Thinning effects on stand and tree growth : different perspectives on same old questions Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/x920g1036

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  • The search for the stand density that optimizes growth and hence generates the maximum amount of wood is one of the philosopher’s stone in modern forestry. Since scientific methods were applied to forestry, numerous generations of foresters have studied the relations between stocking density and growth, obtaining diverse results and structuring acquired knowledge into different principles that continue to be debated by supporters and detractors. Three main principles are: 1) any reduction in stand density entails a decreasing in the growing capacity, 2) growth is constant for a range of stand densities and, 3) progressive reductions in stand density increase growth up to an optimum, after which growth starts decreasing. Thinning is the traditional tool that foresters use to manage stand density. Consequently, most of growth-growing stock relations are based on data from thinning experiments. In addition to the potential effects of thinning on growth of residual trees, the erratic behavior of periodic annual increment routinely found in field studies may be contributed by measurement errors, imputation inaccuracies, variation in initial conditions, imprecise treatment implementation, irregular mortality, or variation in definition of density and stocking. These sources of variation in response to thinning lead to different results and different interpretations of conformity to the principles listed above. The Black Rock Unit of the George T. Gerlinger Experimental Forest in Oregon holds one of the largest and oldest Douglas-fir thinning experiments with the native region of this species. The characteristics of the stand, particularly the age when treatments were carried out and the initial dimensions of the trees, broaden our perspective to the on results from other studies in other areas and in younger stands of both Douglas-fir and other species. Reductions in stand density generally lead to a change in the expected growth of the stand, most typically a reduction in growth more or less commensurate with the reduction in stand density, as further controlled by the initial diameter, height, and crown length of the residual trees. The direct effect of thinning is that portion of the growth response that is systematically related to the type, intensity, and timing of the thinning and that is not explained by the relationship of growth to conventional stand, tree, and site predictor variables in unthinned stands. Accurate forecasting of tree and stand growth after thinning therefore requires quantification of the direct effects of thinning and its change over time since thinning. In an effort to isolate the direct effect of thinning treatments at the stand and tree level, from effects of conventional stand, tree, and site variables, regression techniques were applied to data from a set of thinned and unthinned stands. After an initial period of no response, limited response, or even "shock", thinning increased stand growth inversely proportional to the intensity of treatment (if stands were compared at the same initial conditions). The overall direct effect of thinning was consequently an initial decrease relative to the growth rate of an unthinned stand with the same initial conditions, but then an increase growth after several years. Direct effects at the stand level were the aggregate result of direct effects of thinning on the constituent trees. These tree-level direct effects were more complicated than has previously been assumed in growth models. Trees of different diameter classes (correlated with crown classes) differed in their direct response to thinning. In general, thinning had little direct effect on growth patterns in the largest diameter class (largest 28%), increased the growth performance in mid-sized trees (next largest 22% by diameter), and reduced initially the average growth of smallest trees (smallest 19% by diameter). However, most diameter classes under most thinning intensities recovered quickly after a few years and direct thinning effects led to greater growth that expected for the same initial conditions in unthinned stands. The intensity of thinning increased this positive direct response among all size classes. These results help to understand the factors that control growth in Douglas-fir stands. Likewise, quantification of direct growth effects attributable to thinning add to our knowledge base on tree and stand-level responses to thinning. In regard to the stated growth-growing stock principles, the Black Rock thinning trials seemed to support the principle that stand growth declines in direct proportion to thinning intensity. The eventual growth increase stimulated by the direct thinning effect did not compensate for the reduced growing stock imposed by thinning. Furthermore, little evidence was found for uniform growth across a wide range in initial stand density. Finally, analyses of individual trees and percentiles of the diameter distribution corresponding roughly to crown classes demonstrated that the direct effect of thinning within a stand is not uniform across tree size, implying that stand-level multipliers for the direct thinning effect might gainfully be replaced by a tree-level function of relative tree size as well as thinning intensity and time since thinning.
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