Graduate Thesis Or Dissertation
 

The use of a three-stage sample to estimate the effect of commercial thinning on the growth of two stands of Douglas-fir

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  • A three stage sale was taken of two high-site, second growth Doug1as-fir (Pseudtsuga menziesii) stands located in the extreme northwestern portion of the Oregon Coast Range. Permanent plots were established during the first stage variable-plot cruise. A Hartley- List sample, taken from the first stage Douglas-fir in trees, selected second stage sample trees in proportion to their estimated total height. A subsequent equal probability systematic sample was taken to choose the third stage sample trees. First stage variable-plot cruise data provided a conventional estimate of stand volume per acre and the basis for expansion of estimates from subsequent sample stages. Second stage sample trees were measured for form and volume with a Barr and Stroud Type FP15 dendometer. These measurements were converted to tree volume estimates and expanded to volume per acre estimates for the stand. Third stage sample trees were felled. Cross-sectional disks were cut at measured intervals along the bole. Radial increment measurements from these disks provided the basis for estimating growth of individual trees and stand parameters. Findings indicate that the average tarif number, for use with the DNR (Department of Natural Resources, State of Washington) tarif volume tables, computed from stem analysis. data for the present Douglas-fir component of these stands is significantly different from that estimated by conventional methods using tree height and diameter. This suggests that the growth habit and stem form of Douglas-fir may be altered as a result of commercial thinning. It also implies that volume tables constructed for natural stands may underestimate volume in thinned stands Evaluation of King's site index revealed a decreasing trend in apparent site index of about one site class from 15 to 30 years breast height age. This downward trend appears to subside about the tire of the initial thinning operations and remain relatively constant thereafter. Examination of stem analysis data from an unthinned stand about 25 miles east of the project area revealed a similar trend in site index. Present volume per acre for an unthinned stand was predicted using past stand parameters and the DNR yield tables for the Douglas-fir zone. Results indicate that volume in the surviving stand plus that removed by commercial thinnings exceeds the predicted stand yield. This amounted to about 5 percent in one stand and about 16 percent in the higher density, more heavily thinned stand. This comparison implies that total yield in Douglas-fir stands 50 to 55 years of age may be slightly increased by repeated commercial thinnings.
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