flore indigène après éradication des griffes de sorcière (Carpobrotus sp.) sur une île méditerranéenne[END_REF].

Removing the litter from the ground along with the live parts of Carpobrotus leads to major soil erosion most likely due to increased rain splash and runoff-driven erosion processes. The soil was almost completely bare during the autumn and winter following the removal, apart from a few germinations of Carpobrotus and Lotus cytisoides and a few resprouts of Aetheoriza bulbosa and Arisarum vulgare. The soil is therefore highly sensitive to the erosion induced by autumn and winter precipitation. Similar increases in runoff and erosion rates were observed by [START_REF] Chapman | Intact ash and charred litter reduces susceptibility to rain splash erosion post-wildfire[END_REF] when the charred litter from a prescribed fire was removed and the bare soil was exposed to simulated rainfall. The water repellency of sandy soil after a long period of Carpobrotus presence has also been observed (pers. obs.). Sowing seeds of native species can be considered to accelerate native plant colonization but does not counteract soil erosion.

Sown species require the autumn rain to germinate, and this means that the native plants would only appear as germinating seedlings, which are too young and small to provide much erosionpreventing benefit at a time of year when erosion control is needed the most (pers. obs.).

On other hand, this study clearly demonstrates that leaving a 50 cm strip of Carpobrotus downhill from the areas where Carpobrotus and its litter are removed, serves to significantly reduce the amount of soil exported from that area, to the point that it becomes comparable to areas where Carpobrotus was left in place. A Carpobrotus strip can therefore be left in place for a couple of years while waiting for native species to colonize and cover the soil, and must be removed once native colonization has taken place in order to avoid re-invasion. If such an option is taken, funding and capacity must be allocated i) for Carpobrotus strip management while it is in place and ii) for its later removal. Site managers have to make sure that the Carpobrotus strip is not a source of seeds and re-colonization while it is in place: cutting all the flowers as they appear to prevent seed production and trimming the strip edge if it grows too quickly. To further mitigate soil loss by runoff, another option would be to cover bare ground with geotextile on the steeper areas and/or on the edges of patches where Carpobrotus and its litter have been removed [START_REF] Bhattacharyya | Use of palm-mat geotextiles for rainsplash erosion control[END_REF]. In case native seeds are sowed, this should most probably be combined with a way to prevent seeds from being washed away by the first heavy rains, before they germinate: it may include geotextile or seed burying or drilling, but would have to be investigated [START_REF] Bochet | How can we control erosion of roadslopes in semiarid Mediterranean areas? Soil improvement and native plant establishment[END_REF][START_REF] Álvarez-Mozos | Evaluation of erosion control geotextiles on steep slopes. Part 2: Influence on the establishment and growth of vegetation[END_REF]).

Novoa and co-workers [START_REF] Novoa | Effects of Soil Characteristics, Allelopathy and Frugivory on Establishment of the Invasive Plant Carpobrotus edulis and a Co-Occuring Native, Malcolmia littorea[END_REF][START_REF] Novoa | Constraints to native plant species establishment in coastal dune communities invaded by Carpobrotus edulis: Implications for restoration[END_REF] have shown that Carpobrotus edulis produces allelopathic substances that accumulate in the litter and prevent seed germination, thus potentially limiting native and exotic plant reestablishment. They therefore advise against leaving fresh or dry Carpobrotus litter on restored areas. By contrast, we have found that species richness and native vegetation percent cover actually increase slightly when Carpobrotus is removed and its litter remains. Our results are however not significant and different from the seed bank study that was carried out at the same site [START_REF] Chenot | Consequences of iceplant (Carpobrotus) invasion on the vegetation and seed bank structure on a Mediterranean island: response elements for their local eradication[END_REF]).

These differences may result from the short time span of our study.

If the litter is left on site, colonization will occur from the seeds contained in Carpobrotus litter, some of which come from native species (22.4%), such as Atriplex prostrata, Frankenia hirsuta and Sonchus asper. However, most of the seeds (77.6%) are Carpobrotus [START_REF] Chenot | Consequences of iceplant (Carpobrotus) invasion on the vegetation and seed bank structure on a Mediterranean island: response elements for their local eradication[END_REF]). Although selecting the treatment which leaves the litter on the ground is less complicated and less expensive to implement, subsequent control of Carpobrotus germinations and resprouts may actually be more intensive. In the soil seed bank, Carpobrotus is also present, but native species seeds are more abundant (62.4% vs. 37.6% for Carpobrotus) than in the litter (see [START_REF] Chenot | Consequences of iceplant (Carpobrotus) invasion on the vegetation and seed bank structure on a Mediterranean island: response elements for their local eradication[END_REF] for list of species).

