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Description and prediction of mortality in some coastal douglas fir stands Paillé, Gilbert

Abstract

This study is based on 68 permanent sample plots long established throughout the Coastal Douglas fir Region of the Pacific Northwest, in both natural and planted stands of Douglas fir (Pseudotsuga menziesii (Mirb.) Franco). These plots, established by seven agencies, cover 27 acres of land and contain 13 thousand trees, of which 10 thousand had been located in the Cartesian system of coordinates. Trees were individually measured, usually every 5 years, during periods varying between 10 and 30 years. Four computer programs have been written in Fortran IV to compile detailed information about forest growth and yield, stocking and stand density, frequency distributions of tree parameters, their spatial arrangements and probabilities of mortality. Multiple regression techniques were used to determine relationships among mortality, and stand and site characteristics. The objectives of the study were to fully describe "regular" mortality caused primarily by competition in second-growth stands, and to develop some methods to facilitate and improve its prediction. The problem has been tackled by using both the stand approach and the tree approach. Results show that regular mortality can be described and predicted by making use of the stand parameters entering growth functions, i.e. density, age and site index. It is best expressed in number of stems per acre. However, first- and second-order linear models including these variables could not account for more than 43 percent of the variation in mortality in natural stands. Six groups of mortality tables are presented, indicating the annual probability of individual tree mortality by age classes, based on their relative size, increment, or position with regard to the stand in which they grow. These probabilities could be used to devise marking rules for thinning operations, and to replace or supplement competition formulae built into most forest growth simulators. As such, they serve in quantifying changes in the relative growth capability of each tree with time. All percentage diameter distributions of dead trees studied fit to negative binomial probability distributions. Spatial arrangements of dead trees were clumpy only in very dense stands, in mixed stands of Douglas fir and other conifers, or in stands affected by irregular mortality. Five methods are given to predict mortality on a stand basis. A new semi-stochastic stand model, based on mortality tables, is presented as a tool to investigate growth and mortality in actual or hypothetical forest stands. In addition to providing much information about mortality, its estimates are as precise as those from other current methods for yield prediction. The input consists of a tally of tree diameters and a height-diameter equation; the output is composed of histograms of dead tree diameters, maps of their most likely spatial arrangement, and stand growth and yield information for 10-year prediction periods. Moreover, the model allows probabilities of irregular and catastrophic mortality to be taken into account.

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