Thomas Vanneste
email: thomas.vanneste@ugent.be.

Sanne Van Den Berge

Enya Riské

Jörg Brunet

Guillaume Decocq

Martin Diekmann

Bente J Graae

Per-Ola Hedwall

Jonathan Roger Michel

Henri Lenoir

Jaan Liira

Per- Ola Hedwall

Jonathan Lenoir

Sigrid Lindmo

Kathrin Litza

Tobias Naaf

Anna Orczewska

Monika Wulf

Kris Verheyen

Pieter De Frenne

Hedging against biodiversity loss: Forest herbs' performance in hedgerows across temperate Europe

Keywords: Agricultural landscapes, contrasting environmental conditions, forest herbs, functional traits, hedgerows, forest fragmentation, metapopulation dynamics, microclimate, migration corridors, plant performance, source-sink dynamics, understorey vegetation

published or not. The documents may come

Introduction

Forest ecosystems across the globe are increasingly threatened by human activities [START_REF] Curtis | Classifying drivers of global forest loss[END_REF], with severe implications for global biodiversity [START_REF] Haddad | Habitat fragmentation and its lasting impact on Earth's ecosystems[END_REF][START_REF] Tracewski | Toward quantification of the impact of 21st-century deforestation on the extinction risk of terrestrial vertebrates[END_REF]) and ecosystem functioning (van der [START_REF] Van Der Plas | Biotic homogenization can decrease landscape-scale forest multifunctionality[END_REF] as well as the delivery of essential supporting (e.g. primary production), provisioning (e.g. wood and medicinal plants), regulating (e.g. carbon sequestration and pest control) and cultural (e.g. recreational activities) services [START_REF] Valdés | High ecosystem service delivery potential of small woodlands in agricultural landscapes[END_REF]. Forests harbour about two-thirds of all terrestrial species on Earth (Millenium Ecosystem Assessment 2005). Among vascular plants in temperate forests, the large majority is concentrated in the herb layer [START_REF] Gilliam | The Ecological Significance of the Herbaceous Layer in Temperate Forest Ecosystems[END_REF][START_REF] Landuyt | The functional role of temperate forest understorey vegetation in a changing world[END_REF]. Many herb-layer species are characterized by slow colonization rates and specific habitat requirements [START_REF] Honnay | Possible effects of habitat fragmentation and climate change on the range of forest plant species[END_REF][START_REF] Matlack | Slow plants in a fast forest: local dispersal as a predictor of species frequencies in a dynamic landscape[END_REF], making them particularly susceptible to the effects of habitat loss and fragmentation [START_REF] Flinn | Recovery of forest plant communities in post-agricultural landscapes[END_REF][START_REF] Honnay | Forest fragmentation effects on patch occupancy and population viability of herbaceous plant species[END_REF]).

These species are often poor at crossing open habitats such as grassland and arable land, and their long-term survival in agricultural landscapes depends on the availability of remnant habitat patches as well as the connectivity among remnant and newly established patches.

Wooded corridors (e.g. hedgerows and other linear woody habitats) are traditional components of agricultural landscapes across Europe [START_REF] Baudry | Hedgerows: an international perspective on their origin, function and management[END_REF], and have been repeatedly advocated as a tool for land managers to enhance habitat connectivity [START_REF] Davies | Are hedgerows effective corridors between fragments of woodland habitat? An evidence-based approach[END_REF]. Indeed, a myriad of studies have shown that hedgerows may function as a refuge habitat or dispersal route for forest herbs among otherwise isolated forest fragments [START_REF] Corbit | Hedgerows as habitat corrdiors for forest herbs in central New York, USA[END_REF][START_REF] Sitzia | Hedgerows as corridors for woodland plants: a test on the Po Plain, northern Italy[END_REF][START_REF] Liira | Do forest-dwelling plant species disperse along landscape corridors?[END_REF][START_REF] Van Den Berge | Contrasting vegetation change (1974-2015) in hedgerows and forests in an intensively used agricultural landscape[END_REF]. From this perspective, hedgerows may be crucial to the long-term persistence of forest herbs in forest devoid agricultural landscapes. Moreover, their importance for species movements through the landscape is expected to increase given the predicted range shifts of species under climate change [START_REF] Mcguire | Achieving climate connectivity in a fragmented landscape[END_REF].

Even so, over the past decades, the efficacy of hedgerows as migration corridors has been the subject of considerable debate [START_REF] Gilbert-Norton | A meta-analytic review of corridor effectiveness[END_REF][START_REF] Paal | Both spatiotemporal connectivity and habitat quality limit the immigration of forest plants into wooded corridors[END_REF]. Most importantly, for hedgerows to act as efficient dispersal conduits, they must support persistent populations of forest herbs with the ability to reproduce, thus allowing the progressive movement of successive generations along the corridor [START_REF] Corbit | Hedgerows as habitat corrdiors for forest herbs in central New York, USA[END_REF]). Yet, due to their linear structure, hedgerows are generally characterized by a reduced habitat quality for many understorey forest herbs owing to edge effects, e.g. high soil nutrient levels combined with high light levels and less buffered microclimates, stimulating the growth of highly competitive ruderal species at the expense of typical forest herbs [START_REF] Roy | Evaluating hedgerow corridors for the conservation of native forest herb diversity[END_REF][START_REF] Wehling | Hedgerows as an environment for forest plants: a comparative case study of five species[END_REF]Vanneste et al. 2020a). These deteriorated conditions compared to forest habitats could further aggravate the risk of local extinction in hedgerows, because long-term persistence is potentially hampered for forest herb species that have colonized these wooded linear structures (see [START_REF] Schmucki | Population structures and individual performances of Trillium grandiflorum in hedgerow and forest habitats[END_REF].

