Characterizing plant functional trait profiles to infer drivers of ecological change in Wisconsin forest plant communities

Background/Question/Methods

Traditional studies of plant communities track shifts in species composition over space or time in response to differences in local conditions and/or disturbance. Plant species and communities, however, respond to factors acting at broader spatial scales including differences in herbivory, invasions of invasive species, nitrogen deposition, landscape fragmentation, and climate change. To disentangle how these extrinsic factors drive ecological change, we sought to predict how each is likely to affect species with particular functional traits. We track shifts in species and associated trait profiles among >300 forested sites dispersed across Wisconsin surveyed in both the 1950s and the 2000s. Using a species-based approach, we ask which traits are associated with increases or decreases in abundance and site occupancy. At the community level, we ask how sites are changing in trait means and variances. We are also analyzing shifts in trait profiles across communities to determine whether these forest communities are collectively converging, diverging, or moving in a consistent direction in trait space. Finally, we use observed shifts in community functional trait profiles to infer the factors that may be driving shifts in community composition. 

Results/Conclusions

Differences in functional traits among species are associated with their population trends within and among sites. Growth form, specific leaf area (SLA), leaf nitrogen, and modes of dispersal are related to shifts in species abundance or range. Across the fragmented landscapes of southern Wisconsin, rarer species with limited dispersal and higher habitat specificity are declining suggesting failures to recolonize following local extinction. Associations between herbaceous SLA and soil and canopy conditions have weakened here since the 1950s. In more continuous northern forests, plants with more leaf protein have declined in apparent response to deer herbivory. Functional beta diversity does not (yet) appear to be declining among sites despite declines in species beta diversity. Although some general trends occur across communities, these vary in extent and direction among regions and community types suggesting variation in the factors driving ecological change. Understanding the significance of particular traits and drivers in different ecological contexts will improve our ability to predict ecological change and manage these forests wisely.