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INFLUENCE OF COMMERCIAL THINNING ON GROWTH AND STABILITY OF SILVER BIRCH IN HEMIBOREAL FORESTS

Baiba Jansone

Abstract

Silver birch (Betula pendula Roth.) regenerates well naturally and is not much affected by different biotic and abiotic factors, except wind. It is increasingly recognized as fast growing species, important for veneer production, thus establishment and management of its stands are promoted. In contrast to coniferous trees, there is much less experience in birch reaction to thinning, thus the optimal management regime. Therefore aim of our study was to assess the influence of thinning on increment of silver birch and its wind stability. For the study pure birch stands, growing on fertile fresh mineral soils and fertile drained mineral soils (forest types Oxalidosa and Myrtillosa mel., respectively) were randomly selected across Latvia. From these stands 18 were thinned 8-12 years before the measurement (at the age of 20 to 46 years) and 10 were un-thinned (control). Three sample plots (area 500m2) were established in each stand, altogether 84 sample plots, and tree diameter at breast height (DBH) measured. In each sample plot 15 trees from different DBH groups were randomly selected, their height measured and increment core collected (altogether 1260 cores). Ring-width was measured and additional increment was calculated: pairs of trees from two groups of stands (thinned and unthinned) with similar annual increment trends in reference period (before the thinning) selected and differences in their increment after the treatment estimated. Basal area and mean DBH in the stand had weak (statistically insignificant) correlation with annual additional increment after the thinning, suggesting that stand density at younger (<20 years) age (thus the amount of green crown) is of critical importance for diameter growth. There was no significant impact on slenderness (thus ? wind stability) of the thinning. Additional increment varied widely between the stands and cumulatively over 6 years reached 0.94 m3ha-1, over 10 years: 4.28 m3ha-1.

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