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Aims: The influence of priority effects on plant community succession is increasingly discussed in community ecology. However, most grassland restoration approaches involving seed addition transfer all target species at the same time. Our objective was to tackle the question: does the order of arrival influence the establishment of restored communities? Location: La‐Bâtie‐Neuve, Southern Alps, France. Methods: We applied sequential sowing using two groups, one set of dominant species and one set of subordinate species, each comprising three different perennial plant species. We tested four sowing treatments: control (without any sowing), two sequential sowing treatments (dominants first or subordinates first) and synchronous sowing. We analysed plant cover each year for three years after sowing and calculated priority and earliness indices (the third year) for each group and each sown species. Results: Manipulating the order of arrival shaped community composition and trajectories. Some species of both groups were positively affected by being sown first compared to being sown synchronously. However, dominant and subordinate groups differed in their earliness index, showing a significant benefit for subordinates to be sown first. The subordinate species Onobrychis viciifolia and Plantago lanceolata established only when they were sown first, while Festuca cinerea showed greater establishment when sown first and simultaneously, compared to late sowing. The dominant Anthyllis vulneraria was not affected by date or type of sowing. However, the cover of the most dominant Bromopsis erecta was lower when being sown second, allowing a control of its dominance by delayed sowing. Conclusion: The strength of priority effects differs between species, which may depend on niche characteristics or microenvironment, influencing (negatively or positively) the establishment of late‐arriving species and affecting their competitive abilities. Our study provides evidence that plant community assembly was influenced by the order of arrival, but demonstrated a strong species‐specific response to priority effects. [ABSTRACT FROM AUTHOR]

The separate and combined effects on calcareous grassland communities of habitat conditions and management regimes are analyzed using constrained ordination techniques. Measuring the niche breadth of species along constrained ordination axes corresponding to habitat and management gradients allows to hierarchize conservation priorities between species within a given biogeographic area and to focus conservation strategies on species with particular life traits.

Aim: Climate warming reshuffles biological assemblages towards less cold‐adapted but more warm‐adapted species, a process coined thermophilization. However, the velocity at which this process is happening generally lags behind the velocity of climate change, generating a climatic debt the temporal dynamics of which remain misunderstood. Relying on high‐resolution time series of vegetation data from a long‐term monitoring network of permanent forest plots, we aim at quantifying the temporal dynamics – up to a yearly resolution – of the climatic debt in the understorey of temperate forests before identifying the key determinants that modulate it. Location: France. Time period: 1995–2017. Taxa studied: Vascular plants. Methods: We used the community temperature index (CTI) to produce a time series of understorey plant community thermophilization, which we subsequently compared to a time series of mean annual temperature changes over the same period and for the same sites. The direction and magnitude of the difference (i.e., the climatic debt) was finally analysed using linear mixed‐effect models to assess the relative contributions of abiotic and biotic determinants, including forest stand characteristics. Results: We found a significant increase in CTI values over time (0.08–0.09 °C/decade), whereas the velocity of mean annual temperature changes was three times higher over the same period (0.22–0.28 °C/decade). Hence, the climatic debt increased over time and was greater in forest stands with higher basal area or older trees as well as under warmer macroclimate. By contrast, a greater frequency of anthropogenic disturbances decreased the climatic debt, while natural disturbances and herbivory had no impact. Conclusions: Although often overlooked in understanding the climatic debt of forest biodiversity, changes in forest stand characteristics may modulate the climatic debt by locally modifying microclimatic conditions. Notably, the buffering effect of the upper canopy layer implies microclimate dynamics that may provide more time for understorey plant communities to locally adapt. [ABSTRACT FROM AUTHOR]