Hitherto, we know little about the performance of forest herb species once they have colonized hedgerows, and how their performance relates to the prevailing abiotic and biotic conditions in these corridors. Some studies (e.g. [START_REF] Endels | Population structure and adult plant performance of forest herbs in three contrasting habitats[END_REF][START_REF] Wehling | Hedgerows as an environment for forest plants: a comparative case study of five species[END_REF][START_REF] Schmucki | Population structures and individual performances of Trillium grandiflorum in hedgerow and forest habitats[END_REF] reported that hedgerows may harbour viable populations of forest plants, but that reduced recruitment could hamper their persistence and sustainability as a source population. However, these studies focussed on relatively small study regions, making it difficult to generalize conclusions. A promising approach to resolve this knowledge gap is to compare plant performance traits (i.e. a combination of vegetative and reproductive characteristics) of forest herbs between forest interiors and hedgerows, and repeat this along a large environmental gradient in a systematically paired design (forests vs. hedgerows). Indeed, it is expected that, for instance, macroclimate variables that vary at continental scales will significantly affect hedgerow habitats, e.g. via increased thermal buffering in warmer, southern macroclimates (Vanneste et al. 2020b). Plant performance traits provide vital information on the strategies by which forest plants survive and reproduce in the sub-optimal forest environment of hedgerows [START_REF] Mcgill | Rebuilding community ecology from functional traits[END_REF][START_REF] Zanne | Plant functional traits -linkages among stem anatomy, plant performance and life history[END_REF][START_REF] Diaz | Functional traits, the phylogeny of function, and ecosystem service vulnerability[END_REF][START_REF] Poorter | Can traits predict individual growth performance? A test in a hyperdiverse tropical forest[END_REF], and how these strategies will in turn influence their capacity to persist and migrate along these linear landscape structures (see also [START_REF] Paal | Standardized response signatures of functional traits pinpoint limiting ecological filters during the migration of forest plant species into wooded corridors[END_REF].

Here, we compiled data from eight regions across the European temperate forest biome and quantified intraspecific differences in the functional trait attributes of forest plant species using a systematic paired design of comparison between forest interiors and hedgerows. For this study, four temperate forest herbs were chosen based on differences in colonization capacity and flowering phenology: Anemone nemorosa (spring flowering, slow colonizer), Ficaria verna (spring flowering, fast colonizer), Poa nemoralis (summer flowering, moderate colonizer) and Geum urbanum (summer flowering, fast colonizer). The large-scale environmental gradient along which the study sites are located, assured that the wide climatic variation of temperate Europe was represented adequately in our study and allowed us to test the generality of the observed patterns. Specifically, we addressed the following questions:

(1) Do abiotic and biotic environmental conditions differ in forests vs. hedgerows along a macroenvironmental gradient within temperate Europe?

(2) Can differences in plant performance traits of the four study species be detected between the two contrasting habitats along the gradient?

(3) What are the relationships between plant performance traits and environmental conditions in the two habitats?

(4) Can hedgerow understories support reproductive source populations of forest herbs (including ancient forest specialists), thus potentially allowing the progressive dispersal of successive generations along the corridor?

Materials and methods

Study area

This study was conducted in eight regions across the European temperate forest biome (Fig. 1). These regions were chosen to represent a broad environmental gradient with a mean annual temperature (MAT) range of 5.1-10.4 °C and mean annual precipitation (MAP) range of 601-1251 mm year -1 (long-term average values for the period 1970-2000; Fick & Hijmans 2017) (Fig. S1). In each region, we selected four paired combinations of hedgerows connected to ancient forests (i.e. no land-use change since 1850), whereof the canopy mainly consisted of broadleaved woody species. The selected hedgerows were at least 50 years old (critical age to be colonized by typical forest herbs; [START_REF] Brunet | Migration of vascular plants to secondary woodlands in southern Sweden[END_REF], had a diverse vertical structure (herbs, shrub and tree layer) and were surrounded by a non-wooded habitat (e.g. road, cropland, etc.) (see also Vanneste et al. 2020b).

Study species

Four model plant species were specifically selected for this study based on their distribution range (present in a majority of the study regions), reproductive traits (both sexual and clonal propagation), and their flowering phenology (two early spring vs. two summer flowering species) and colonization capacity (along a gradient of very slow to relatively fast colorizers; Table S2 Anemone nemorosa and F. verna are both vernal geophytes [START_REF] Taylor | Ranunculus Ficaria L. (Ficaria verna Huds.; F. Ranunculoides Moench)[END_REF][START_REF] Shirreffs | Anemone Nemorosa L[END_REF]. Flowers of A. nemorosa are hermaphroditic and mostly pollinated by insects, though selfing occurs sporadically [START_REF] Müller | Variation in breeding system among populations of the common woodland herb Anemone nemorosa (Ranunculaceae)[END_REF].

Each flowering ramet produces 10-30 flask-shaped, single-seeded fruits (often referred to as 'achenes') per year.

The seeds are mainly gravity-dispersed, although dispersal via slugs or ants can occur as well [START_REF] Türke | Seed consumption and dispersal of ant-dispersed plants by slugs[END_REF].

A. nemorosa also propagates clonally via rhizomes [START_REF] Philipp | Long-term study of dry matter allocation and rhizome growth in Anemone nemorosa[END_REF]. The species is known as an 'ancient forest specialist', and has very slow colonization rates [START_REF] Verheyen | Response of forest plant species to landuse change: a life-history trait-based approach[END_REF]. Ficaria verna starts its growth in winter, and the flowers appear in early spring. The carpels develop into achenes, each containing a single seed. The diploid subspecies of F. verna produces a larger number of seeds, while the tetraploid subspecies (not sampled here) produces only very few seeds [START_REF] Taylor | Ranunculus Ficaria L. (Ficaria verna Huds.; F. Ranunculoides Moench)[END_REF]. Ficaria verna also spreads vegetatively via adventitious roots or by means of bulbils or tubers, which separate easily and develop into new plants allowing the species to colonize new habitats rapidly. The species is often associated with moist woodland soils and frequently occurs along streams or river banks, but is sometimes also found on drier, upland soils [START_REF] Taylor | Ranunculus Ficaria L. (Ficaria verna Huds.; F. Ranunculoides Moench)[END_REF].

