The effect of variable snowpack and tree girdling on soil carbon cycling in a subalpine forest

Background/Question/Methods Snowpack depth and the timing of spring melt are fundamental drivers of carbon uptake in western high elevation forests. Previous analyses have shown that earlier springs and longer growing seasons, correlated with lower snowpack levels and earlier melts, result in less annual CO₂ uptake for the Niwot Ridge AmeriFlux subalpine forest. However, the role of soil respiration in this climate- carbon coupling remains unclear. To explore the dynamics of changing snowpack on soil respiration we amended and removed snow on experimental plots to 125% and 75% of control plot depth averages, respectively. We also girdled the lodgepole pines (Pinus contorta) present on a subset of plots to isolate potential snowpack effects on the autotrophic and heterotrophic components of soil respiration. Over the 2008 growing season, we measured plot soil CO₂ efflux, extractable soil carbon and microbial biomass carbon, and the ¹³C/¹²C of soil respired CO₂. In addition, we collected related environmental variables including xylem sap flux, soil temperature, and soil moisture. Results/Conclusions Our results indicate that although girdling produced significant decreases in soil CO₂ efflux, extractable soil carbon, and microbial biomass carbon over the growing season, the snow treatments had no effect when not accounting for environmental covariates. In contrast, we found that snow amendments to plots with non-girdled trees produced a relatively more depleted d¹³C signature of soil respiration, and that this signal persisted through the growing season as compared to non-girdled control plots and plots where snow was removed. Also, girdling plot trees produced a more enriched d¹³C signature in soil respiration as compared plots with non-girdled trees. Our findings demonstrate that changes in snow pack depth and timing of melt can be detected by changes in the autotrophic component of soil respiration before changes in overall respiration rate.