Patrick Saccone

Climate change increases plant species richness in alpine ecosystems. However, to what extent this diversity enrichment masks extinction dynamics of resident species remains elusive. In this study, we used floristic resurvey data from 896 permanent vegetation plots across 62 European mountain summits to show that local extinctions have increased over the past 21 years. Extinction rates rose with the magnitude of warming, and species were more likely to go extinct toward their low-elevation range margins and in communities undergoing stronger thermophilization. Moreover, local extinctions were significantly related to preceding abundance declines, which can serve as an early warning signal. These findings suggest that despite increasing plant species richness, plant assemblages above the treeline face an accelerating but so far neglected loss of their most characteristic species.

Recent changes in alpine vegetation are often attributed to climate warming, particularly community composition shifts towards more warmth-associated species, or thermophilization. Here we assess the link between thermophilization and warming across 53 European mountain summits. We combine long-term macroclimatic and microclimatic temperature time series with vegetation surveys in 724 permanent plots, monitored over 21 years, to evaluate a possible thermophilization signal and relate it to rates of change in 10 temperature metrics. We find evidence of both thermophilization of alpine plant communities and an increase in temperatures. However, although these two trends are related when averaged across mountain regions, their relationship is weak at the individual plot scale, especially when considering microclimatic temperature metrics. Instead, substrate conditions and particularly the availability of thermophilic colonizers in the surrounding vegetation have a major influence on plot-level thermophilization rates. We conclude that the response of plant communities to climate change strongly depends on the abiotic and biotic context, and intensified monitoring efforts are needed to reduce the resulting uncertainties.

Climate change is driving significant changes in alpine ecosystems, where temperature‐sensitive plant communities are particularly vulnerable. As plants migrate to higher elevations, understanding the mechanisms behind these changes is critical for predicting ecosystem dynamics. Morphological, physiological, and chemical functional traits may help understand species' responses to environmental change. This study examines whether functional traits can explain recent changes in presence and cover of alpine plant species in permanent plots. We analyzed vegetation data from repeated surveys of permanent plots at two sites. Morphological traits (e.g., plant height, leaf area) and physiological traits (e.g., frost and drought resistance) were measured locally for common species and supplemented with data from the TRY trait database to broaden species coverage. Linear models were applied to assess the relationships between traits and abundance changes (presence and cover), using multiple metrics. Our results reveal that plant height and leaf area are significant predictors of species abundance changes, emphasizing the role of morphological traits in shaping alpine plant communities. In contrast, physiological traits showed limited explanatory power. Notably, leaf carbon content emerged as a key predictor, suggesting that conservative strategies may provide advantages under warming conditions. Measures describing biomass dynamics (e.g., cover) differed from those describing establishment (e.g., presence), highlighting the multifaceted nature of species responses to environmental changes. Although our findings emphasize the importance of competitive interactions and resource acquisition, the study is limited by the absence of traits related to heat tolerance and prolonged warming. Future research should address these gaps to better understand the impacts of sustained temperature increases on alpine ecosystems.

The increasingly acknowledged and consequently also better understood microclimatic variability in terrestrial ecosystems has motivated a call for finer spatial resolution in species distribution modelling, especially in the case of sedentary low‐stature organisms such as plants. In contrast, less attention has so far been paid to the way climate should be represented in these models. In fact, most modelling applications rely only on a handful of so‐called bioclimatic variables (i.e. essential climatic variables designed for ecological applications), which is at odds with the hypothesised variation in the sensitivity of individual plant species to different facets of climate. We argue that the recent shift towards microclimate modelling provides a window of opportunity for re‐evaluating the predominant reliance on the small set of bioclimatic variables. We used a unique dataset of 895 1‐m ² plots with vascular plant species observations and in situ soil temperature measurements across a high‐mountain landscape spanning 1700 elevational metres. From the hourly temperature measurements, we calculated bioclimatic variables as well as 188 additional ‘grid variables' arising from an aggregation of various summary statistics over different parts of the year. We then used those ‘grid variables', their subsets, and the bioclimatic variables to fit species distribution models for 101 plant species with combinations of one, two or three predictors. We found that bioclimatic variables consistently delivered less accurate models than many of the grid variables. Models based on subsets of grid variables only slightly decreased in accuracy and remained superior over bioclimatic variables even after the set was narrowed from the initial 188 to only six variables on the basis of a cluster analysis. These results highlight that modelling species distributions with only a few climatic variables is a viable strategy. However, the most suitable variables may often be different from those that are commonly used nowadays.

Boreal and tundra plant communities are expected to change in biodiversity due to increasing global change pressures such as climate warming. One long‐term scenario is increasing compositional similarity, i.e. biotic homogenization, which has been relatively little studied in high‐latitude plant communities. Here, we study how the composition and diversity of heathland and tundra plant communities have changed in northern Fennoscandia over several decades. In 2013–2023, we resurveyed 275 historic vegetation plots, originally surveyed in 1964–1975, with percentage covers for vascular plant, bryophyte and lichen species. We analyzed temporal changes in community composition and diversity across the study area and in different biogeographic zones, continentality‐humidity classes and habitat types. We found a strong homogenization trend across the study area, with plant communities becoming more similar in composition over the decades when all taxa were treated together. The observed homogenization was driven especially by the increased similarity of vascular plant and lichen communities and was largely independent of biogeographic zones or continentality‐humidity gradient. Homogenization was particularly associated with the drastic encroachment of the evergreen dwarf shrub Empetrum nigrum in habitat types originally dominated by other species, and with the decrease in lichen cover. In general, our findings suggest that Fennoscandian heathland and tundra vegetation is transforming towards a more homogeneous evergreen dwarf shrub‐dominated system, which may threaten ecosystem multifunctionality. Our results highlight the importance of exploring biodiversity among different metrics and growth forms to understand the overall changes in heathland and tundra biodiversity.

