|ST.LOUIS, David - MISSISSIPPI STATE UNIV|
Submitted to: Agroforestry Systems
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 14, 2001
Publication Date: September 1, 2003
Citation: ARES, A., ST.LOUIS, D.G., BRAUER, D.K. 2003. TRENDS IN TREE GROWTH AND UNDERSTORY YIELD IN SILVOPASTORAL PRACTICES WITH SOUTHERN PINES. AGROFORESTRY SYSTEMS. 9:27-33. Interpretive Summary: Agroforestry is a new land use practice in the temperate United States. An agroforestry estate model has been adapted from a farm level simulator used in New Zealand. New Zealand agroforestry systems are rather simple involving pine and perennial ryegrass for sheep grazing. Agroforestry systems in the United States will encompass a much greater variety of trees and understory crops. Currently, the agroforestry estate model uses tree growth data collected from trees growing in native stands. The objective of this study was to determine if the output from the agroforestry estate model based on tree data from native stands closely matches that found in silvopastoral systems with pines in the southeastern United States. The results indicate that height-diameter relationships of trees and the dependancy of tree height on tree spacing and understory type are different in agroforestry systems than what is predicted by the estate model. These results suggest that tree growth data from agroforestry systems will need to be incorporated into the agroforestry estate model to accurately predict economic returns. These results are of interest to natural resource professionals and landowners considering agroforestry practices.
Technical Abstract: Farm-level agroforestry simulations such as the Agroforestry Estate Model (AEM) use as yield inputs either tables developed for a particular system or simulations from forest modeling tools. Specific tables with data for relatively long periods are often unavailable for temperate agroforestry systems and predictive functions are needed. Forest models, on the other hand, rely upon a set of assumptions related to site index (i.e., dominant height at a base age), stem diameter (DBH) and wood product distribution, wood production and tree mortality, which may not apply to agroforestry systems. Differences may arise because of the effects on tree growth of different spacings and configurations, the understory component and fertilizer and grazing regimes. We examined data from published or ongoing field trials to determine trends in tree growth and understory yields in silvopastoral systems with southern pines (P. taeda, P. elliotti) in the United States. Tree DBH and height were generally higher in systems with improved pastures than in those with volunteer grasses. Effects of understory characteristics were more marked on DBH than on height. DBH-height relationships differed among systems. Sigmoidal models predicted that tree height will peak at different age depending on tree spacing. These changes may affect the accuracy and applicability to agroforestry systems of site indices and wood yield predictions generated by forest models.