The risk of reinvasion from a site where an invasive species has been removed is lower on uninhabited islands or on islands managed for biodiversity conservation sake, such as Bagaud Island [START_REF] Ruffino | Eradications as scientific experiments: progress in simultaneous eradications of two major invasive taxa from a Mediterranean island[END_REF]. The chance of reinvasion and spread is expected to be reduced once Mediterranean mattoral native plants, forming dense stands, have re-established [START_REF] Ruffino | Eradications as scientific experiments: progress in simultaneous eradications of two major invasive taxa from a Mediterranean island[END_REF]. Therefore, once the native vegetation has recolonized the environment, and in the absence of further disturbances, one of the expected results is a lack of Carpobrotus germination in the newly established native vegetation understory. Indeed, the study of [START_REF] Chenot | Consequences of iceplant (Carpobrotus) invasion on the vegetation and seed bank structure on a Mediterranean island: response elements for their local eradication[END_REF] showed that Carpobrotus seeds were found in significant quantities within the native seed bank on Bagaud Island, even though Carpobrotus was not expressed in the aboveground vegetation.

However, Carpobrotus have consequent long-term seed banks (at least up to 5 years) and can also resprout many years after removal (up to 8 years) from underground stems and can thus quickly reinvade [START_REF] Affre | Ecological and Evolutionary Insights from Plant Invasions: The Case of Carpobrotus spp. (Aizoaceae) in the Mediterranean Basin[END_REF][START_REF] Ruffino | Eradications as scientific experiments: progress in simultaneous eradications of two major invasive taxa from a Mediterranean island[END_REF]. Successful eradications of such plant species thus requires regular and long-term controls of treated areas [START_REF] Mack | Eradicating invasive plants: hard-won lessons for islands[END_REF]; on Bagaud Island, 45 man-days / ha were necessary to remove Carpobrotus germinations and resprouts one year after large scale removal (2012; [START_REF] Ruffino | Eradications as scientific experiments: progress in simultaneous eradications of two major invasive taxa from a Mediterranean island[END_REF]. Therefore, sufficient funding and capacity have to be planned before the management program is implemented, to monitor the treated areas in the long-term and to take necessary actions (germination or resprout removal) to ensure the ultimate success of the eradication.

Figure 4 ,

 4 Figure 4, axis 1 delineated the quadrats sampled in the native vegetation, characterized by Pinus halepensis, Pistacia lentiscus and Rosmarinus officinalis, from the other quadrats. Axis 2 separated the control quadrats and the quadrats sampled before Carpobrotus removal, characterized by Carpobrotus, from quadrats where Carpobrotus was removed with and without its litter. The latter is characterized by Geranium ssp. and Sonchus asper, while quadrats where Carpobrotus and its litter were removed are characterized by Frankenia hirsuta and Sonchus asper glaucescens.

Figure 1 .

 1 Figure 1. A. Experimental eradication set-up in four sites on Bagaud Island. Four treatments regrouped in blocks were replicated three times. B. Position of the plastic boxes with respect to the 2m × 2m experimental quadrat, to the 2m × 2m vegetation monitoring quadrat and to the Carpobrotus strip.

Figure 2 .

 2 Figure 2. Effects of the three treatments (1: Carpobrotus presence (control), 2: living Carpobrotus removal, 3: living Carpobrotus and litter removal) and of the position of the box downhill from the plots on soil erosion (in g / box) (maxT = 4.07, p-value = 0.001). *Upper boxes are placed directly below the quadrats while **lower boxes are placed below a 50 cmwide Carpobrotus strip (see Figure 1 for detailed set-up). Soil erosion monitoring could not be carried out on the fourth treatment (native vegetation -reference) because this would have required the destruction of small patches of native vegetation in a strict nature reserve. Error bars represent ±SE, bars having a common letter are not significantly different (multiple comparison tests recalculated with the BH adjustment; p>0.05).

Figure 3 .

 3 Figure 3. Effects of the four treatments (1: Carpobrotus presence (control), 2: living Carpobrotus removal, 3: living Carpobrotus and litter removal, 4: native vegetation (reference)) on the percent cover of total vegetation, Carpobrotus and native vegetation and on species richness (N=12). Error bars represent ±SE, bars having a common letter are not significantly different (multiple comparison tests recalculated with the BH adjustment; p>0.05).

Figure 4 .

 4 Figure 4. Non-Metric Multidimensional Scaling (NMDS) analysis of plant cover depending on treatments and year (81 samples × 47 species) (NMDS stress: 0.12). For clarity-sake, only the most correlated species are shown. Samples are grouped by combinations of treatment and year and written in boxes.

Figure 1 Figure 2

 12 Figure 1

Figure 4

 4 Figure 4

Material and Methods

  living Carpobrotus removal; (2) living

	Carpobrotus and litter removal; (3) Carpobrotus presence (control) and (4) native vegetation
	(reference).

This study was made possible through the major support of Port-Cros National Park, the Institut Méditerranéen de Biodiversité et d'Ecologie (IMBE) and funding partners (the European Union FEDER, Conservatoire du littoral et des rivages lacustres, Fondation Total, Parc National de Port Cros). We thank the staff at Port-Cros National Park, Eric Vidal and Frédéric Médail (IMBE) for their commitment and logistic support. We also thank all the people who helped with our field work, in particular Teddy Baumberger, Noellie Fonvieille and Renaud Jaunatre for removing Carpobrotus from our quadrats, Alain Bence, Solène Masson and Isabelle Muller for collecting the soil from the boxes, and Daniel Pavon, who provided assistance in identifying plants. We also thank Kolo D. Wamba from Proediting who revised the English.