Abstract
The objective of this study was to elaborate a statistical method for estimating the total average density of infestation of silver fir (Abies alba Mill.) windfalls by the bark beetle (Pityokteines curvidens Germ.) that allows calculation of estimation errors and does not require debarking of the whole stem. The study was conducted in near-natural forests with A. alba growing in the Świetokrzyski National Park in Central Poland. In total, 56 windfalls of this species were examined. Two main patterns (A and B) of spatial distribution of P. curvidens egg galleries on stems of A. alba were observed. In pattern A, egg galleries were present in the first 1-m length of stem (measured from the base), and the probability of infestation in the first, second, and third 1-m-long stem section was 100%; it then decreased as the distance from the tree base increased. In pattern B, egg galleries were absent from the first 1-m length, but the probability of infestation in successive 1-m lengths increased (from the second to the ninth sections); then it decreased (from the 10th to the 18th sections). The linear regression equations were calculated for trees with the infested (pattern A) and uninfested (pattern B) first 1-m-long stem section measuring from the tree base. The most significant correlation between the number of P. curvidens egg galleries in individual 1-m-long stem sections and the total infestation density in the case of pattern A was found for the sixth and seventh sections counting from the tree base (coefficients of determination were 0.8551 and 0.8516, respectively, while the mean relative errors of estimation did not exceed 28%), and in the case of pattern B for the tenth section (coefficient of determination was 0.7992, and the mean relative error of estimation was 39.7%). The method presented in this paper facilitated quick estimation of the density of infestation of A. alba stems by P. curvidens. Together with survey sampling, it allows estimation of the P. curvidens numbers in the area investigated, also in stands situated in zones under strict protection in nature reserves and national parks.
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