Pruning young Douglas-fir (Pseudotsuga menziesii [mirb.] Franco) in a western Oregon agroforestry setting : changes in tree water relations and effects on forage production

Graduation date: 1995There is renewed interest in agroforestry as a management strategy for marginal agricultural lands in western Oregon. Silvopastoral systems combine tree and\ud forage production, which involve crops and practices familiar to the area. The objective of this study was to determine how\ud management influences the physiology and ecological interactions of two silvopastoral crop components. Changes in water relations and growth of 9 to 12 year-old Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) trees\ud were examined in response to pruning and pasture competition. Four buffered nine-tree treatment plots were installed at each of the four western Oregon sites, in a\ud randomized complete block design, each site was a block. Approximately 50% of the crown length was removed in the pruning treatment. The unimproved pasture community was either retained or removed chemically. Forage production beneath pruned and unpruned trees was measured and modeled. With no understory competition, Pruned trees showed consistently lower moisture stress than Unpruned trees. The differences were generally small and were significant only twice in 1992, and once in 1993. With pasture\ud competition, pruning appeared to have the opposite effect, and Pruned trees had more negative pressure potentials than Unpruned trees once in 1992, and three times in 1993.\ud No differences were found between trees growing with and without pasture when the trees were unpruned. Both pruning and pasture removal treatments had an influence on tree growth. Pruning appeared to have a\ud stronger effect on the radial growth than pasture competition. The growth rate of Pruned trees with Pasture was lower than all others. The regression models of forage production in the Unpruned treatments explained more variability each year (69% and 73%) than did the models for the Pruned\ud treatments (60% and 45%). The models for the Unpruned treatments explained similar amounts of the total variability in both years although the models were quite\ud different. The amount of variability explained by models fitted to the pruned treatments declined from the first to\ud the second year. The results of this study are consistent with the general notion that removing foliage from trees will reduce water use and tree growth. It also appears that forage communities continue compete with trees for moisture. The measured trends were, however, small and of little concern from a management perspective. Thus, pruning to increase wood value and maintain forage\ud production appears to have few negative effects. Pruning should be undertaken when managers feel that the expense will be paid for by increased wood value