50-years of Change in Forest Tree Species Diversity and Composition in Wisconsin: The Role of Native and Exotic Generalists

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Many concerned with conserving forests emphasize retaining blocks of forest habitat. Often within intact forests, however, some tree species dramatically decline in abundance because their seedlings fail to recruit into adults. Such losses make clear that to protect forest diversity, we must identify patterns of forest change, even if patterns take decades to detect. Here, we explore 50-years of forest community change in WI. Our goals are to examine changes in woody species diversity and abundance, and to test the hypotheses: (1) Some specialist or sensitive species (e.g.,Quercus and Taxus) have declined in abundance while a few generalist species (e.g.,Acer rubrum) have increased in abundance, and (2) declines in woody species richness are associated with increases in weedy species abundance. In 1946-1956, John Curtis and colleagues censused the vegetation at 1,000 terrestrial sites in WI and we recensused 240 of these sites in 2000-2005 using more intense sampling. Species presence/absence data were collected in the understory (2; 1950s), and in 60-120 quadrats (2000s). Canopy tree data were collected using a point quarter, random pairs, or fixed area plots at each site. We adjusted for sampling differences in our analyses. Site-level woody species richness declined by 2.23±0.52 species (mean±SE; P< 0.005; N =60). This loss was due to declines in tree seedling richness (P0.5; N=48) and vines were too rare to matter (1 or 2 occurrences at 5/60 sites). Interestingly, at the quadrat scale, richness increased (0.35±0.07 species/quadrat; P< 0.005; N=60). This result suggests that quadrat-level increases may be due to local increases in weedy species abundance, which contributed to site-level declines of rare species. Together with other results, we find that forests homogenization was due to increases in native and exotic weedy species and decreases in endemic species.


Presented at the 91st Ecological Society of America Annual Meeting, Memphis, TN.

COS 55 - Forest Ecology III: Historical and Environmental Changes.

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