Poa nemoralis and G. urbanum are respectively tussock-and rosette-forming hemicryptophytes [START_REF] Taylor | Geum Urbanum L[END_REF]. P. nemoralis produces flowers arranged in panicles from June to August. Fruits (caryopses) are mostly dispersed via epizoochory. The bristly diaspore (i.e. with straight appendages) is an adaption to disperse by adhesion to animals' fur, mainly to mid-sized mammals [START_REF] Heinken | Do Wild Ungulates Contribute to the Dispersal of Vascular Plants in Central European Forests by Epizoochory? A Case Study in NE Germany[END_REF]. Zoochory and a high quantity of seeds ensures the quick colonisation success of P. nemoralis in new forest plantations within abandoned arable lands [START_REF] Brunet | Migration of vascular plants to secondary woodlands in southern Sweden[END_REF] and abandoned wooded grasslands [START_REF] Dahlgren | Specific leaf area as a superior predictor of changes in field layer abundance during forest succession[END_REF]. Even so, the species mainly spreads clonally via tussock growth, and therefore has slow vegetative colonization rates [START_REF] Heide | Long Day Control of Flowering in Poa nemoralis in Controlled and Natural Environments[END_REF]Plue et al. in press).

For G. urbanum, however, vegetative spread is rather limited and sexual reproduction via seeds is more common.

It is self-compatible and flowers from July to September. Each flowering head can produce up to several hundreds of achenes. These achenes typically have a hooked awn, which enables them to attach to the fur of animals and hence assist seed dispersal over large distances via epizoochory [START_REF] Taylor | Geum Urbanum L[END_REF]).

Plant performance traits

In each of the selected forest-hedgerow pairs, we sampled populations of each species, both in the hedgerow and in the adjacent, connected forest patch. Specifically, we located a point in both the hedgerow and forest patch at 50 m from their connection, and defined the search area as a circle with 30 m radius around these points. For the hedgerows, the sampling was not performed outside the hedgerow canopy area (Fig. S2). Notably, we strictly focused on the forest-hedgerow paired combinations. So, if a species occurred only in the forest and not in the corresponding hedgerow (or vice versa), the species was not sampled in this particular site (Table S3). All trait measurements were performed on healthy individuals at seed maturity, following the standardized protocol of [START_REF] Pérez-Harguindeguy | New handbook for standardised measurement of plant functional traits worldwide[END_REF]. Individuals with obvious symptoms of pathogen attack or insect predation were avoided. Depending on the phenology of the species and the geographical region, the sampling was performed on different dates (see Table S4).

Both in the forest and in the corresponding hedgerow, we randomly selected 10 healthy-looking individuals of each species at the time of seed maturity (10 individuals × 2 plots × 4 sites = 80 individuals of each species per region). Because not all species were available in all regions, our total number of individuals was 1032. Subsequently, the following set of functional life-history traits was measured on each of these individuals: plant height, specific leaf area (SLA), dry aboveground biomass, seed releasing height, seed number, seed mass and resource investment in reproduction (RIR). We avoided performing measurements on neighbouring individuals (i.e. less than 50 cm from each other; especially for clonal species) to avoid a potentially strong spatial autocorrelation signal in plant traits.

Plant height was measured as the vertical distance between the ground level and upper boundary of the main photosynthetic tissues, excluding inflorescences (i.e. the highest leaf). Plant height at maturity is associated with a species' competitive vigour as well as its ability to cope with various forms of environmental stress [START_REF] Westoby | A leaf-height-seed (LHS) plant ecology strategy scheme[END_REF].

Next, seed-releasing height was measured as the vertical distance between the ground level and highest elevation of seeds or fruits. For species with multiple flowers per individual (e.g. G. urbanum), this measurement was repeated and averaged for all flowering heads. This trait is strongly related to a species' dispersal potential [START_REF] Tackenberg | Assessment of wind dispersal potential in plant species[END_REF].

For the SLA measurements, we collected a young but fully expanded, healthy leaf for each plant individual. Leaves were oven-dried to constant mass (carefully flatted between drying papers) in a drying stove at 60 °C for 48 h. The leaf area (mm²) was measured with a LiCor LI-3000C Area Meter (LI-COR Biosciences, Nebraska, USA). Leaf dry matter (mg) was weighed to the nearest 0.1 mg with a Mettler Toledo AG204 DeltaRange Analytical Balance (Mettler Toledo, Ohio, USA). Specific leaf area (mm² mg -1 ) was then calculated as leaf area divided by leaf dry matter. Specific leaf area typically reflects the trade-off between leaf photosynthetic capacity and leaf longevity [START_REF] Wright | The worldwide leaf economics spectrum[END_REF]). High SLA species usually have shorter leaf lifespans but higher photosynthetic rates. Besides, increasing SLA is generally associated with higher potential growth rates and higher resilience against various disturbance regimes (Bernhardt-Römermann et al. 2011). Finally, we harvested all aboveground biomass of each individual. The plant material was oven-dried to constant mass in a drying stove at 60 °C for 48 h, and subsequently weighed to the nearest 0.1 mg.

