Using Tree Rings to Study the Effects of Invasive Earthworms on Tree Growth

Long-known to be beneficial species in agricultural ecosystems, it has only recently been realized that the invasion of European earthworms into previously earthworm-free hardwood forests of the Great Lakes are profoundly influencing nutrient cycles, understory plant diversity, and the patterns of spread among invasive plant species. The effects of invasive earthworms on overstory forest dynamics are less certain, however, because trees grow and live to ages that are measured in decades or centuries and little is known about the timing of past earthworm invasions. Our research will include the identification of an active earthworm invasion front in the Chequamegon National Forest of northern Wisconsin, sampling of earthworms to determine population levels and species diversity across the invasion front, and collection of tree-ring samples to determine how growth rates and climate-tree growth relationships change in sugar maple trees following earthworm invasions. Our research will help establish a method that can be applied to other invasion fronts in order to better understand the long-term influences of earthworms on forest growth and health, and will inform our understanding of how invaded forests may respond differently to future climate change than those forests without earthworms. This information will be important for developing adaptive management strategies to ensure healthy forests in the future.