Questions: Tree diversity is key to the functioning of forest ecosystems. However, which components of tree diversity are responsible for tree diversity effects on associated organisms, and in which context, is poorly understood. Location: ORPHEE Experimental site, Cestas–Pierroton in the southwest of France. Methods: We used a large‐scale tree diversity experiment in which we controlled water availability by an irrigation treatment to address tree diversity and water stress effects on the diversity and height of forest understorey vegetation. We assessed the species richness and height of understorey vegetation in irrigated and non‐irrigated 20 m × 20 m plots, either being monocultures of Pinus pinaster, Betula pendula or Quercus robur, or either mixtures of two to five species among Pinus pinaster, Betula pendula, Quercus robur, Quercus ilex and Quercus pyrenaica. Results: Tree species composition, i.e. the proportion of birch or pine, had a significant effect on understorey plant assemblage, species richness and diversity. The proportion of the fast‐growing deciduous angiosperm Betula pendula was negatively correlated to understorey plant richness and diversity, and positively correlated with understorey vegetation height. Understorey vegetation was higher in irrigated plots than in non‐irrigated plots, but irrigation had no clear effect on the species richness, diversity or composition of understorey plant assemblages. Conclusions: Forest tree species composition and in particular the relative proportions of different tree species had stronger effects on understorey plants than tree species richness per se. These effects were consistent across irrigation treatments. Even in young forest plantations, effects of tree mixture on understorey vegetation may be observed and seem mainly driven by the functional type of tree in the canopy and initial dynamics of plant regeneration in planted forests. [ABSTRACT FROM AUTHOR]

Abstract: Large wild ungulates are a major biotic factor shaping plant communities. They influence species abundance and occurrence directly by herbivory and plant dispersal, or indirectly by modifying plant‐plant interactions and through soil disturbance. In forest ecosystems, researchers’ attention has been mainly focused on deer overabundance. Far less is known about the effects on understory plant dynamics and diversity of wild ungulates where their abundance is maintained at lower levels to mitigate impacts on tree regeneration. We used vegetation data collected over 10 years on 82 pairs of exclosure (excluding ungulates) and control plots located in a nation‐wide forest monitoring network (Renecofor). We report the effects of ungulate exclusion on (i) plant species richness and ecological characteristics, (ii) and cover percentage of herbaceous and shrub layers. We also analyzed the response of these variables along gradients of ungulate abundance, based on hunting statistics, for wild boar (Sus scrofa), red deer (Cervus elaphus) and roe deer (Capreolus capreolus). Outside the exclosures, forest ungulates maintained higher species richness in the herbaceous layer (+15%), while the shrub layer was 17% less rich, and the plant communities became more light‐demanding. Inside the exclosures, shrub cover increased, often to the benefit of bramble (Rubus fruticosus agg.). Ungulates tend to favour ruderal, hemerobic, epizoochorous and non‐forest species. Among plots, the magnitude of vegetation changes was proportional to deer abundance. We conclude that ungulates, through the control of the shrub layer, indirectly increase herbaceous plant species richness by increasing light reaching the ground. However, this increase is detrimental to the peculiarity of forest plant communities and contributes to a landscape‐level biotic homogenization. Even at population density levels considered to be harmless for overall plant species richness, ungulates remain a conservation issue for plant community composition. [ABSTRACT FROM AUTHOR]

A dynamic coupled biogeochemical–ecological model was used to simulate the effects of nitrogen deposition and climate change on plant communities at three forest sites in France. The three sites had different forest covers (sessile oak, Norway spruce and silver fir), three nitrogen loads ranging from relatively low to high, different climatic regions and different soil types. Both the availability of vegetation time series and the environmental niches of the understory species allowed to evaluate the model for predicting the composition of the three plant communities. The calibration of the environmental niches was successful, with a model performance consistently reasonably high throughout the three sites. The model simulations of two climatic and two deposition scenarios showed that climate change may entirely compromise the eventual recovery from eutrophication of the simulated plant communities in response to the reductions in nitrogen deposition. The interplay between climate and deposition was strongly governed by site characteristics and histories in the long term, while forest management remained the main driver of change in the short term. [ABSTRACT FROM AUTHOR]

