Graduate Thesis Or Dissertation
 

Thinning response in Alnus rubra and Arbutus menziesii : effects of spacing, light, and moisture

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/gm80hx57v

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  • In western Oregon, hardwood trees occupy 20% of the timberland but account for less than 1% of the timber harvest. Information about how to manage them effectively is limited. The objective of this study was to examine: 1) effect of thinning on tree growth, plant moisture stress, and crown cover expansion and 2) the inter-relationships of these three factors. The study site for red alder was on the western slope of the central Oregon Coast Range. The trees were about 20 years old when thinned in 1986. The red alder experiment was designed as a completely randomized design and each treatment was replicated three times. The average stem densities per ha were 1466(100%), 535(36%), and 412(28%). The Pacific madrone study site was about 8 km west of Central Point in southwestern Oregon. Tree ages ranged from 40 to 45 years when thinning was done in 1984. Measurements continued for 5 years. The study was designed as a randomized block design and composed of three blocks. The average stem densities per ha were 2290(100%), 486(21%), and 272(12%). The data were analyzed for two groups of sample trees, average plot trees and crop trees. The average plot trees were random samples of each plot. The crop trees were dominant trees that would be left to harvest at the end of the rotation. The effect of thinning on soil moisture availability was evaluated by measuring predawn plant moisture stress (PMS) with a pressure chamber once in a summer. To determine the effect of thinning on light availability, five fisheye photographs of crown cover were taken per each plot and analyzed using an automated fisheye photograph analysis system. For tree basal area, red alder crop trees in the heavily thinned plot increased 66% (13.1 cm²) more than those in the control stand but average plot trees did not grow more. Tree growth of red alder was negatively correlated with stem density, but was not significantly correlated with plant moisture stress. For Pacific madrone, average plot trees in the heavily thinned stand increased 286% more than those in the control stand, while crop trees increased 589% (13.2 cm²) more. Tree growth of Pacific madrone was significantly correlated not only with stem density but also with plant moisture stress. However, it was not significantly correlated with crown cover. Plant moisture stress might be a reason for marked effects of thinning on Pacific madrone tree growth. Total height and merchantable height growth were not significantly different among the treatments for both species. For red alder, however, height growth of the heavily thinned plot was significantly less than that of the control plot. Total height and merchantable height growth were not significantly correlated with stem density or plant moisture stress. There were no significant differences in plant moisture stress among the treatments for red alder. For Pacific madrone, however, plant moisture stresses of the thinned plots were 37-42% lower than those of the control plot in the third and fifth year after thinning. The increases of stand basal area and volume were not significantly different among the treatments for both species. If the management objective is to increase growth of crop trees without losing stand productivity, it is recommended that the stands should be managed at a low density range (stand density index 300-360 for red alder, 200-290 for Pacific madrone).
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