Abstract
AbstractAimLand 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.LocationFinland.Time period1968–2019.Major taxa studiedVascular plants.MethodsWe 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.ResultsIntensively 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 conclusionsFunctional 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.
Highlights
There is an urgent need to develop better tools for monitoring biodiversity change, as the most commonly used metrics, species richness and compositional turnover, do not capture its full extent (Blowes et al, 2019; Hillebrand et al, 2018)
When external drivers such as land-use change affect the optimal allocation of resources to vertical growth and to leaves, they are expected to exert directional selection pressure on the composition of plant communities, which should manifest as changes in average functional traits
Our results highlight the way changes in average traits can be informative of changing selection pressures on vegetation, as most observed changes in functional composition were directly linked to land-use and canopy changes in ways that are consistent with theory
Summary
There is an urgent need to develop better tools for monitoring biodiversity change, as the most commonly used metrics, species richness and compositional turnover, do not capture its full extent (Blowes et al, 2019; Hillebrand et al, 2018). Changes in average functional traits can be directly informative of changing selection pressures on communities and resulting ecosystem consequences (Lavorel & Garnier, 2002; Violle et al, 2007). Plant size is often described with vegetative height, and the LES with traits such as specific leaf area (SLA) and leaf dry matter content (LDMC) (van der Plas et al, 2020). When external drivers such as land-use change affect the optimal allocation of resources to vertical growth and to leaves, they are expected to exert directional selection pressure on the composition of plant communities, which should manifest as changes in average functional traits
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