In restored grasslands of southern Europe, perennial plants remain highly underrepresented compared with the reference ecosystems. We tested various treatments to reintroduce common perennial plant species ( Brachypodium retusum, Poaceae, and Thymus vulgaris, Lamiaceae), which are usually not or poorly reintroduced via soil and hay transfer. Treatments included microenvironmental manipulations (rock cover and plant interactions) and two grazing intensities. Target perennial species were transplanted in 2002 in the reference grassland ecosystem (intact grassland area used as a control) and in two abandoned fields. Survival was assessed in June 2003 and June 2004. Target species shoot and root biomass were measured in June 2004. Grazing greatly reduced the survival and biomass of both target species and its effects were reinforced by summer drought: plants that did not establish well enough during the autumn and spring did not survive summer. The restored rock cover had a mild positive effect, particularly on B. retusum. There were no negative or positive plant neighbor interactions in the steppe, while there was competition in both abandoned fields. Competition was particularly intense in the abandoned melon field, composed of a dense sward of annual grasses ( Bromus sp.). In order to reintroduce perennial species to dry grasslands, the ideal combination of treatments is to exclude or reduce grazing during the first year to allow seedlings to establish and to recreate adequate microenvironmental conditions. Reducing competition from arable weeds may help but is not essential in such dry grasslands. [ABSTRACT FROM AUTHOR]

Nutrient pollution presents a serious threat to biodiversity conservation. In terrestrial ecosystems, the deleterious effects of nitrogen pollution are increasingly understood and several mitigating environmental policies have been developed. Compared to nitrogen, the effects of increased phosphorus have received far less attention, although some studies have indicated that phosphorus pollution may be detrimental for biodiversity as well. On the basis of a dataset covering 501 grassland plots throughout Europe, we demonstrate that, independent of the level of atmospheric nitrogen deposition and soil acidity, plant species richness was consistently negatively related to soil phosphorus. We also identified thresholds in soil phosphorus above which biodiversity appears to remain at a constant low level. Our results indicate that nutrient management policies biased toward reducing nitrogen pollution will fail to preserve biodiversity. As soil phosphorus is known to be extremely persistent and we found no evidence for a critical threshold below which no environmental harm is expected, we suggest that agro-environmental schemes should include grasslands that are permanently free from phosphorus fertilization. [ABSTRACT FROM AUTHOR]

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Questions Can we adequately model beech distribution based on accompanying vegetation, and how does this compare with a climate model? To what extent does accompanying vegetation predict larger extension of beech at its SW margin compared with current distribution? Do human factors explain the discrepancy between current and potential distribution? Location Lowland forests at the SW European range margin of beech. Methods We built a logistic regression model of beech presence based on accompanying understorey vegetation using 66 976 plots in the French National Forest Inventory ( NFI) network. Explanatory variables were obtained by multivariate reduction of 252 species in NFI plots. The model was calibrated in regions where human impact on beech has been comparatively low. The probability of beech presence was estimated at its SW margin and compared with current distribution. We tested whether forest management factors could explain beech absence in locations where the flora predicted its presence. Performance of this model was compared with a classical climatic-envelope model. Results Modelled potential distribution of beech along its SW margin was larger than observed distribution, suggesting area shrinkage under man's influence. Sites where beech was predicted as present but was currently absent were significantly less common in high forest stands, and more common in coppices, plantations and private forests. Characteristics of species replacing beech (early-successional, plantation or exotic species) confirmed the role of forest management and disturbance in beech disappearance. The floristic model provided a finer resolution distribution map than the climatic-envelope model. Conclusions Plant communities, together with precise tree species maps, allowed us to estimate manipulation of beech by man, and main silvicultural causes of its disappearance. The improvement in quality of current distribution models has important implications for modelling of niches under future climate scenarios. [ABSTRACT FROM AUTHOR]