To determine the seed number and seed mass, we collected all seeds per individual in the field. The number of seeds was simply counted by hand, and the seed mass was calculated as the total mass of all seeds divided by the seed number. Seed traits play a key role in a species' colonization capacity, and are associated with dispersal distances, seed bank persistence, germination, establishment and survival potential of the seeds [START_REF] Coomes | Colonization, tolerance, competition and seed-size variation within functional groups[END_REF]. In general, smaller seeds can be produced in larger number with the same reproductive effort. Heavier seeds, however, typically have a higher survival and establishment rate upon emergence, notably in the face of environmental stress (e.g. drought, herbivory, etc.) [START_REF] Moles | Global patterns in seed size[END_REF]). To account for this trade-off, we calculated the resource investment in reproduction (RIR) of each individual as the seed mass multiplied by the seed number

Site characteristics

Five variables were used to describe the canopy structure in each forest and hedgerow: the total canopy cover, the total shrub and tree cover (quantified as the sum of cover percentages of the individual shrub and tree species, respectively), and the average shrub and tree height. Height measurements were performed with a vertex (Haglöfs Vertex IV), while the total canopy cover was estimated with a convex spherical densiometer (Forestry Suppliers, Model A). Furthermore, the composition of the canopy was characterized by the shade-casting ability (SCA) and litter quality (LQ) of all overstorey (shrub and tree) species. For each forest and hedgerow, these variables were calculated as the cover-weighted average of SCA and LQ scores of the individual canopy species, respectively (sensu [START_REF] Verheyen | Driving factors behind the eutrophication signal in understorey plant communities of deciduous temperate forests[END_REF]. These scores range between '1' (very low SCA or LQ) and '5' (very high SCA or LQ) (see Table S5).

Mineral topsoil (0-10 cm) samples were collected as mixed-soil samples from three random locations in each forest and hedgerow. The samples were subsequently dried to constant weight at 40 °C for 24 h and sieved through a 2 mm mesh before analysing pH-H20, carbon-to-nitrogen (C/N) ratio, bio-available soil phosphorous (P) concentrations (mg kg -1 ) and sum of basic cations (Ca 2+ + Mg 2+ + K + ) (mg kg -1 ) (see Appendix S1; Table S1).

Sub-canopy (microclimate) temperatures were recorded in each forest and hedgerow using "HOBO Pendant" data loggers. These loggers were mounted at 1 m height in a radiation shield, and recorded the air temperature every two hours from 1 st September 2017 to 1 st September 2018. For each logger, we computed daily summary statistics (that is, minimum, mean and maximum temperatures). Corresponding 'free-air' (macroclimate) temperature data were retrieved for each study site from nearby weather stations (Table S6). Temperature offset values were calculated for each summary statistic as microclimate temperatures minus macroclimate temperatures. In this study, we mainly focused on the effect of maximum summer temperature offsets, because this summary statistic has been shown to differ most between forests and hedgerows [START_REF] Zellweger | Seasonal drivers of understorey temperature buffering in temperate deciduous forests across Europe[END_REF]Vanneste et al. 2020b). Finally, it should be noted that sub-canopy temperature measurements were available for all sampled sites, except for the four forest-hedgerow combinations in Poland. For these sites, the maximum summer temperature offset was predicted based on the available temperature data from the 28 remaining sites taking into account the corresponding canopy characteristics and macroclimate temperature (see Appendix S1).

Data analyses

To test for differences in abiotic and biotic environmental conditions between forests and hedgerows, we fitted linear mixed-effect models (LMM) with restricted maximum-likelihood model estimation (REML) [START_REF] Zuur | Mixed effects models and extensions in ecology with R[END_REF]. In these models, the environmental variables (canopy cover, tree and shrub cover, tree and shrub height, SCA, LQ, soil pH, soil C/N ratio, soil P, sum of basic cations and maximum summer temperature offset; Table S7) were included as response variable and the factor variable 'habitat' ('forests' vs. 'hedgerows') as fixed effect. In addition, a nested random intercept term ('region/site') was included to account for the hierarchical structure of the dataset and spatial autocorrelation between populations of the same geographical region or study site.

Next, we assessed whether the functional traits of each species differed between the two contrasting habitats. As above, we built LMMs with REML including the functional traits as response variable, 'habitat' ('forests' or 'hedgerow') as fixed effect, and 'region/site' as nested random intercept term. Finally, we tested the relationship between the functional traits of each species and the different environmental variables in both habitats. For each species and for each functional trait, we ran a series of separate univariate LMMs, one per predictor variable as fixed effect. We specifically used this univariate regression approach to avoid multicolinearity issues. As above, the models were fit with REML including the functional traits as response variable, the environmental variable as fixed effect, and 'region/site' as nested random intercept term. All predictors were centred to a mean of zero and scaled to a standard deviation of one prior to analysis. P values were adjusted for multiple testing via the false discovery rate (FDR) correction, i.e. the adjusted P values depict the expected proportion of type I errors (rejections of a true null hypothesis or "false discoveries") when the threshold for rejecting the null hypothesis is set at the original P value (sensu [START_REF] Benjamini | Controlling the False Discovery Rate: A Practical and Powerful Approach to Multiple Testing[END_REF]. The FDR-based correction procedure for multiple comparisons is more appropriate for detecting significant results than the traditional methods (e.g. Bonferroni), and has been shown to be especially useful for studies in ecology and evolution [START_REF] Verhoeven | Implementing false discovery rate control: increasing your power[END_REF][START_REF] Pike | Using false discovery rates for multiple comparisons in ecology and evolution[END_REF]).

