The carbon isotopic composition of soil respiration in the decade following disturbance by bark beetle or stem girdling

Recent outbreaks of mountain pine beetle have caused large-scale tree mortality in western North America, which can lead to fundamental changes in carbon cycling. When a tree is infested, the flow of photosynthate is disrupted. This causes the roots and their symbionts to die, eliminating the autotrophic component of soil respiration. Mycorrhizal fungi are enriched in 13C compared to plant tissues. As the dead fungal biomass is consumed by soil heterotrophs, the δ13C of CO2 in heterotrophic soil respiration may become more enriched as the fungal biomass is consumed. We investigated this response by measuring soil respiration in chronosequences of stem-girdled plots at the Niwot Ridge AmeriFlux site, and beetle-killed plots at the Fraser Experimental Forest, both in Colorado. Stem girdling was used to simulate beetle attack because it kills trees by a similar mechanism. Plots at Niwot Ridge included live trees and 7 years of girdled plots extending back to 2002. Plots at Fraser included live trees and three age classes of beetle-killed trees, within a similar chronosequence. We used manual soil-gas sampling at three depths, during the summers of 2011 and 2012, to determine if there is an isotopic effect associated with disturbance. Consistent with our expectations, in 2011, we found an enrichment in δ13C of approximately 1‰ in the two years following girdling which was absent in subsequent years. Although this pattern was also evident in 2012, the enrichment in δ13C during the same time period was about half that in 2011. At both Niwot and Fraser, in 2011, seasonal mean δ13C decreased by about 1‰ at all depths 3-4 years after disturbance, but returned to values close to control plots in the following 4-6 years. While we found a similar pattern at Fraser in 2012, we measured an enrichment of 1-1.5‰ at the OA interface at Niwot 8-10 years after disturbance, which was not found in 2011. It is possible this is due to the decomposition of woody biomass. At both sites and in both years, seasonal mean δ13C was enriched by about 1‰ at the OA interface compared to the 10 and 30 cm depths, which were similar. Overall, these results lend support to the hypothesis that mycorrhizal biomass is consumed in the first few years following major disturbance to their plant hosts.