Although atmospheric nitrogen ( N) deposition and climate changes are both recognized as major components of global change, their interaction at ecosystem level is less well understood. A stratified resampling approach was used to investigate the potential impact of changing levels of atmospheric nitrogen deposition and climate change on species composition of nutrient-poor acid grasslands within the French Atlantic Domain ( FAD). The study was based on a comparison, over a period of 25 years, of 162 past and present vegetation records assigned to the species-rich Nardus grasslands and distributed in regional community types ( CTs). Similarly, the characterization of N deposition and climate was stratified according to (i) past (1980-1990) and present (1995-2005) periods, and (ii) FAD and CT scales. Despite the relatively short time span between sampling periods, significant N deposition and climate changes were detected as well as vegetation changes. Correspondence analysis showed that the relative importance of N deposition and climate in explaining vegetation changes depended on the spatial scale of investigation ( FAD vs. local CTs) and the CT. At the FAD scale, the increase of annual mean temperature and decrease of water availability were clearly related to the changes in floristic composition. At the local scale, the most stable CT experienced no significant climate change and a stable load of N deposition, whereas the CTs characterized by the largest floristic changes were associated with dramatic climate changes and moderate loads in both oxidized and reduced N deposition. Despite the narrow gradient of deposition investigated, N deposition was related to significant grassland community changes, depending on the region, i.e. climate context, and on whether N deposition was in the oxidized or reduced form. Our results suggest that N deposition drives grassland composition at the local scale, in interaction with climate, whereas climate changes remain the predominant driver at the FAD scale. [ABSTRACT FROM AUTHOR]

A survey of 153 acid grasslands from the Atlantic biogeographic region of Europe indicates that chronic nitrogen deposition is changing plant species composition and soil and plant-tissue chemistry. Across the deposition gradient (2–44 kg N ha−1 yr−1) grass richness as a proportion of total species richness increased whereas forb richness decreased. Soil C:N ratio increased, but soil extractable nitrate and ammonium concentrations did not show any relationship with nitrogen deposition. The above-ground tissue nitrogen contents of three plant species were examined: Agrostis capillaris (grass), Galium saxatile (forb) and Rhytidiadelphus squarrosus (bryophyte). The tissue nitrogen content of neither vascular plant species showed any relationship with nitrogen deposition, but there was a weak positive relationship between R. squarrosus nitrogen content and nitrogen deposition. None of the species showed strong relationships between above-ground tissue N:P or C:N and nitrogen deposition, indicating that they are not good indicators of deposition rate. [Copyright &y& Elsevier]

Evidence from an international survey in the Atlantic biogeographic region of Europe indicates that chronic nitrogen deposition is reducing plant species richness in acid grasslands. Across the deposition gradient in this region (2–44 kg N ha−1 yr−1) species richness showed a curvilinear response, with greatest reductions in species richness when deposition increased from low levels. This has important implications for conservation policies, suggesting that to protect the most sensitive grasslands resources should be focussed where deposition is currently low. Soil pH is also an important driver of species richness indicating that the acidifying effect of nitrogen deposition may be contributing to species richness reductions. The results of this survey suggest that the impacts of nitrogen deposition can be observed over a large geographical range. [Copyright &y& Elsevier]

Question: What is the most appropriate combination of treatments to reintroduce Nassella pulchra, a perennial bunchgrass, into degraded mediterranean coastal grasslands? Location: Central coast of California, USA. Methods: N. pulchra was sown from seeds and transplanted into a degraded grassland in a multi-factorial experiment testing the effects of (1) two grazing intensities (lightly grazed by native mammal species or ungrazed); (2) topsoil removal and (3) reduction of plant neighbours. The experiment was carried out on two types of surrounding vegetation (exotic annual grasses and exotic forbs). Results: Topsoil removal greatly enhanced establishment from seeds and transplant survival, mainly because it reduced the exotic vegetation and thus reduced competition. While removing neighbours was essential when topsoil was left intact, it had a negative effect on N. pulchra when surrounding species included exotic forbs ( Brassica spec, and Asteraceae) at low density (after topsoil removal). Moderate grazing by native mammals (deer, rabbits and gophers) did not affect N. pulchra. Conclusion: Our results suggest that seeding after topsoil has been removed is a promising method to reintroduce N. pulchra to highly degraded sites where there is little to no native seed bank. [ABSTRACT FROM AUTHOR]