All data analyses were performed in R Version 3.5.1 (R Core Team 2019), using the functions p.adjust of the stats package (R Core Team 2019), lmer of the lme4 package [START_REF] Bates | Fitting Linear Mixed-Effects Models Using lme4[END_REF] and r.squaredGLMM of the MuMin package [START_REF] Barton | MuMIn: Multi-Model Inference[END_REF]. Several response variables required a transformation prior to analysis to achieve normality of their distribution [START_REF] Zuur | Mixed effects models and extensions in ecology with R[END_REF]. Shrub height, shrub cover, and bio-available P were transformed with natural logarithm. The seed releasing height, seed number, seed mass and RIR of Ficaria, the seed number, RIR and SLA of Poa, and the seed number, RIR and biomass of Geum were square root transformed.

Results

Abiotic and biotic environment

Trees in hedgerows were 10.9 (±1.9 SE) m smaller than in forests, while shrub cover was higher in hedgerows.

Tree cover and SCA were marginally lower in hedgerows (Fig. 2; Fig. S4). The total canopy cover, shrub height, litter quality, soil C/N ratio and sum of basic cations did not differ between habitat types. As to soil properties, only bio-available P concentrations differed significantly between habitats, with consistently higher values in hedgerows than in forests. Maximum summer temperature offset was 0.941 (±0.053 SE) °C less negative (i.e. lower thermal buffering) in hedgerows (Fig. 2; Fig. S4).

Plant performance traits

In general, we found higher vegetative performance in hedgerows than in forests in all study species. For instance, plant individuals of Anemone were 0.751 (±0.268 SE) cm taller in hedgerows than in forests. In addition, all species showed a higher biomass and all species except Ficaria showed a significantly lower SLA in hedgerows than in forests (Fig. 3; Fig. S6-S9). The reproductive performance was higher in hedgerows than in forests in Poa and Geum but lower in Anemone and not significantly different between habitats in Ficaria. To illustrate, we observed a higher seed releasing height, seed number per individual and seed mass in hedgerows for Poa as well as a higher RIR in hedgerows for both Poa and Geum. For Anemone, however, we detected a reduced RIR in hedgerows, notably due to the lower seed mass (Fig. 3; Fig. S5; Fig. S6-S9).

Trait-environment relationships

Plant height of the studied forest herbs was mainly affected by overstorey variables. In general, we found smaller plant individuals under denser canopies. To illustrate, plant height decreased with increasing tree cover in Anemone and with increasing total canopy cover in Ficaria and Poa. In Poa, we also detected a negative effect of soil C/N ratio on plant height. Furthermore, SLA showed consistent negative relationship with tree height in all study species. As to the biomass, no such consistent results among species were found, albeit individual negative effects of tree cover and microclimatic buffering on the biomass could still detected in Anemone and Poa, respectively.

Finally, reproductive output (RIR) of the vernal species was not affected by any of the environmental variables, whereas for both summer-flowering species a strong positive effect of lower microclimatic buffering was found.

In Poa, RIR also correlated positively to soil P levels (Fig. 4; see Table S8-S10 for additional analyses with hedgerow-specific characteristics).

Discussion

Using a systematic paired design among forest interiors and hedgerows across temperate Europe, we show that contrasting environmental conditions among the two habitat types led to differences in the life-history traits as well as vegetative and regenerative performance of understorey forest herbs. Overall, we found a higher biomass of the studied forest plants in hedgerows than in forests, while the reproductive fitness showed a divergent response being higher in hedgerows than in forests for the summer-flowering species but equal or slightly lower for the vernal species. This suggests that hedgerows will likely act as suitable habitats and, on a longer time scale, also as effective corridors for some (albeit not all) forest plant species to migrate among remnant and newly formed woodland patches. These findings underpin the importance of preserving, managing and establishing hedgerows in intensively managed agricultural landscapes with few remaining forest habitats, given their potential to contribute to the long-term conservation and migration of forest herbs in the face of global environmental change.

Divergent environmental conditions of forests and hedgerow habitats

As expected, the abiotic and biotic environment of hedgerows and forest interiors differed consistently. For instance, trees in the studied hedgerows were consistently smaller than in the adjacent forests. This can, at least partly, be attributed to contrasting growth strategies of trees in both habitats [START_REF] Loehle | Height growth rate tradeoffs determine northern and southern range limits for trees[END_REF][START_REF] Sterck | Performance of trees in forest canopies: explorations with a bottom-up functional-structural plant growth model[END_REF].

Hedgerows also tended to have a higher cover of shrubs compared to forest interiors, likely because of the higher light availability in hedgerow environments allowing the formation of a dense and structurally diverse shrub layer.

The higher soil P content in hedgerows than in forest interiors is likely caused by the inflow of fertilizers and runoff from adjacent agricultural lands [START_REF] Wehling | Hedgerows as an environment for forest plants: a comparative case study of five species[END_REF]. Here it is noteworthy that the soil P values in our hedgerows (~40 mg kg -1 ) were markedly higher than the ones reported by [START_REF] Sitzia | Topsoil organic matter properties in contrasted hedgerow vegetation types[END_REF] (~12 mg kg -1 ), presumably owing to a difference in soil fertility and soil pH being neutral in our study vs. weakly alkaline in [START_REF] Sitzia | Topsoil organic matter properties in contrasted hedgerow vegetation types[END_REF]. Finally, linear habitats such as hedgerows are also more prone to edge effects, including greater wind speeds and higher levels of lateral radiation reaching the understorey. These conditions directly affect the nearground microclimate of hedgerows, most notably causing consistently higher temperature extremes than in forests interiors (Vanneste et al. 2020b).

