Thinning prolongs growth duration of European beech (Fagus sylvatica L.) across a valley in southwestern Germany

摘要

Analyses of seasonal growth patterns of trees permit an increased understanding on growth responses to year-to-year weather variability and site-specific conditions. In this study, growth responses of European beech (Fagus sylvatica L) to thinning were analyzed in a narrow valley in southwestern Germany over a six-year study period (2001-2006). We considered three thinning treatments (unthinned control, intermediate thinning and heavy thinning), two aspects (north-east and south-west) and three social classes (predominant, dominant and co-dominant). The growth of 72 beech trees was monitored using automatic point dendrometers. To obtain seasonal curve parameters for statistical analyses, we fitted cumulative Weibull functions to standardized annual dendrometer series. The scale and shape parameter of this function, T and m, were used as indicators of timing and duration of growth, respectively. Together with annual radial growth, ir, they were analyzed in linear mixed models. Temporal variability in growth reflected inter-annual weather fluctuations; with generally lower growth levels and shorter growth duration in dry years. Further, the extreme dry year 2003 showed a remarkable early growth onset, followed by a late onset in 2004. Effects of thinning treatment highlighted the importance of resource limitation, e.g., water availability, as lowest growth levels were found in trees subjected to high competition in control plots. In addition, results indicated growing season prolongation with thinning. Effects of tree social class and aspect upon ir, T and m were absent or of minor importance, respectively. For both wet and dry years, reduced competition through thinning was found to stimulate growth and to prolong growth duration. Hence, thinning may be suggested as an adaptive forest management tool that may reduce stand vulnerability to drought. Further, this study illustrates that comparable growth levels can be achieved through very different seasonal growth patterns, thereby stressing the importance of intra-annual studies to increase the understanding on growth determinism of beech