Short-term reaction of European beech stem taper due to weather extremes

This study analyses a tree's short-term allocation pattern under varying weather conditions. Based on a sample of 311 stem discs of the stem boles from 92 European beech (Fagus sylvatica L.) trees, the annual stem taper was calculated retrospectively over multiple decades and linked to the annual weather conditions. The bole shape results from long-term local density and stand structure. A general understanding of short-term tree allometry due to weather conditions is even more relevant as future climate developments are assumed to cause drought stress in Central-Europe more frequently. Taper was computed by linear regression analysis and normalized with a medium-stiff spline that considers mid-term impacts of thinning, other disturbances such as insect attacks and age, but neglected short-term impacts of weather conditions. Subtracting the standardized taper by one, the percentage change of taper was confronted with the temperature and precipitation of the vegetation period. Compared to years of normal weather conditions, median taper changed by +0.090% in favorable and −0.080% in unfavorable years. In particular, rainfall influenced significantly annual taper change by +4.15 × 10⁻⁴% mm⁻¹, i.e. larger amount of precipitation during the vegetation period accelerated bole taper. Bole taper sums up many past aspects of tree environment like competition situation and growing conditions over time. Even when the annual weather impact on taper demonstrated in this study was small, it relates to tree stability and timber quality, such as fiber deviation in boards.