Trait-based responses to forestry and grazing modify long-term changes in biodiversity hot-spots of boreal forests

The current biodiversity decline is primarily caused by human land use. Boreal forests have been managed for a long time, but the long-term effects of this management on boreal forest understories remain unclear. Changes apart from trends in species richness are especially poorly understood. To increase understanding about the effects of common land-use practices in boreal forests we resurveyed 245 vegetation plots in boreal herb-rich forest understories, originally sampled in 1968-1975, and investigated the effects of forest management and semi-domesticated reindeer herding on changes in seven community-level metrics: species richness, Shannon diversity, species evenness, vegetation height, leaf dry matter content (LDMC), specific leaf area (SLA), and temporal turnover. Changes in species evenness and Shannon diversity correlated negatively with shifts in vegetation height, resulting in increased diversity inside the reindeer herding area. Canopies in managed forests had higher cover in the long-term, which correlated with higher SLA and lower LDMC of the understory vegetation. Forest management intensity also correlated negatively with understory species richness trends. Compositional turnover was higher in managed forests, and lower inside the reindeer herding area. The apparent stability of understory species richness, SLA and height at the scale of the entire study area resulted from opposing trends in different parts of the study area cancelling each other out when viewed at a larger scale. We conclude that even though the long-term effects of human land use on plant communities can be complex, complementing approaches based on species richness and dissimilarity metrics with functional trait-based perspectives has the potential to unearth mechanistic explanations for observed patterns, such as livestock grazing selectively affecting plants based on their height, and forest management filtering species based on their light-interception traits.