PS 40-17 - Impacts of oak wilt on temperate oak forests

Background/Question/Methods

Forest diseases reduce plant fitness and kill trees, causing downstream effects on ecological communities. Forest pathology typically focuses on pathogens as destructive agents; however, some theory suggests that disease could be a diversifying agent. To explore this gap in knowledge, we investigated how oak wilt, a non-native disease caused by a fungal pathogen, influences forest succession and plant diversity though its creation of canopy gaps. As the range of oak wilt continues to spread, we must understand how this disease and its management are changing forest diversity and impacting regeneration of oak, a keystone species in many forests. Our study characterizes the impacts of disease and various management strategies on plant communities within oak wilt gaps. We surveyed four types of gaps: 1) unmanaged disease, 2) removal of diseased trees, 3) stem-girdle treatment of diseased trees, and 4) healthy stands. We conducted vegetation surveys along transects spanning from the middle of the infection center to beyond the gap edge in each of the four cardinal directions. Along each transect, we recorded seedling and herbaceous cover (1-m² plots) as well as sapling and shrub density (1/1000-ha plots).

Results/Conclusions

Compared to healthy stands, gaps with unmanaged disease or where diseased trees were removed had higher mean species density of seedling (66 and 39% increase, respectively) and saplings (29 and 47% increase, respectively). In general, the composition of seedlings and saplings in managed and unmanaged gaps appeared to shift away from abundant red oak species towards more shade tolerant species. Herbaceous species cover increased by at least 50% in unmanaged gaps and gaps where diseased trees were removed compared to healthy, disease-free stands. These preliminary results suggest that certain tree diseases and their management can trigger increases in species diversity and change successional trajectories. Red oak species may fail to reach to pre-disturbance levels without further intervention, indicating that disease may increase overall biodiversity at the expense of a keystone species. Our study demonstrates the complex ecological impacts of disease, and the tradeoffs between managing for a keystone species versus overall forest diversity.