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The elemental allelopathy hypothesis states that the metal‐rich litter of hyper‐accumulating species may have a detrimental impact on neighbouring plants giving a competitive advantage to hyper‐accumulators. Here, we aim to specify the conditions of application of this hypothesis in realistic field conditions, and its relative importance compared to other positive effects found in metalliferous systems.We disentangled the litter‐induced elemental allelopathy and canopy effects of two metallophyte species (Arenaria multicaulis and Hutchinsia alpina) with different levels of leaf Zn and Cd accumulation on two ecotypes of Agrostis capillaris with different levels of metal tolerance. The experiment was conducted in two habitats with contrasting pollution levels in a former mining valley in the Pyrenees (France).The metallophyte species that accumulates more metals (Hutchinsia alpina) showed a strong elemental allelopathy effect on the target with lower metal tolerance in the habitat with lower pollution level, while the metallophyte species that accumulates less metals (Arenaria multicaulis) had no litter effect. Both metallophyte species had positive canopy effects likely due to improvement of micro‐climatic conditions. The drought that occurred during the experiment may have influenced these canopy effects, increasing their importance during the course of the study. For Hutchinsia alpina, the positive canopy effects were stronger than the negative litter effects, resulting in overall positive effects on both target ecotypes.Synthesis. Our results brought a better understanding of the occurrence of elemental allelopathy in metallophyte communities and its relative importance as compared to micro‐climatic facilitation in a global warming context. [ABSTRACT FROM AUTHOR]

Assisted tree migration has been proposed as a conceptual solution to mitigate lags in biotic responses to anthropogenic climate change. The rationale behind this concept is that tree species currently growing under warmer and drier climates will be more resistant and resilient to the new climatic conditions than tree species naturally growing in currently wetter and colder climates. However, we hypothesize that, by being more stress‐tolerant to warmer and drier conditions, translocated species should exhibit different functional attributes, which could induce important ecological and societal costs and overcome the desired benefits of maintaining wood production and other ecosystem services. We used principal component analysis (PCA) to analyze variation in seven traits of 106 tree and tall shrub species from contrasting latitudinal distributions in western North America and Europe to predict the potential functional changes of forest ecosystems due to the translocation of tree species from low to high latitudes. We show that species from both continents differed primarily by their position on the leaf economy spectrum (LES) and their size traits. Even though, in Europe, differences in LES were significantly correlated to species southern latitudinal positions, in both continents differences in size traits were significantly correlated to latitude. These results suggest that assisted migration by translocating more conservative species of shorter stature in currently cooler climates should decrease the buffering capacity of forest canopies, decrease facilitation for understory species, and increase wildfire risks, whose effects have the potential to accelerate climate warming through negative atmospheric feedback processes. As an alternative solution to assisted migration that may accelerate rather than mitigate climate change, we recommend that foresters gradually diversify the vertical structure and layering of the existing forest canopy to maintain a sustainable water cycle and energy balance between the soil, the tree and the atmosphere without increasing the wildfire risk. [ABSTRACT FROM AUTHOR]

Questions: Plant communities have been shown to include several functional groups of species that may have contrasting responses to the effects of dominant neighbours, although potentially balancing at the community level. We aimed to assess the potential of species‐group vs community‐level responses of subordinate species to explain variation in the effects of the dominant shrub Artemisia sieberi on understorey species along a grazing intensity gradient in arid steppes of northeast Iran. We also aimed to assess whether species‐group responses help explain variation in community composition. Locations: An Artemisia steppe community in Golestan National Park (northeast Iran). Methods: We used the relative interaction index (RII) to quantify the effects of a shrub on the cover of the 12 most frequent subordinate species. We conducted a first principal component analysis (PCA) on species RII followed by cluster analysis to group species depending on their responses to the shrub, and a second PCA on subordinate species composition. Results: At the community level, subordinate species showed strong competition at the low grazing level, weak facilitation at the intermediate level and weak competition at the high grazing level; a unimodal pattern with a switch back to competition in extremely disturbed conditions that is inconsistent with ecological theories. However, species‐group analyses showed contrasting subordinate species responses, supporting either the decrease in competition with increasing disturbance scenario or the collapse of facilitation, but never a switch back to competition as predicted by recent facilitation theory along resource gradients. Moreover, these contrasting species‐group responses significantly explained community composition. Conclusions: Our study provides new evidence, along a grazing intensity gradient, that communities include different species groups with contrasting responses to dominant species. [ABSTRACT FROM AUTHOR]

Questions: We assessed interactions between climate change, bedrock types and snow cover duration on the trajectories of taxonomic and functional composition of subalpine plant communities. We predict (i) an increase in species richness on siliceous bedrock due to a reduced competition and a decrease in richness on calcareous bedrock due to increasing drought stress; (ii) decreasing snow cover duration should induce a higher shrub encroachment in hollows as compared to ridges; and (iii) increasing growing season temperature should induce taller sizes and more conservative growth traits, in particular in hollows. Location: Subalpine belt of the Grandes Rousses mountain range, southwestern Alps (France). Methods: 189 vegetation plots were sampled in 1997 and 2017–2018. The duration of snow cover was assessed during two years in 1995–1997 and five functional traits were measured on 108 species in 2021. We performed multivariate analyses, quantified community‐weighted means (CWM) of traits and used ANOVAs to detect responses to local‐scale factors and changes in snow cover, temperature and precipitation since 1997 according to a nearby meteorological station. Results: Overall, taxonomic composition weakly changed and changes were more dependent on the position of communities along the snow cover duration gradient than on their bedrock type. The abundance of drought‐tolerant species increased at the border of hollows and there was, over all communities, a slight increase in the abundance of dwarf shrubs and tall herbaceous species, a strong decrease in short herbaceous species and, thus, an overall decrease in species richness. There were important overall changes in CWM of size traits, in particular leaf area which increased the most in hollows irrespective of bedrock types. Conclusion: In this subalpine site the effects of decreasing snow cover duration overwhelmed the effects of bedrocks, which may explain the overall increase in competitive species and decrease in species richness. [ABSTRACT FROM AUTHOR]

Plant interactions in extreme environments are often inferred from spatial associations and quantified by means of paired sampling. Yet, this method might be confounded by habitat‐sharing effects. Here, we address whether paired and random sampling methods provide similar results at varying levels of environmental heterogeneity. We quantified spatial associations with the two methods at three sites that encompass different micro‐environmental heterogeneity and stress levels: Mediterranean environments in Canary Islands, Spain, and Sardinia, Italy, and a cold alpine environment in Hokkaido, Japan. Then, we simulated plant communities with different levels of species micro‐habitat preferences, environmental heterogeneity, and stress levels. We found that differences in species associations between paired and random sampling were indistinguishable from zero in a homogeneous space. When simulating codispersion over a decreasing abundance gradient, both sampling methods correctly identified facilitation and distinguished it from codispersion. Yet, the pairwise method provided higher facilitation estimates than the random one. At each site, there were strong differences between beneficiary species in their spatial association with nurse species, and associations became more positive with increasing stress in Spain. Most importantly, there were no differences in results yielded by the two methods at any of the different stress levels at the Spanish and Japanese sites. At the Italian site, although micro‐environmental heterogeneity was low, we found weakly significant differences between methods that were unlikely due to habitat‐sharing effects. Thus, the paired sampling method can provide significant insights into net and long‐term effects of plant interactions in spatially conspicuous environments. [ABSTRACT FROM AUTHOR]