Grasslands are hot spots of biodiversity but are now widely threatened by changes in patterns of disturbances, such as grazing and fire regimes, exotic species invasions, and cultivation. The goal of this experiment was to find the most appropriate combination of treatments to reintroduce Danthonia californica, a formerly dominant perennial bunchgrass, into degraded California coastal prairies. Danthonia californica was sown from seed and transplanted at two sites and at two grazing intensities (grazed/ungrazed) in a multifactorial experiment testing the effects of (1) local versus nonlocal seed sources; (2) topsoil removal; and (3) reduction of plant neighbors. Seed emergence was very low, suggesting that transplanting may be a better option to reintroduce D. californica. Although transplants grown from nonlocal seeds survived better initially at both sites, transplants from local seeds had higher survival after 1.5 year at one site. This suggests that short-term plant establishment studies may be misleading. Topsoil removal greatly enhanced transplant survival, and neighbor removal primarily increased transplant growth. Our results suggest that removing topsoil prior to transplanting seedlings grown from local seeds is the most promising method to reintroduce D. californica. However, the benefits of removing topsoil to provide safe sites for plant establishment should be weighed carefully against potential negative effects on the native seed bank and microbial communities on a site-specific basis. [ABSTRACT FROM AUTHOR]

We compared the effects of herbivory by grasshoppers and neighbourhood competition on two dominant grasses, Bromus erectus and Brachypodium pinnatum, in a calcareous grassland in the French Alps. In a fully factorial design, herbivory was reduced by insecticide spraying and competition was reduced by removal of neighbouring plants. The effects of herbivory and competition were species-dependent. Bromus, a stress-tolerant species, was strongly affected by competition, but not by herbivory. In contrast, the more competitive species, Brachypodium, was negatively affected by herbivory, but only when neighbouring vegetation was removed. The greatest herbivory pressure on isolated targets of Brachypodium is likely to be due to the indirect effects of experimental gaps, i.e. more favourable microclimatic and foraging conditions for grasshoppers. This suggests that herbivory by insects may be a confounding factor in many plant removal experiments. Field experiments designed to study the combined effects of competition and herbivory should take into account the indirect effects induced by experimental gaps. [Copyright &y& Elsevier]

In Boulanger et al. (2018), we investigated the effects of ungulates on forest plant diversity. By suggesting a revisit of our conclusions regarding ecosystem dynamics since the late Pleistocene, Fløjgaard et al. (2018) came to the conclusion that moderate grazing in forest should be a conservation target. Since major points of our paper were mis‐ or over‐ interpreted, we put the record straight on our study system and on the scope of our conclusions. Finally, we advocate for an assessment of the conservation issues of ungulates in forests not only regarding hypothetical and still debated states of past ecosystems but also considering timely challenges for forest ecosystems. [ABSTRACT FROM AUTHOR]

Résumé La régénération forestière est un processus clé de persistance des forêts, particulièrement en marge chaude d'aire de répartition. L'influence du contexte thermique (tempéré ou chaud), de la disponibilité en eau du sol et des interactions chimiques par allélopathie de différentes espèces ont été testées sur la germination de Fagus sylvatica en conditions climatiques contrôlées. Le taux de germination de faînes de Fagus vernalisées est amélioré par des températures relativement chaudes (20 °C), mais bloqué de manière réversible sous contrainte thermique (27 °C). Le taux de croissance relatif des plantules de Fagus est plus important en conditions tempérées que chaudes. Les extraits foliaires de Hedera helix montrent d'importants impacts allélopathiques sur la régénération de Fagus , surtout en conditions tempérées. Nos résultats suggèrent une limitation de régénération de Fagus en marge chaude de son aire de répartition, et une modulation des succès de régénération selon le type de voisinage végétal. Abstract Recruitment is a key process for forest sustainability, especially in warm margin of distribution area. The influence of climate (temperate or warm), of soil water availability, and of allelopathic interactions from different forest species have been tested on the germination of Fagus sylvatica in controlled climatic conditions. Germination rates of non-dormant Fagus seeds were improved by relatively warm temperatures (20 °C), but reversibly stopped under heat constraint (27 °C). The relative growth rate of Fagus seedlings was better under temperate climatic conditions. Foliar extracts of Hedera helix showed the highest allelopathic effect on Fagus recruitment, especially in temperate conditions. Our results suggest a limitation of Fagus recruitment in warm margin of its distribution area, and a modulation of recruitment success according to the identity of plant neighbourhood. [ABSTRACT FROM AUTHOR]

