How drivers of the forest understory shift between biotic and abiotic controls based on deer browsing and oak mortality

Background/Question/Methods

Disturbances are ubiquitous in forest ecosystems and often modulate ecosystem development and nutrient processes. In Northeastern forests, Sudden Oak Death and large-herbivore populations, are altering forest communities through large-scale die-off of dominant canopy tree species and browsing preferences. In this experiment, we aim to understand how multiple disturbance types impact biotic and abiotic controls over forest regeneration. Large-scale pathogens like Sudden Oak Death can alter abiotic factors such as light and nutrient availability through canopy tree die off, causing cascading effects on understory growth and species composition by decreasing competition for light availability and soil resources. Intensity and preferences of large herbivore browsing could mediate the abiotic effects of canopy mortality on the understory. To assess these effects, we collected data on abiotic conditions and understory vegetation in plots where all canopy oak trees were girdled, 50% of oaks were girdled, non-oak trees were girdled, or no girdling occurred. Within each plot, we sampled inside and outside deer exclosures. Through assessing the environmental conditions and vegetation characteristics of forest plots that were exposed to simulated pathogen die-off, deer browsing, or both we addressed how multiple interacting disturbance types influence the environmental conditions and vegetation responses of the forest understory.

Results/Conclusions

A suite of abiotic factors such as soil pH, soil moisture, and canopy openness were significantly influenced by tree girdling. There was higher soil pH in oak-girdled plots vs. non-oak girdled plots. The mortality of oak trees can increase soil pH through the decrease of tannic acids present in oak’s foliage. Soil moisture was significantly influenced by girdling treatment, and was highest in oak and non-oak girdled plots. This suggests that quantity of canopy mortality influences soil moisture more than species-specific mortality. Canopy openness was significantly influenced by girdling, but the effect was not strong. Girdling and exclosure treatments significantly influenced plant community diversity. Species diversity was greatest in oak-girdled plots followed by non-oaks. These results suggest that abiotic and biotic factors play a role in mediating species community diversity. The total foliage cover of understory species was significantly influenced by girdling treatments, suggesting that abiotic factors exert more of an influence over foliage than biotic factors. This could be attributed to a decrease in nutrient competition between species in girdled plots, resulting in more energy for above-ground growth. This research will provide valuable information for forest managers trying to plan for the increasing risk of multiple simultaneous disturbance regimes.