Stand-scale correspondence in empirical and simulated labile carbohydratesin loblolly pine

As investment into intensive forestry increases, the potential trade-offs b etween productivity and sustainability should be scrutinized. Because of th eir important role in internal carbon (C) budgets, labile C pools may provi de a measure of the potential ability of trees and stands to respond to str ess. We modified the process model BIOMASS to examine daily C budgets of mi drotation nonfertilized and fertilized loblolly pine stands. We tested whet her the absolute difference between daily simulated net canopy assimilation (GPP minus maintenance respiration) and our empirical estimates of product ion, or daily gross carbon balance, mimics the labile carbohydrate C pool. We compared this labile pool surrogate to independent, empirical analyses o f total nonstructural (starch and soluble sugars) carbohydrates from an ind ividual whole-tree analysis scaled to the stand level. Of particular interest, the simulated daily gross C balance indicated perio ds of carbon deficit during the growing season that lasted from 1 to 40 day s. Simulated daily net C balance was met from labile C storage during these periods. Fertilized plots had similar time-period trends as the control pl ots, but exhibited a twofold increase in C assimilation and use, Simulated and empirical estimates of the labile carbohydrate pools displayed similar seasonal trends, although their correspondence depended on the time of year . Simulations indicated a winter/early spring "recharge" period; concentrat ions peaked at similar to 50 and similar to 60 mg C g biomass-l in control and fertilized plots, respectively, in 1995. The overall correlation betwee n predicted and empirical estimates was low to moderate (r = 0.51). The bes t agreement was with the empirical data from April through June as concentr ations declined; however, predicted minimum concentrations (15 and 5 mg C g biomass-l in control and fertilized plots, respectively) were lower, and o btained earlier in the year than the empirical data (similar to 20 mg C mg biomass(-1)). These analyses quantify the strong extent that loblolly pine exhibits a buffered capacity to balance the C budget when current photosynt hesis occasionally cannot meet daily C requirements. Further development of our approach may lead to a tool for analyzing potential risks associated w ith intensive forest management.