This data set consists of vascular plant and bryophyte species composition and plant and soil biogeochemical data from 153 acid grasslands located in the Atlantic biogeographic region of Europe. Data were collected between 2002 and 2007. The grasslands all belong to the Violion caninae association and were managed by grazing or cutting but had not received fertilizer inputs. These data provide plant composition from five randomly located 2 × 2 m quadrats at each site with all vascular plants and bryophytes identified to species level with cover estimates for each species. Topsoil and subsoil were collected in each quadrat, and data are provided for pH, metal concentrations, nitrate and ammonium concentrations, total carbon and N, and Olsen extractable phosphorus. Aboveground plant tissues were collected for three species (Rhytidiadelphus squarrosus, Galium saxatile, and Agrostis capillaris), and data are provided for percentage N, carbon, and phosphorus. These data have already been used in a number of research papers focusing on the impacts of atmospheric N deposition on grassland plant community and biogeochemistry. The unique data set presented here provides the opportunity to test, theories about the effect of environmental variation on plant communities, biogeochemistry, and plant-soil interactions, as well as spatial ecology and biogeography. [ABSTRACT FROM AUTHOR]

The objective of this study was to examine the interactive influence of tree diversity and water availability on microbial functioning in surface soil (0–10 cm). The study was conducted at two field experiments (ages 4.2 and 9.0 years, respectively) of the TreeDivNet platform that established plantations with a gradient of tree species richness exposed to high and low water availability treatments. Soil microbial biomass, activity, functional diversity and functional identity were estimated using the MicroResp™ method. Soil organic carbon and several other variables potentially associated with soil processes (soil moisture, soil and air temperature, soil pH, soil C:N ratio, tree biomass) were also measured. Our results indicate that tree species richness significantly increased soil microbial biomass and, to a lesser extent, soil microbial activity in the high water availability treatment, at both trial sites. However, tree species richness effects on microbial processes were not expressed in the low water availability treatment, contrary to the stress-gradient hypothesis. Tree species richness and water treatment interaction impacted microbial biomass and activity through their effects on microbial functional diversity and identity at one site and air temperature at the other site. In conclusion, the mechanisms through which tree diversity creates bottom-up effects on biological soil processes depend on environmental conditions and time following the establishment of tree communities. Future research relating diversity and water availability effects on soil microbial processes should consider functional traits of tree species and a larger range of trophic levels including herbivores and soil fauna to better understand biodiversity-soil ecosystem function relationships in forest ecosystems. • Soil was sampled in 2 tree diversity experiments that also manipulated water supply. • Microbial biomass increased with diversity, but only under high water availability. • Soil organic carbon was not affected by tree diversity nor by water availability. • Tree diversity impacted soil through microbial functional diversity and temperature. • Future research should consider functional traits of trees and larger food web. [ABSTRACT FROM AUTHOR]

Highlights: [•] We studied the change in dry calcareous grassland vegetation over the past 70years. [•] We compiled 1186 plots from the sub-oceanic regions in North-western Germany. [•] Plot species richness did not, or only marginally change, over time. [•] Species composition changed, with typical dry grassland species decreasing. [•] Atmospheric nitrogen deposition did not show clear effects on the vegetation. [ABSTRACT FROM AUTHOR]