Environmental change shapes understory plant diversity and dominance in boreal forests.

Ongoing environmental change threatens ecosystems worldwide, yet little is known about its effect on understory plant diversity, which underpins ecosystem functioning and sustainability. Here, we use Canada's National Forest Inventory database to evaluate decade-long changes in local plant diversity within understory communities. Species richness of shrubs and bryophytes increases by 8 and 11% per decade, while species evenness of herbs and bryophytes declines by 14 and 8%, respectively. Temporal increases in species richness and declines in species evenness are both associated with rising temperature, nitrogen deposition, water availability, and increased temperature seasonality. Additionally, the proportion of bryophyte biomass increases, whereas that of shrub biomass decreases over time, with the effects of temperature seasonality and water availability on these temporal shifts strongly dependent on overstory basal area. Species richness is positively associated with biomass across shrubs, herbs, and bryophytes, suggesting that changes in diversity alter understory biomass distribution under environmental changes. Contrary to the common view that climate warming uniformly reduces biodiversity, our findings show that understory communities undergo complex and dynamic shifts in plant diversity and composition. We suggest that environmental change-driven shifts in resource availability and heterogeneity may shape understory composition and species dominance, ultimately influencing forest ecosystem function.