Performance of forest herbs in hedgerows

Interestingly, all studied forest herbs displayed a higher vegetative performance in hedgerows than in forest interiors. This is clearly demonstrated by the consistently higher aboveground biomass of plant individuals growing in the hedgerow habitats. A similar observation for post-agricultural vs. ancient forests was made by [START_REF] Baeten | Plasticity in response to phosphorus and light availability in four forest herbs[END_REF], who reported that the biomass of several forest herb species tended to respond positively to the higher resource availability as well as higher light levels in post-agricultural forest stands. In our study, we indeed found that several structural canopy characteristics related to a higher light availability and warmer microclimate in hedgerows than in forests (e.g. lower and more open tree canopy as well as lower SCA) positively affected the biomass production of the studied forest herb species. Additionally, the SLA of all species (albeit not significant for Ficaria) was lower in hedgerows, and displayed a consistent negative relationship with tree height (shown to be lower in hedgerows). Low SLA values are a clear response to higher light availability and may reflect a greater physical strength of the leaves, and allow the plant individuals to cope with the higher irradiance and lower humidity in hedgerow understories [START_REF] Dahlgren | Specific leaf area as a superior predictor of changes in field layer abundance during forest succession[END_REF]). Alternatively, their lower SLA values could also reflect higher herbivore pressure in hedgerows, because plants have to invest more in protection tissues [START_REF] Schädler | Palatability, decomposition and insect herbivory: patterns in a successional old-field plant community[END_REF][START_REF] Agrawal | PLANT DEFENSE SYNDROMES[END_REF]. Finally, we also found larger plant individuals of Anemone in the hedgerow habitats. On the one hand, plant height can be linked to competition strategies, with hedgerow plants thus being competitively stronger. In this respect, the higher stature could be perceived as a potential response of the species to competition from generalist and grassland species in the linear habitats [START_REF] Roy | Evaluating hedgerow corridors for the conservation of native forest herb diversity[END_REF][START_REF] Paal | Both spatiotemporal connectivity and habitat quality limit the immigration of forest plants into wooded corridors[END_REF]). On the other hand, plant height has also been related to light limitation and nutrient stress [START_REF] Westoby | A leaf-height-seed (LHS) plant ecology strategy scheme[END_REF], which is reflected by the negative effect of overstorey cover (in Anemone, Ficaria and Poa) as well as C/N ratio of the topsoil (only in Poa) in our systems.

As to their reproductive performance in the understorey of both habitat types, the studied forest plant species seemed to respond in three different ways. First, the 'ancient forest herb' Anemone showed a lower reproductive output in the hedgerows than in the adjacent forest patches. A potential explanation could be the strong competition with opportunistic competitors such as Urtica dioica and Rubus sp. that benefit from the higher nutrient availability (particularly phosphate) in hedgerows [START_REF] Endels | Population structure and adult plant performance of forest herbs in three contrasting habitats[END_REF][START_REF] Hipps | The effects of phosphorus nutrition and soil pH on the growth of some ancient woodland indicator plants and their interaction with competitor species[END_REF][START_REF] Baeten | Environmental limitation contributes to the differential colonization capacity of two forest herbs[END_REF]). Indeed, several earlier studies have reported the vigorous growth of these species as a key factor limiting the successful colonization and establishment of many ancient forest species in the herb layer of nutrient-enriched, postagricultural forests (Verheyen & Hermy 2004;[START_REF] Orczewska | Migration of Herbaceous Woodland Flora into Post-Agricultural Black Alder Woods Planted on Wet and Fertile Habitats in South Western Poland[END_REF]. Besides, while De Frenne et al. ( 2010) found consistently positive effects of warmer temperatures on the sexual reproduction of A. nemorosa, it is not unlikely that the species' reproductive success might still be reduced by drought in the hotter and drier microclimate of hedgerows (see e.g. [START_REF] Ali | Post-dispersal embryo growth and acquisition of desiccation tolerance in Anemone nemorosa L. seeds[END_REF][START_REF] Mondoni | Habitat-correlated seed germination behaviour in populations of wood anemone (Anemone nemorosa L.) from northern Italy[END_REF][START_REF] Leuschner | Air humidity, soil moisture and soil chemistry as determinants of the herb layer composition in European beech forests[END_REF]. Owing to the stronger competition and less favourable environment, we argue that Anemone individuals in hedgerows are more likely to allocate a greater amount of energy to growth and persistence, and thus invest less in their generative reproductive output. Alternatively, the reduced reproductive success of this forest specialist in hedgerows could also be a consequence of its generally smaller and more sparsely distributed populations in these habitats. Smaller populations tend to attract fewer pollinators, which could in turn negatively affect seed production (Jacquemyn et al. 2002;[START_REF] Brys | Reduced reproductive success in small populations of the self-incompatible Primula vulgaris[END_REF][START_REF] Hofmeister | Strong influence of long-distance edge effect on herb-layer vegetation in forest fragments in an agricultural landscape[END_REF].

Even so, it should be noted that, as a typically clonally spreading species, Anemone does not so much rely on sexual reproduction via seeds, but could also spread vegetatively via rhizomes and thus still maintain a viable population in hedgerows. Moreover, echoing [START_REF] Herben | Species traits and plant performance: functional trade-offs in a large set of species in a botanical garden[END_REF], we could expect that Anemone will compensate the unfavourable conditions for sexual reproduction by investing more in vegetative reproductive organs and thus even enhance its clonal spread in hedgerow habitats. This is further corroborated by [START_REF] Klimešová | Evolutionary and organismic constraints on the relationship between spacer length and environmental conditions in clonal plants[END_REF], who highlighted that (i) extensive lateral spread is typical for plants growing in more productive habitats with high levels of aboveground competition where seedling establishment could be hampered, and that (ii) this higher investment vegetative reproduction tends to be strongly correlated with plant height. Although speculative, the later could partly explain the higher stature of Anemone individuals found in hedgerows. Yet, this hypothesis remains to be confirmed, for instance, by comparing clonal traits for this species (or even a larger set of clonally spreading species) among forest and hedgerow habitats (e.g. [START_REF] Lohmus | Long-term colonization ecology of forest-dwelling species in a fragmented rural landscape -dispersal versus establishment[END_REF][START_REF] Paal | Standardized response signatures of functional traits pinpoint limiting ecological filters during the migration of forest plant species into wooded corridors[END_REF].

