| Knowledge of ecosystem carbon (C) dynamics is critical in predicting the response of ecosystems to climate change and how these responses will feedback and influence net ecosystem C losses or gains. As much as 80% of grassland plant production is belowground; therefore C allocation to roots can be the single most important component of grassland plant community C dynamics. Yet, this is the least understood component of C dynamics in ecosystems. In these studies, I have focused my research efforts on determining the plant, soil, and consumer controls on C allocation to rates of root respiration. Field experiments incorporated analysis of spatiotemporal variation of root respiration in grasslands of Yellowstone National Park (YNP). I also measured other ecosystem C fluxes such as rates of assimilation and soil respiration, which allowed for the examination of the proportion of assimilated C respired by roots and the contribution of root respiration to soil respiration. Results revealed that the greatest proportion of the observed variation in root respiration were associated with green biomass and shoot N, indices of photosynthate supply, and direct measures of C assimilation. Simulated grazing by clipping reduced the proportional allocation of assimilated C to root respiration. In addition, simulated grazing reduced root respiration and had no effect on soil respiration, thus, by subtraction, stimulated microbial activity. These studies indicate that although root respiration in YNP was associated with a combination of plant, soil, and consumer factors, the spatiotemporal variation in grassland root respiration was predominantly determined by patterns of plant C assimilation. Thus, changes in climate or herbivore populations that affect assimilate supply have the potential to alter CO2 efflux from root systems and C available to the heterotrophic food web. In addition, findings suggest that defoliation by herbivores has the potential to reduce the proportional allocation of assimilated C to root respiration allowing for a greater proportion of assimilated C available for other plant processes, e.g. growth. Observed increases in the proportion of C available for growth and microbial activity may be two mechanisms in which grassland plant communities compensate lost shoot tissue due to herbivory. |
