Drought sensitivity and stem growth variation of nine alien and native tree species on a productive forest site in Germany

Many non-native tree species have been introduced to Europe to improve forest productivity. It is assumed that some of these species are better able than native species to mitigate negative effects of climate change. A high growth-related tolerance to climatic extremes is essential to qualify a tree species’ suitability for cultivation and must be quantified before initiating adaptation measures. This study investigated basal area and volume increment (BAI and VI) data at various stem height positions to evaluate inter-annual growth variation (mean sensitivity) and drought tolerance of seven alien tree species (Acer rubrum L., Betula maximowicziana Regel, Castanea sativa Mill, Cryptomeria japonica D. Don, Metasequoia glyptostroboides Hu et Cheng, Thuja plicata Donn and Tsuga heterophylla Sarg.) which are considered stress tolerant, and two native species (Fagus sylvatica L., Picea abies H. Karst.) in the Arboretum Burgholz in West Germany. We found that mean sensitivity and response to drought (resistance, recovery, resilience) were related; i.e., sensitive species exhibited greater drought response than less sensitive species. In the drought years 2003, 2006 and 2010/2011, VI of the highly sensitive species C. japonica and P. abies and rather moderate sensitive A. rubrum decreased most strongly (36%), while less sensitive C. sativa and T. heterophylla were the most resistant tree species (25% decrease). B. maximowicziana, F. sylvatica, M. glyptostroboides and T. plicata were moderately sensitive to drought events (growth depression by 29%). Recovery after drought showed mainly a reverse response pattern; species with lower resistance recovered faster, but this trade-off was not uniform among species. Across drought events, we observed high variation in the response of individual trees and between different tree species. This finding indicates that species’ drought sensitivity depends strongly on the drought’s onset, duration and frequency. Along tree stems, mean sensitivity and response to drought in 2003 decreased species-specifically from lower to upper stem section height. Thus, quantifying drought sensitivity based solely on breast height measures may result in biased estimates of production declines.