Second, Ficaria seemed to be indifferent to both habitat types, albeit with a marginally higher RIR in hedgerows. This is not surprising given that the species has been shown to exhibit a broad ecological amplitude [START_REF] Taylor | Ranunculus Ficaria L. (Ficaria verna Huds.; F. Ranunculoides Moench)[END_REF]. For instance, Ficaria performs equally well in ancient and newly established forests on former agricultural land (Verheyen & Hermy 2004;[START_REF] Baeten | Environmental limitation contributes to the differential colonization capacity of two forest herbs[END_REF], with both forest types displaying highly divergent habitat characteristics (e.g. pH, nutrient and light availability) [START_REF] Flinn | Recovery of forest plant communities in post-agricultural landscapes[END_REF]. Besides, as a very early vernal geophyte Ficaria has the major part of its photosynthetic active period before the canopy has flushed, so that the effect of forest canopy density and shading on its performance is almost negligible.

Third, the reproductive success of the summer-flowering species Poa and Geum was consistently higher in the linear habitats. For Geum, this could be mainly attributed to the higher diurnal temperature maxima during the growing season, whereas for Poa also the higher soil P levels and lower tree height played an important role. Indeed, both species have been shown to occur abundantly in hedgerows. For instance, [START_REF] Endels | Population structure and adult plant performance of forest herbs in three contrasting habitats[END_REF] demonstrated that the fast-colonizing G. urbanum is able to cope relatively well with both edaphic and microclimatic conditions experienced in hedgerow habitats. Even more, in many across Northwest Europe, G. urbanum is much more a species of hedges and woodland margins than of woodland interiors (see also [START_REF] Schmidt | Effects of landscape structure on genetic diversity of Geum urbanum L. populations in agricultural landscapes[END_REF]). The species' preference for hedgerow environments is also corroborated by [START_REF] Taylor | Geum Urbanum L[END_REF], who reported that the reproductive success and viable seed production of G. urbanum was enhanced as a response to increased light availability and higher soil temperatures in the rooting zone after canopy disturbance. For P. nemoralis, [START_REF] Plue | Climatic control of forest herb seed banks along a latitudinal gradient[END_REF] found that seed production increases as populations grow larger over time. In addition, the authors detected a negative effect of increasing canopy cover on population size, leaving them to conclude that populations growing in shaded habitats produce fewer seeds. Furthermore, it has been repeatedly documented in literature that the species tends to forage after the available light at the forest floor, and therefore avoids darker forests or sites with varying light intensities (see e.g. Diekmann 1994;Tinya and Odor 2016). This also suggests that the higher irradiance level in hedgerows may actually be beneficial for its growth and essential for its regeneration. Finally, it should be noted that, apart from a higher seed production, the ability of Poa and Geum to benefit from hedgerows as movement conduits may be further enhanced by their dispersal mode. Seeds of these species may attach to the feet or fur of vertebrates that move through hedgerows, hence achieving dispersal over large distances or across gaps in these linear features. Ant-or slug-dispersed forest herbs such as A. nemorosa, on the other hand, have much lower rates of population expansion and may experience difficulty crossing gaps in hedgerows [START_REF] Matlack | Slow plants in a fast forest: local dispersal as a predictor of species frequencies in a dynamic landscape[END_REF].

Conclusion

Hedgerows are common forest-like elements in agricultural landscapes [START_REF] Baudry | Hedgerows: an international perspective on their origin, function and management[END_REF], and have been repeatedly advocated as an effective surrogate habitat for many forest herbs [START_REF] Corbit | Hedgerows as habitat corrdiors for forest herbs in central New York, USA[END_REF][START_REF] Roy | Evaluating hedgerow corridors for the conservation of native forest herb diversity[END_REF][START_REF] Liira | Old manor parks as potential habitats for forest flora in agricultural landscapes of Estonia[END_REF]Vanneste et al. 2020a). Furthermore, given the predicted range shifts under contemporary climate change, hedgerows may be crucial to allow the migration of species among isolated habitat fragments. The occurrence of forest herbs in hedgerows has often been considered as evidence for habitat suitability (see [START_REF] De Blois | Factors affecting plant species distribution in hedgerows of southern Quebec[END_REF][START_REF] Wehling | Importance of hedgerows as habitat corridors for forest plants in agricultural landscapes[END_REF]. However, to enhance the function of hedgerows as migration corridors for forest plants, it is important that they also support the progressive, short-distance dispersal of multiple generations. This is particularly the case for many forest specialist herbs, which are more likely to proximally colonize a hedgerow attached to a forest patch, and then gradually migrate along the wooded corridor through successive generations (see [START_REF] Lenoir | Historical continuity and spatial connectivity ensure hedgerows are effective corridors for forest plants: evidence from the species-time-area relationship[END_REF]. Our findings suggest that overall plant performance (i.e. a combination of vegetative traits and reproductive fitness) of the summer-flowering forest herbs tends to be better in hedgerows than in forest interiors, most probably due to the direct effect of the higher light and soil phosphorous availability as well as more benign microclimate (i.e. higher growing season temperatures). The ancient forest specialist A. nemorosa, however, showed a reduced reproduction from seeds in the hedgerows, but could potentially maintain a viable population in these linear habitats by relying on its vegetative spread. Yet, since sexual reproduction is still considered most important for population spread in this species [START_REF] Brunet | Migration of vascular plants to secondary woodlands in southern Sweden[END_REF], it remains unclear whether its progressive dispersal along hedgerow corridors will be limited on the longer term. At large geographical scale, we demonstrate that hedgerows may contain reproductive populations of some but not all forest herbs, and could therefore act as functional migration corridors to connect isolated forest fragments (see also [START_REF] Endels | Population structure and adult plant performance of forest herbs in three contrasting habitats[END_REF][START_REF] Paal | Both spatiotemporal connectivity and habitat quality limit the immigration of forest plants into wooded corridors[END_REF][START_REF] Lenoir | Historical continuity and spatial connectivity ensure hedgerows are effective corridors for forest plants: evidence from the species-time-area relationship[END_REF]. We underpin that strategies to conserve, manage and establish hedgerow corridors in degraded agricultural landscapes across the globe are urgently needed, given their potential to host diverse plant communities, and benefit the long-term persistence and migration of species in an era of environmental change. These measures should not only focus on improving the habitat quality within the hedgerows themselves, but also consider the surrounding landscape context as well as spatial positioning of the hedgerows (i.e. proximity to historically continuous seed-source habitats; de [START_REF] De Blois | Factors affecting plant species distribution in hedgerows of southern Quebec[END_REF][START_REF] Lenoir | Historical continuity and spatial connectivity ensure hedgerows are effective corridors for forest plants: evidence from the species-time-area relationship[END_REF]Vanneste et al. 2020a).

