Predicted Effects of Prescribed Burning and Timber Management on Forest Recovery and Sustainability at Fort Benning, Georgia

The objective of this work was to use a simple compartment model of soil carbon (C) and nitrogen (N) dynamics to predict forest recovery on degraded soils and forest sustainability, following recovery, under different regimes of prescribed fire and timber management. This report describes the model and a model-based analysis of the effect of prescribed burning and forest thinning or clearcutting on stand recovery and sustainability at Fort Benning, GA. I developed the model using Stella{reg_sign} Research Software (High Performance Systems, Inc., Hanover, NH) and parameterized the model using data from field studies at Fort Benning, literature sources, and parameter fitting. The model included (1) a tree biomass submodel that predicted aboveground and belowground tree biomass, (2) a litter production submodel that predicted the dynamics of herbaceous aboveground and belowground biomass, (3) a soil C and N submodel that predicted soil C and N stocks (to a 30 cm soil depth) and net soil N mineralization, and (4) an excess N submodel that calculated the difference between predicted plant N demands and soil N supplies. There was a modeled feedback from potential excess N (PEN) to tree growth such that forest growth was limited under conditions of N deficiency. Twomore » experiments were performed for the model-based analysis. In the first experiment, forest recovery from barren soils was predicted for 100 years with or without prescribed burning and with or without timber management by thinning or clearcutting. In the second experiment, simulations began with 100 years of predicted forest growth in the absence of fire or harvesting, and sustainability was predicted for a further 100 years either with or without prescribed burning and with or without forest management. Four performance variables (aboveground tree biomass, soil C stocks, soil N stocks, and PEN) were used to evaluate the predicted effects of timber harvesting and prescribed burning on forest recovery and sustainability. Predictions of forest recovery and sustainability were directly affected by how prescribed fire affected PEN. Prescribed fire impacted soil N supplies by lowering predicted soil C and N stocks which reduced the soil N pool that contributed to the predicted annual flux of net soil N mineralization. On soils with inherently high N availability, increasing the fire frequency in combination with stand thinning or clearcutting had little effect on predictions of forest recovery and sustainability. However, experiments with the model indicated that combined effects of stand thinning (or clearcutting) and frequent prescribed burning could have adverse effects on forest recovery and sustainability when N availability was just at the point of limiting forest growth. Model predictions indicated that prescribed burning with a 3-year return interval would decrease soil C and N stocks but not adversely affect forest recovery from barren soils or sustainability following ecosystem recovery. On soils with inherently low N availability, prescribed burning with a 2-year return interval depressed predicted soil C and N stocks to the point where soil N deficiencies prevented forest recovery as well as forest sustainability following recovery.« less