Mischung und Produktivität von Waldbeständen. Ergebnisse langfristiger ertragskundlicher Versuche:Aus dem Lehrstuhl für Waldwachstumskunde der Technischen Universität München

The paper reviews theoretical considerations and empirical facts on the productivity of mixed species forest stands versus pure stands. The theoretical part first draws attention on the combined effect of facilitation and competition and the net result for both individual and mean tree growth in pure and mixed forest stands (Figs. 1-3). By species mixing, competition as well as facilitation may be changed. Mixing effects can modify the productivity of individual trees and the productivity of the mean tree in a stand, and they can also modify the carrying capacity, i.e., the stand density of mixed versus pure stands (Figs. 4). The combination of mean tree productivity and density results in the overall stand productivity. Stand productivity of mixed versus pure stands can be quantified by ratios of relative productivity (LER, RYT, RPR) and illustrated by cross-diagrams (Fig. 5). How mixed stands come off in terms of over-or underyielding on different sites is conceptualized by the stress-gradient-hypothesis. Latter predicts that positive effects dominate on poor sites due to increased facilitation, while neutral or negative mixing effects in terms of productivity are predicted for rich sites where competition dominates (Fig. 6). In the second part of the paper these theoretical concepts are scrutinized on the basis of long-term experimental plot data from mixed stands experiments in Norway spruce/European beech, Sessile and Common oak/European beech, Scots pine/European beech, and Norway spruce/Silver fir. The included mixed stand trials consist of triplets of plots with pure stands of both species as well as one or more mixed species stands differing in mixing portion, mixing structure or stand density. The oldest of these plots are under survey since about 100 years. The experiments are located along an ecological gradient from Denmark through Poland and Germany to Switzerland; with a concentration in Bavaria and Lower Saxony in Germany. Evaluation of above experim