Even so, deriving generalized conclusions from our results remains difficult because the response of forest plant performance to hedgerow conditions is undoubtedly highly species specific (see [START_REF] Endels | Population structure and adult plant performance of forest herbs in three contrasting habitats[END_REF][START_REF] Schmucki | Population structures and individual performances of Trillium grandiflorum in hedgerow and forest habitats[END_REF]. While some forest herbs perform better in hedgerows by taking advantage of the greater access to resources and withstanding microclimatic stress, others may still prefer the environment of forest interiors where potential competitors are supressed due to reduced light and lower nutrient supply. Finally, even though our findings suggest that well-established populations of forest herbs may thrive in hedgerows, it must be kept in mind that recruitment was not explicitly accounted for in this study. [START_REF] Schmucki | Population structures and individual performances of Trillium grandiflorum in hedgerow and forest habitats[END_REF] found significantly less seedlings and juveniles of Trillium grandiflorum in hedgerows than in forests, and suggested that the greater microclimatic and edaphic stress observed in the linear habitats adversely affected the recruitment of this species.

Ultimately, lower germination rates as well as reduced survivorship of seedlings and juveniles may limit long-term population persistence and hamper migration of forest herbs along hedgerow corridors. More detailed demographic studies during multiple growing seasons are required to fully unravel the demography of forest herbs once they have colonized these linear habitats. offset (temperature difference with a nearby weather station) (f) among forests and hedgerows. Differences between both habitats were tested with a linear mixed-effect model (LMM), and are indicated as "***" for P < 0.001, "**" for P < 0.01, "*" for P < 0.05 and "." for P < 0.1. RIR) (q, r, s, t) among forests and hedgerows for Anemone nemorosa, Ficaria verna, Poa nemoralis and Geum urbanum, respectively. Differences between both habitats were tested with a linear mixed-effect model (LMM), and are indicated as "***" for P < 0.001, "**" for P < 0.01, "*" for P < 0.05 and "." for P < 0.1. The relationships between plant performance traits and environmental variables were tested with linear mixed-effect models (LMM). The colours represent the standardized parameter estimates (SPEs) and direction of the relationships; red and blue squares denote positive and negative relationships, respectively. Statistically significant relationships after correction for false discovery rate (FDR) are indicated as "*" at P < 0.001.

(

  sensu De Frenne et al. 2009).

FiguresFig. 1 .

 1 Figures

Fig. 2 .

 2 Fig. 2. Violin plots showing the difference in tree height (a), tree cover (b), shrub cover (c), shade-casting ability (SCA) (d), bio-available soil phosphorous (P) concentrations (e) and maximum summer temperature (Tmax-summer)

Fig. 3 .

 3 Fig. 3. Violin plots showing the difference in plant height (a, b, c, d), specific leaf area (SLA) (e, f, g, h), biomass (i, j, k, l), seed releasing height (m, n, o, p) and resource investment in reproduction (seed number × seed mass;

Fig. 4 .

 4 Fig. 4. Heat maps showing the predictive pairwise relationships between plant performance traits and environmental variables for Anemone nemorosa (a), Ficaria verna (b), Poa nemoralis (c) and Geum urbanum (d). The

Acknowledgements

We thank Luc Willems and Greet De bruyn for performing the chemical analyses and Matteo Tolosano, Déborah Closset-Kopp, Fabien Spicher, and Audrey Peiffer for their assistance during the fieldwork.

Funding information

We are grateful to the Research Foundation -Flanders (FWO) for funding the scientific research network FLEUR.

TV was supported by the Special Research Fund (BOF) from Ghent University (01N02817), PDF by the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (ERC Starting Grant FORMICA 757833) and KV through ERC Consolidator Grant (PASTFORWARD 614839). JL was funded by the European Union through the European Regional Development Fund as the Centre of Excellence (EcolChange).

Data availability statement

Data available from Figshare, with identifier 10.6084/m9.figshare.8104499.

Author contributions

TV, PDF and KV conceived and designed the study. All authors were involved in the site selection and data collection. TV and PDF conducted the statistical analyses. TV and PDF wrote the first draft of the paper, while all authors contributed to revisions.

Supporting Information

Additional Supporting Information may be found in the online version of this article: Appendix S1. Supplementary Materials & Methods, Tables andFigures.