We studied the effects of annual fertilization (F) and eliminating inter-specific competition via herbicides (H) on loblolly pine productivity and growth allocation. Both treatments increased above ground growth, alone and in combination, relative to the control treatment. The primary effect seems to have been the result of a reallocation of growth above ground from roots, due especially to a reduction in fine root production. Fertilization with nitrogen had the greatest single effect, and the stands we studied are continuing to respond to annual fertilization, largely through the positive effect nitrogen is having on leaf area (and hence photosynthetic area) and canopy nitrogen (which is directly related to ability of the canopy to fix carbon). Herbicide, alone or with fertilizer, caused the greatest reduction in fine root mass and production, but the above ground effects were relatively short-lived. In the combined treatment, trees benefited from the significant reduction in roots associated with herbicide and the enhanced canopy development associated with fertilization. We have used the results of our field research to both construct a new process-based model of forest growth, and to parameterize an existing model for use in our ecosystems. The results of our field experiments and modeling efforts are helping forest managers to better understand the effects of forest management practices on forest growth, and ultimately will help improve the efficacy of such practices.
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