We followed how forest thinning, repeated twice during a period of 93 years, altered understorey plant community composition, affected the succession of forest understorey vegetation and the accumulation of logs in the long-term. The study was carried out in northern Finland by resampling 20 permanent experimental plots, established after wildfire in 1920. Understorey vegetation was inventoried in 1961, 1986 and 2013 with forest thinning treatments done in 1953 and 1987, using four and three different harvesting intensities, respectively. We found succession to override the effects of forest logging until the latest study period (2013). We observed negligible long-term effects of logging on understorey communities during the two mid-successional stages (1961, 1986), when the forest was 41 and 66 years old, respectively. The impacts of logging on understorey vegetation were strongest in the latest successional stage (2013), the forest being at the age of 93 years. In the latest successional stage (2013) logged plots had less coarse woody debris than unlogged plots. Forest management thus influenced the key feature for forest biodiversity and potential habitats for endangered species. These findings are of major interest since the studies of long-term impacts of less intensive forest management practices are scarce. Our results suggest that in addition to possible immediate impacts, harvesting treatments have legacy effects (subtle or delayed inherited effects of forestry in the past) that influence the forest understorey vegetation community composition and the amount of coarse woody debris. This finding deserves special attention when planning species conservation, multiple use of forests and sustainable forestry.

Aim Land use is the foremost cause of global biodiversity decline, but species do not respond equally to land‐use practices. Instead, it is suggested that responses vary with species traits, but long‐term data on the trait‐mediated effects of land use on communities are scarce. Here we study how forest understorey communities have been affected by two land‐use practices during 4–5 decades, and whether changes in plant diversity are related to changes in functional composition. Location Finland. Time period 1968–2019. Major taxa studied Vascular plants. Methods We resurveyed 245 vegetation plots in boreal herb‐rich forest understories, and used hierarchical Bayesian linear models to relate changes in diversity, species composition, average plant size, and leaf economic traits to reindeer abundance, forest management intensity, and changes in climate, canopy cover and composition. We also studied the relationship between species evenness and plant size across both space and time. Results Intensively managed forests decreased in species richness and had increased turnover, but management did not affect functional composition. Increased reindeer densities corresponded with increased leaf dry matter content, evenness and diversity, and decreased height and specific leaf area. Successional development in the canopy was associated with increased specific leaf area and decreased leaf dry matter content and height in the understorey over the study period. Effects of reindeer abundance and canopy density on diversity were partially mediated by vegetation height, which had a negative relationship with evenness across both space and time. Observed changes in climate had no discernible effect on any variable. Main conclusions Functional traits are useful in connecting vegetation changes to the mechanisms that drive them, and provide unique information compared to turnover and diversity metrics. These trait‐dependent selection effects could inform which species benefit and which suffer from land‐use changes and explain observed biodiversity changes under global change.

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.