MODELING SPATIAL PATTERNS OF TREES ATTACKED BY BARK-BEETLES

Many species of bark-beetles kill trees in clusters. One of the most d estructive forest insect pests with this colonization pattern is the m ountain pine-beetle. Heavy losses caused by this beetle are of primary concern to national forest managers in western North America. An unde rstanding of the process of tree selection by the beetles is useful fo r prescribing measures that might minimize beetle damage. Here, an aut ologistic model is used to study the spatial patterns of lodgepole pin e trees attacked by the beetles. The model is used to describe the con ditional probability that a tree of a given size, age and vigour is at tacked given the status of all other trees in the stand. The spatial c orrelation between trees is modelled by constructing a covariate that is a measure of the angles from attacked trees to other trees in the s tand. Some surprising difficulties in modelling conditional probabilit ies are discussed. Parameter estimates are obtained by maximizing a ps eudolikelihood function, and estimates of standard errors are obtained from iteratively simulated samples.