COS 83-5 - Forest type alters the trajectory of short-term carbon dynamics in temperate hardwood forests subjected to deer herbivory and insect invasions

Background/Question/Methods

Temperate forests play an important role in global climate change, storing a significant amount of carbon in both aboveground and belowground pools. While the direct effects of white-tailed deer (Odocoileus virginianus) on vegetation as well as the effects of vegetation on decomposers have been intensively studied, less is known about the indirect effects of herbivory on lower trophic levels such as soil microbes and their processing of soil carbon. It is also unclear if these effects are modulated by other alterations of the watershed such as recovery from acid rain or insect invasions including the hemlock woolly adelgid (Adelges tsugae). Our goal is to elucidate the effects of deer herbivory on soil carbon processing in oak-maple and hemlock-dominated stands upslope of glacial lakes that are recovering from acid rain. We measured soil carbon pools and fluxes (e.g., CO₂ and CH₄ fluxes, microbial carbon substrate use, control litter decomposition, and soil organic matter) in spatially distributed plots in two 0.65 ha 25+-year-old deer exclosures and 30 plots across the Lake Lawacac watershed in the Glacial Poconos High Plateau, Pennsylvania, USA.

Results/Conclusions

We found that the effects of white-tailed deer varied based on forest type. In an oak-maple stand, the presence of deer herbivory led to lower understory cover, lower decomposition rates of labile control litter, and lower CO₂ fluxes. The opposite trend was found in a hemlock stand; in this stand, there was a more open canopy, higher understory cover, higher decomposition rates, and higher CO₂ fluxes in the presence of deer herbivory. The higher understory cover was due primarily to hay-scented fern (Dennstaedtia punctilobula), a native invasive species that can reduce seedling regeneration. Thus, deer may contribute indirectly to the creation of canopy gaps in hemlock stands. Across forest types, soil organic matter was consistently higher in our stands without deer but also exhibited high fine-scale spatial variation. Soil organic matter was tightly coupled with soil moisture. It was also a strong predictor of microbial functional group structure. Stands with deer herbivory differed in their microbial functional group structure between oak-maple and hemlock stands. While more work is needed, our findings suggest that interactions between environmental stressors can lead to shifts in the trajectory of short-term carbon dynamics in these temperate forests.