Title: Belowground Carbon Pools and Fluxes in a Northern Temperate Deciduous Forest and their Response to Stand Disturbance
Description: Understanding the processes that govern terrestrial carbon fluxes and sequestration are fundamental to improving our understanding of climate feedbacks and ecosystem biogeochemistry. Biotic disturbances such as pest or pathogen attacks can have a large impact on forest carbon storage yet our knowledge of how these perturbations impact forest carbon cycling is limited. The goals of this dissertation were to gain insights into the processes governing soil respiration at Black Rock Forest (southeastern NY, USA) by quantifying the relative contributions of autotrophic and heterotrophic activity to soil respiration, to assess the short-term impact of mimicking a pathogen attack on soil carbon pools and fluxes, and to develop the first soil carbon budget for Black Rock Forest. These goals were addressed by utilizing a large-scale manipulative experiment, which induced tree mortality through girdling. Trees on twelve plots (75m by 75m) were girdled according to four treatments: girdling all oaks, girdling half of the oaks, girdling all non-oaks, and a control.

Additionally, one circular plot was created where all trees were girdled. Soil respiration was measured before the girdling and for three years afterwards. Forest floor litter and soil organic carbon at five depth intervals were measured on all plots two years after girdling. The results from the first year of the experiment provided an initial estimate of 50% for the autotrophic component of soil respiration but continued declines in soil respiration rate into the second year provided a more accurate estimate of 58 %. Rapid declines in soil CO2 flux from the fully girdled plot (37%) and from the oaks girdled treatment (33%) within two weeks following girdling demonstrate a fast turnover of recently fixed carbon. The three-year time series of respiration measurements provided insights into the short-term impact of mimicking a pathogen attack on soil carbon fluxes. Respiratory rates on plots where all oaks, half of the oaks, and all trees were girdled declined for two years following treatment before attaining a full recovery of belowground activity in the third year.

Soil respiration from the non-oak girdled treatment was similar to control for the duration of the study. The short-lived respiratory response on plots where all oaks, half of the oaks, and all trees were girdled suggests that belowground activity is highly resilient to disturbance. It also aligns with reported recovery patterns of net ecosystem production after a pest or pathogen attack. Overall, the reduction in soil respiration was not proportional to the degree of canopy loss, the magnitude of the respiratory response varied interannually, and was specific to the plant taxon impacted. No changes across treatments in soil organic carbon storage were observed two years after the mimicked attack. These findings do not support a recent hypothesis that suggests disturbance should reduce soil carbon pools (Peltzer et al. 2010). Instead, it is proposed that shifts in the composition of carbon substrates within the belowground carbon pool occurred and that the changes may offset each other.

This could result in a similar quantity of soil carbon storage between the disturbed and undisturbed forest stands. The first estimate for soil carbon storage (to 30cm depth) at Black Rock Forest is 3.0 ± 0.5 kg C m-2, which is 32% of the aboveground carbon storage. Together, the findings from this dissertation contribute to the limited knowledge of respiratory partitioning and of short-term impacts on soil carbon storage and fluxes following a partial stand disturbance in northeastern deciduous forests.