Plant communities have undergone dramatic changes in recent centuries, although not all such changes fit with the dominant biodiversity-crisis narrative used to describe them. At the global scale, future declines in plant species diversity are highly likely given habitat conversion in the tropics, although few extinctions have been documented for the Anthropocene to date (<0.1%). Nonnative species introductions have greatly increased plant species richness in many regions of the world at the same time that they have led to the creation of new hybrid polyploid species by bringing previously isolated congeners into close contact. At the local scale, conversion of primary vegetation to agriculture has decreased plant diversity, whereas other drivers of change-e.g., climate warming, habitat fragmentation, and nitrogen deposition-have highly context-dependent effects, resulting in a distribution of temporal trends with a mean close to zero. These results prompt a reassessment of how conservation goals are defined and justified.

Aims Many studies of vegetation change over multiple decades have focused on vascular plants, but very few on bryophytes, despite the importance of bryophytes for overall plant biodiversity and ecosystem functioning. Using a repeated survey of vascular plants and bryophytes in a forest ecosystem, we tested predictions of the hypotheses that: (1) vegetation change has been driven by N deposition and climate warming, and (2) bryophytes are more responsive to environmental change than vascular plants. Location Lowland temperate forest, northwest France. Methods In forest plots initially surveyed in 1976, we re‐surveyed both vascular plants and bryophytes in 2009 and 2012, respectively. We analysed changes in α‐diversity, β‐diversity, and species composition, and we used community‐weighted mean values of species affinities for temperature, light, pH, soil moisture and N to assess the temporal responses potentially caused by warming, N deposition, or possibly a changing light regime. Results We observed significantly increased species richness of bryophytes and decreased richness of vascular plants. Community affinities to N, pH and temperature increased significantly for bryophytes, but not for vascular plants, although the change over time in N affinities for vascular plants was qualitatively in the predicted direction. Bryophytes showed a higher magnitude of temporal community change than vascular plants, both in terms of overall species composition and environmental affinities, indicating a higher responsiveness of bryophytes to environmental change. Conclusion Overall, the result of more marked temporal community change for bryophytes suggests that the many studies of changes in vascular plant communities over time might underestimate the sensitivity of the broader plant community (including cryptogams) to environmental change.

Many studies of individual sites have revealed biotic changes consistent with climate warming (e.g., upward elevational distribution shifts), but our understanding of the tremendous variation among studies in the magnitude of such biotic changes is minimal. In this study, we resurveyed forest vegetation plots 40 years after the initial surveys in three protected areas along a west‐to‐east gradient of increasingly steep recent warming trends in eastern Canada (Québec). Consistent with the hypothesis that climate warming has been an important driver of vegetation change, we found an increasing magnitude of changes in species richness and composition from west to east among the three parks. For the two mountainous parks, we found no significant changes in elevational species’ distributions in the easternmost park (raw mean = +11.4 m at Forillon Park) where warming has been minimal, and significant upward distribution shifts in the centrally located park (+38.9 m at Mont‐Mégantic), where the recent warming trend has been marked. Community Temperature Indices (CTI), reflecting the average affinities of locally co‐occurring species to temperature conditions across their geographic ranges (“Species Temperature Indices”), did not change over time as predicted. However, close examination of the underpinnings of CTI values suggested a high sensitivity to uncertainty in individual species’ temperature indices, and so a potentially limited responsiveness to warming. Overall, by testing a priori predictions concerning variation among parks in the direction and magnitude of vegetation changes, we have provided stronger evidence for a link between climate warming and biotic responses than otherwise possible and provided a potential explanation for large variation among studies in warming‐related biotic changes.

17Many studies of individual sites have revealed biotic changes consistent with climate 18 warming (e.g., upward elevational distribution shifts), but our understanding of the 19 tremendous variation among studies in the magnitude of such biotic changes is minimal. 20In this study we re-surveyed forest vegetation plots 40 years after the initial surveys in 21 three protected areas along a west-to-east gradient of increasingly steep recent warming 22However, close examination of the underpinnings of CTI values suggested a high 32 sensitivity to uncertainty in individual species' temperature indices, and so a potentially 33 limited responsiveness to warming. Overall, by testing a priori predictions concerning 34 variation among parks in the direction and magnitude of vegetation changes, we have 35 provided stronger evidence for a link between climate warming and biotic responses than 36 otherwise possible, and provided a potential explanation for large variation among studies 37 in warming-related biotic changes. 38

