Northern Upland Forests
Initial resurveys carried out by Don Waller and Shannon Wiegmann focused on mesic northern forests in the early 2000's. They found that most understory plant communities have experienced substantial biotic impoverishment over the past 50 years. The average site lost 18.5% of its original native flora, with native species density declining at >70% of resurveyed sites.
Species that increased at northern sites tend to be abiotically pollinated and dispersed, and many are clonal or exotic. In particular, graminoid species (grasses and sedges) were far more likely to increase over the past 50 years than other plant groups.
Drivers of Change
The changes occurring in northern mesic forests appear unrelated to succession. The loss of uncommon native species and the gain of exotic species are driving biotic homogenization in these forests. Fire suppression, the loss of animal mutualists, and the invasion of European earthworms into northern forests are all likely contributing to the changes that have been observed. However, a major factor simplifying forest understories is likely deer herbivory. Deer densities average 3-5 times pre-settlement values; deer depend on understory vegetation, particularly herbaceous species, for most of their diet.
Around 30 xeric forest sites were resurveyed in 2005. The Waller lab is currently using these data to compare 50-year plant community changes along the mesic-xeric continuum. In addition the lab is expanding the suite of plant traits that have been quantified on important northern species. We plan to use these data to further elucidate how plant traits interact with land-use change and deer herbivory to result in plant community changes.