Aim Climate change causes major shifts in species distributions, reshuffling community composition and favouring warm‐adapted species (“thermophilization”). The tree community response is likely to be affected by major disturbances, such as fire and harvest. Here, we quantify the relative contributions of climate change and disturbances to temporal shifts in tree composition over the last decades and evaluate whether disturbances accelerate community thermophilization. Location Québec, Canada. Time period 1970–2016. Taxa studied Trees. Methods Using 6,281 forest inventory plots, we quantified temporal changes in species composition between a historical (1970–1980) and a contemporary period (2000–2016) by measuring temporal β‐diversity, gains and losses. The effects of climate and disturbances on temporal β‐diversity were quantified using multiple regressions and variation partitioning. We compared how community indices of species temperature preference (CTI) and shade tolerance (CSI) changed for forests that experienced different levels of disturbance. We quantified the contribution of species gains and losses to change in CTI. Results Temporal β‐diversity was mainly driven by disturbances, with historical harvesting as the most important predictor. Despite the prevailing influence of disturbances, we revealed a significant thermophilization (ΔCTI = +.03 °C/decade) throughout forests in Québec. However, this shift in community composition was weakly explained by climate change and considerably slower than the rate of warming (+.14 °C/decade). Importantly, thermophilization was amplified by moderate disturbances (+.044 °C/decade), almost a threefold increase compared with minor disturbances (+.015 °C/decade). The gains and losses of a few tree species contributed to this community‐level shift. Conclusions Our study provides evidence that disturbances can strongly modify tree community responses to climate change. Moderate disturbances, such as harvesting, might reduce competition and facilitate gains of warm‐adapted species, which then accelerate thermophilization of tree communities under climate change. Although accelerated by disturbances, community thermophilization was driven by the gains and losses of a small number of species, notably gains of maples.

Several temperate tree species are expected to migrate northward and colonize boreal forests in response to climate change. Tree migrations could lead to transitions in forest types, but these could be influenced by several non-climatic factors, such as disturbances and soil conditions. We analysed over 10,000 forest inventory plots, sampled from 1970 to 2018 in meridional Québec, Canada, to identify what environmental conditions promote or prevent regional-scale forest transitions. We used a continuous-time multi-state Markov model to quantify the probabilities of transitions between forest states (temperate, boreal, mixed, pioneer) as a function of climate (mean temperature and climate moisture index during the growing season), soil conditions (pH and drainage) and disturbances (severity levels of natural disturbances and logging). We further investigate how different disturbance types and severities impact forests' short-term transient dynamics and long-term equilibrium using properties of Markov transition matrices. The most common transitions observed during the study period were from mixed to temperate states, as well as from pioneer to boreal forests. In our study, transitions were mainly driven by natural and anthropogenic disturbances and secondarily by climate, whereas soil characteristics exerted relatively minor constraints. While major disturbances only promoted transitions to the pioneer state, moderate disturbances increased the probability of transition from mixed to temperate states. Long-term projections of our model under the current environmental conditions indicate that moderate disturbances would promote a northward shift of the temperate forest. Moreover, disturbances reduced turnover and convergence time for all transitions, thereby accelerating forest dynamics. Contrary to our expectation, mixed to temperate transitions were not driven by temperate tree recruitment but by mortality and growth. Overall, our results suggest that moderate disturbances could catalyse rapid forest transitions and accelerate broad-scale biome shifts. K E Y W O R D S climate change, continuous-time multi-state Markov model, equilibrium, natural disturbances and logging, Québec, temperate-boreal ecotone, transient dynamics, transition probabilities | 4419 BRICE Et al. | INTRODUC TI ONGlobal climate warming has led to altitudinal and latitudinal migration of species across the globe (Chen, Hill, Ohlemüller, Roy, & Thomas, 2011;Parmesan & Yohe, 2003). In ecotones, where transition between vegetation biomes occurs, these shifts in species distributions entail far reaching consequences for forest ecosystems (Evans & Brown, 2017). In some cases, climate-induced shifts in tree species distributions might trigger a 'regime shift' (Scheffer, Carpenter, Foley, Folke, & Walker, 2001) and transform treeless tundra into boreal forests (Harsch, Hulme, McGlone, & Duncan, 2009), tropical forests into savanna (Hirota, Holmgren, Van Nes, & Scheffer, 2011) or coniferous forests into deciduous forests (Boulanger et al., 2019). As e...

Aim:In urbanized areas, exotic invasions, native extinctions, and the alteration of habitats and natural processes drive homogenization, which is a form of biotic impoverishment. This study examines whether urbanization and flooding induce homogenization of herbaceous communities in riparian forests and quantifies the relationships between taxonomic and functional β-diversity.Location: Montréal, Québec, Canada.Methods: Inventories were conducted in 56 riparian forests. Taxonomic and functional β-diversity were calculated as between-site similarities in species or trait composition for three levels of urbanization and flooding. Differences among the disturbance levels were compared using tests for homogeneity in multivariate dispersions. We quantified the correlation between local species richness, exotic proportion, taxonomic and functional β-diversity. We also partitioned taxonomic β-diversity into species turnover and richness difference.Results: Urbanization led to taxonomic and functional differentiation, while increased flooding led to taxonomic and functional homogenization. We found a significant correlation between taxonomic and functional β-diversity. Changes in β-diversity were associated with species and trait turnover among both urbanization and flood levels, and with changes in species richness. Differentiation was associated with low species richness, and homogenization with high species richness. Exotic invasions tended to favour differentiation, but only at a low urbanization level. Main Conclusions:The effect of urbanization on plant diversity in riparian forests was twofold: first, it directly induced taxonomic and functional differentiation through its effect on species loss and turnover (higher β-diversity at high urbanization level); second, differentiation was indirectly favoured through the reduction in flooding (higher β-diversity at low flood level). Taxonomic and functional β-diversity followed similar patterns, likely because species invasions and extinctions are not random, but are related to species traits. Our results underline the need to move our focus from exotic species to the true underlying factors of biodiversity loss and homogenization, notably land use changes and human disturbances. K E Y W O R D S β-diversity, exotic species, flooding, functional traits, herbaceous species, multivariate dispersion, species richness difference, species turnover | 829 BRICE Et al. Funds came from the Natural Sciences and Engineering Research Council of Canada (scholarship to MH Brice, Discovery grant to S. Pellerin: RGPIN-2014-05367 and M. Poulin RGPIN-2014-05663), the Fonds de recherche Nature et technologies and the Quebec Centre for Biodiversity Science (scholarships to MH Brice). Our thanks municipalities and landowners for site access, to A. Langlois, A. Richer, J. S. Mignot, S. Pasquet and Y. Tendland for research assistance, P. Legendre, D. Borcard, E. Laliberté and P. Peres-Neto for statistical advice, K. Grislis and C. Brown for language editing, and Z.E. Taranu and three anonymous ...