The relative contribution of bryophytes to plant diversity, primary productivity, and ecosystem functioning increases towards colder climates. Bryophytes respond to environmental changes at the species level, but because bryophyte species are relatively difficult to identify, they are often lumped into one functional group. Consequently, bryophyte function remains poorly resolved. Here, we explore how higher resolution of bryophyte functional diversity can be encouraged and implemented in tundra ecological studies. We briefly review previous bryophyte functional classifications and the roles of bryophytes in tundra ecosystems and their susceptibility to environmental change. Based on shoot morphology and colony organization, we then propose twelve easily distinguishable bryophyte functional groups. To illustrate how bryophyte functional groups can help elucidate variation in bryophyte effects and responses, we compiled existing data on water holding capacity, a key bryophyte trait. Although plant functional groups, can mask potentially high inter- and intraspecific variability, we found better separation of bryophyte functional group means compared to previous grouping systems regarding water holding capacity. This suggests that our bryophyte functional groups truly represent variation in the functional roles of bryophytes in tundra ecosystems. Lastly, we provide recommendations to improve monitoring of bryophyte community changes in tundra study sites.

Despite many studies showing biodiversity responses to warming, the generality of such responses across taxonomic groups remains unclear. Very few studies have tested for evidence of bryophyte community responses to warming, even though bryophytes are major contributors to diversity and functioning in many ecosystems. Here, we report an empirical study comparing long‐term change in bryophyte and vascular plant communities in two sites with contrasting long‐term warming trends, using “legacy” botanical records as a baseline for comparison with contemporary resurveys. We hypothesized that ecological changes would be greater in sites with a stronger warming trend and that vascular plant communities, with narrower climatic niches, would be more sensitive than bryophyte communities to climate warming. For each taxonomic group in each site, we quantified the magnitude of changes in species' distributions along the elevation gradient, species richness, and community composition. We found contrasted temporal changes in bryophyte vs. vascular plant communities, which only partially supported the warming hypothesis. In the area with a stronger warming trend, we found a significant increase in local diversity and dissimilarity (β‐diversity) for vascular plants, but not for bryophytes. Presence–absence data did not provide sufficient power to detect elevational shifts in species distributions. The patterns observed for bryophytes are in accordance with recent literature showing that local diversity can remain unchanged despite strong changes in composition. Regardless of whether one taxon is systematically more or less sensitive to environmental change than another, our results suggest that vascular plants cannot be used as a surrogate for bryophytes in terms of predicting the nature and magnitude of responses to warming. Thus, to assess overall biodiversity responses to global change, abundance data from different taxonomic groups and different community properties need to be synthesized.

The Arctic is warming at an alarming rate. While changes in plant community composition and phenology have been extensively reported, the effects of climate change on reproduction remain poorly understood. We quantified multidecadal changes in flower density for nine tundra plant species at a low- and a high-arctic site in Greenland. We found substantial changes in flower density over time, but the temporal trends and drivers of flower density differed both between species and sites. Total flower density increased over time at the low-arctic site, whereas the high-arctic site showed no directional change. Within and between sites, the direction and rate of change differed among species, with varying effects of summer temperature, the temperature of the previous autumn and the timing of snowmelt. Finally, all species showed a strong trade-off in flower densities between successive years, suggesting an effective cost of reproduction. Overall, our results reveal region-and taxon-specific variation in the sensitivity and responses of co-occurring species to shared climatic drivers, and a clear cost of reproductive investment among arctic plants. The ultimate effects of further changes in climate may thus be decoupled between species and across space, with critical knock-on effects on plant species dynamics, food web structure and overall ecosystem functioning.

The Arctic is warming at an alarming rate. While changes in plant community composition and phenology have been extensively reported, the effects of climate change on reproduction remain poorly understood. We quantified multidecadal changes in flower density for nine tundra plant species at a low- and a high-Arctic site in Greenland. We found substantial changes in flower density over time, but the temporal trends and drivers of flower density differed both between species and sites. Total flower density increased over time at the low-Arctic site, whereas the high-Arctic site showed no directional change. Within and between sites, the direction and rate of change differed among species, with varying effects of summer temperature, the temperature of the previous autumn and the timing of snowmelt. Finally, all species showed a strong trade-off in flower densities between successive years, suggesting an effective cost of reproduction. Overall, our results reveal region- and taxon-specific variation in the sensitivity and responses of co-occurring species to shared climatic drivers, and a clear cost of reproductive investment among Arctic plants. The ultimate effects of further changes in climate may thus be decoupled between species and across space, with critical knock-on effects on plant species dynamics, food web structure and overall ecosystem functioning.