Modeling the effects of forest management on in situ and ex situ longleaf pine forest carbon stocks
Assessment of forest carbon storage dynamics requires a variety of techniques including simulation models. We developed a hybrid model to assess the effects of silvicultural management systems on carbon (C) budgets in longleaf pine (Pinus palustris Mill.) plantations in the southeastern U.S. To simulate in situ C pools, the model integrates a growth and yield model with species-specific allometric and biometric equations and explicitly accounts for the impacts of both thinning and prescribed fire. To estimate the ex situ C pool, the model used the outputs of merchantable products from the growth and yield model with current values of forest product conversion efficiencies and forest product decay rates. The model also accounts for C emissions due to transportation and silvicultural activities. Site productivity (site quality) was the major factor controlling stand C density followed by rotation length. Thinning reduced C sequestration, as the slow growth rate of longleaf pine reduced the potential of C sequestration in forest products. Prescribed burning reduced average C stock by about 16–19%, with the majority of the reduction in the forest floor. In a comparison of longleaf pine C dynamics with slash pine (Pinus elliottii Engelm.), both species reached a similar average C stock at age 75 years, but when averaged across the whole rotation, slash pine sequestered more C. Nevertheless, for medium quality sites, C sequestration was similar between thinned 75-year rotation longleaf pine and unthinned 25-year rotation slash pine. This longleaf pine plantation C sequestration model, based on empirical and biological relationships, provides an important new tool for developing testable research hypotheses, estimating C stocks for regional assessments or C credit verification, and for guiding future longleaf pine management.