Growth response to competition and release measures of alien tree species in Western Germany.
Deep knowledge of the species-specific response of alien tree species to changes in competition due to silvicultural activities is mostly lacking so far. Based on dendrochronological records (stem disk samples) at various height along the stem, we analyzed the annual volume increment and growth response to changes in competition of seven alien and two native tree species (Fagus sylvatica, Picea abies) at experimental tree stands in the Arboretum Burgholz, Wuppertal, in Western Germany. To compute target tree competition, four different competition indices were calculated. Comparisons indicated that none of the tested indices significantly increased the explanatory power for the description of tree growth. Before tree harvest, the 3-year average volume increment of the tree species Acer rubrum, Castanea sativa, F. sylvatica and Tsuga heterophylla could be primarily explained by the strength of competition. However, the increment of Betula maximowicziana und P. abies was triggered by the initial size (total tree volume) of individual target trees. For Abies grandis, Metasequoia glyptostroboides und Thuja plicata a high proportion of the explained variance of the volume growth was due to both, competition and initial tree size. Before the time of crown release, except for A. grandis, strong relationships between the volume increment and the strength of competition were found for all species. In total, volume increment was strongly affected by competition for T. heterophylla and F. sylvatica and moderately for A. grandis, C. sativa, T. plicata and M. glyptostroboides, respectively. Little response to competition was found for A. rubrum, B. maximowicziana and P. abies. Growth of dominant A. rubrum, P. abies and T. plicata trees was significantly affected by crown release measures resulting in an increased volume increment after treatment. The remaining tree species only indicated little or no growth increase due to release events. Dominated trees of all species showed no response to reduced competition. Increasing forestry impact (intensity of release) promoted volume growth of F. sylvatica and T. heterophylla indicating a high phenotypic plasticity or responsiveness in the middle to higher age (47 years). No relationships between release intensity and growth recovery were found for the remaining species. We provided species-specific functional coefficients, age growth models (diameter, height), height curves and competition growth models for classifying the growth and response potentials of various alien tree species in Western Germany.