The mixed deciduous forests of the upper Midwest, USA are approaching an ecological threshold in which early successional dominant aspen and birch trees are reaching maturity and beginning to senesce. These aging forests are predicted to decline in carbon (C) storage; however, little empirical evidence exists to support this hypothesis. In northern lower Michigan, we are combining long-term C cycling measurements with a large-scale experimental manipulation to forecast how ecosystem C storage will change in response to ongoing succession, disturbance, and climatic variation. Our goal is to elucidate biophysical mechanisms that will constrain C storage in future forests of the upper Great Lakes. In the spring of 2008, we began the Forest Accelerated Succession ExperimenT (FASET), in which all aspen and birch (~35% canopy LAI) within 39 ha of an 85 year old forest were stem girdled to accelerate mortality. Soil respiration, the dominant component of ecosystem C loss, was the focus of the present study. We hypothesize that aspen-birch mortality will decrease C allocation to root and microbial pools as a consequence of lower gross primary productivity (GPP), temporarily reducing soil respiration (R_s). We postulate that in the short term, diminished belowground C inputs will decrease heterotrophic respiration while reducing R_s sensitivity to temperature and moisture. 

Results/Conclusions

We measured R_s continuously at FASET using treatment and control arrays of automated soil respiration chambers. We also quantified above and below-canopy radiation, precipitation, air and soil temperature, and soil moisture. At the ecosystem scale, we calculated GPP from measurements of net CO₂ exchange between forest and atmosphere using eddy-covariance methods.These data were analyzed using Granger causality theory, developed in economics, and recently applied to ecological data. Granger causality is a statistical technique for determining whether one time series is useful in forecasting another.  Prior to canopy senescence we found that at the daily timescale soil temperature and GPP both exerted a significant positive effect on R_s. At longer frequencies (> 1 day), soil temperature alone was positively correlated with soil respiration. As canopy senescence proceeds we expect a reduction in GPP and consequently R_s, with altered responses of ecosystem C loss due to changing environmental variables.