Long- term changes in American beech (Fagus grandifolia) forest metacommunities at the edge of its range

Background/Question/Methods

Baseline data from historic ecological surveys represent rare opportunities to estimate long-term dynamics of plant metacommunities. Data collected by the Plant Ecology Lab (PEL) at the University of Wisconsin over the period 1940 – 1955 have been used to document shifts understory diversity of Wisconsin oak and maple forests revealing a general trend toward decreased richness and heterogeneity in response to chronic deer browse, canopy succession, and habitat fragmentation.  Here, I use additional PEL data to assess changes in Wisconsin forests dominated by American beech (Fagus grandifolia), which represents a relatively small component of Wisconsin forests where beech reaches it's western range limit. Species at the edge of their range are generally more sensitive to shifting climate and I hypothesized that this forest type would have experienced greater magnitude of changes than did the forest types previously documented. I re-surveyed 35 such sites, using similar but more intensive methods to quantify shifts in metacommunity composition, structure and diversity. To compare vegetation responses to environmental gradients via ordination, I also collected data on soil texture and nutrients as well attributes of the surrounding landscape including land cover and habitat fragmentation.  Finally, I compiled data on the functional traits of both canopy and understory species to investigate ecological determinants of documented shifts in composition.

Results/Conclusions

American beech experienced a 15% decline in dominance in these forests canopies, although it remained relatively stable in the understory.  Unlike previous resurveys using the same baseline data set, there were no significant changes in the richness (+1.2%, p = 0.356) or heterogeneity (-4.7%, p = 0.125) of beech forest understory communities. Although diversity statistics remained relatively stable, there were significant shifts in understory composition and in how metacommunties were structured relative to environmental and landscape factors. Most significantly, landscape measures of habitat fragmentation and urbanization appear to be increasingly important as predictors of both understory diversity and community composition. Forests within fragmented or urbanized landscapes also showed greater turnover in species composition -  favoring woody species, taller herbaceous species and species with relatively thick leaves. Stands in less fragmented landscapes actually showed increases in floristic quality (+11.2%, p = 0.012) and understory richness (+17.7%, p = 0.008). Such stands also favored herbaceous species with thin, shade tolerant leaves, low fiber content and higher levels of crude protein -  perhaps reflecting recovery from the cut-over era or from historically high deer densities.