Aim: Understanding the resilience and adaptability of alpine flora under climate change is crucial for biodiversity conservation. While functional traits are key to predicting alpine plants' responses to climate change, the role of regeneration traits remains underexplored. We hypothesised that alpine species thriving under climate change produce seeds with higher dispersal ability, longer soil persistence, lower dormancy requirements, and faster germination, while declining species would show opposite traits. Location: Twenty‐three summits across six mountain ranges in Central and Southern Europe: Sierra Nevada, Northern and Central Apennines, and Northeastern, Central, and Southern Alps. Methods: We analysed long‐term data on frequency and abundance changes and eight seed traits related to dispersal, establishment, and soil persistence for 177 alpine species using linear mixed‐effect models. Results: Over two decades, alpine plant populations remained stable, with nearly 90% of species showing minimal frequency change and 70% showing minimal abundance change. However, abundance shifts varied by region: 16%–25% of species declined in Sierra Nevada, the Central Apennines, and the Southern Alps, while the Northeastern Alps and the Northern Apennines showed the largest increases (27% and 17%, respectively). Significant but limited relationships between seed traits and population dynamics were captured, primarily in the Central and Northern Apennines. Species with lower potential for epizoochory or anemochory were more likely to increase in abundance, while smaller seeds were linked to ‘winners’ in some regions. Germination traits, such as broader temperature requirements and slower germination, characterised species with increased abundance in the Northern Apennines. Main Conclusions: Seed traits had limited predictive power in distinguishing ‘losers’ and ‘winners’ of climate change among European alpine plants. This likely reflects the longevity of alpine plants, short observation periods, and potential mismatches between seed‐level microenvironmental conditions and broader climatic trends.

Although variation in effect sizes and predicted values among studies of similar phenomena is inevitable, such variation far exceeds what might be produced by sampling error alone. One possible explanation for variation among results is differences among researchers in the decisions they make regarding statistical analyses. A growing array of studies has explored this analytical variability in different fields and has found substantial variability among results despite analysts having the same data and research question. Many of these studies have been in the social sciences, but one small “many analyst” study found similar variability in ecology. We expanded the scope of this prior work by implementing a large-scale empirical exploration of the variation in effect sizes and model predictions generated by the analytical decisions of different researchers in ecology and evolutionary biology. We used two unpublished datasets, one from evolutionary ecology (blue tit, Cyanistes caeruleus, to compare sibling number and nestling growth) and one from conservation ecology (Eucalyptus, to compare grass cover and tree seedling recruitment). The project leaders recruited 174 analyst teams, comprising 246 analysts, to investigate the answers to prespecified research questions. Analyses conducted by these teams yielded 141 usable effects (compatible with our meta-analyses and with all necessary information provided) for the blue tit dataset, and 85 usable effects for the Eucalyptus dataset. We found substantial heterogeneity among results for both datasets, although the patterns of variation differed between them. For the blue tit analyses, the average effect was convincingly negative, with less growth for nestlings living with more siblings, but there was near continuous variation in effect size from large negative effects to effects near zero, and even effects crossing the traditional threshold of statistical significance in the opposite direction. In contrast, the average relationship between grass cover and Eucalyptus seedling number was only slightly negative and not convincingly different from zero, and most effects ranged from weakly negative to weakly positive, with about a third of effects crossing the traditional threshold of significance in one direction or the other. However, there were also several striking outliers in the Eucalyptus dataset, with effects far from zero. For both datasets, we found substantial variation in the variable selection and random effects structures among analyses, as well as in the ratings of the analytical methods by peer reviewers, but we found no strong relationship between any of these and deviation from the meta-analytic mean. In other words, analyses with results that were far from the mean were no more or less likely to have dissimilar variable sets, use random effects in their models, or receive poor peer reviews than those analyses that found results that were close to the mean. The existence of substantial variability among analysis outcomes raises important questions about how ecologists and evolutionary biologists should interpret published results, and how they should conduct analyses in the future.

Boreal and tundra plant communities are expected to change in biodiversity due to increasing global change pressures such as climate warming. One long-term scenario is increasing compositional similarity, i.e., biotic homogenization, which has been relatively little studied in high-latitude plant communities. Here, we study how the composition and diversity of heathland and tundra plant communities have changed in northern Fennoscandia over several decades. In 2013–2023, we resurveyed 275 historic vegetation plots, originally surveyed in 1964–1975, with percentage covers for vascular plant, bryophyte and lichen species. We analyzed temporal changes in community composition and diversity across the study area and in different habitat types, biogeographic zones, and along the continentality-humidity gradient. We found a strong trend across the study area, with plant communities becoming more similar in composition over the decades. The observed homogenization was associated with compositional changes in vascular plant and lichen communities, and in particular with the encroachment of the evergreen dwarf shrub Empetrum nigrum . In comparison to vascular plants and lichens, the diversity of bryophytes generally remained more stable over time. Our findings suggest that Fennoscandian heathland and tundra vegetation is transforming towards a more homogenous evergreen dwarf shrub dominated system, which may threaten ecosystem multifunctionality. Our results highlight the importance of exploring biodiversity among different metrics and growth forms to understand the overall changes in heathland and tundra biodiversity.