Growth Trends of Forests in Southern Germany

Abstract

Results from leaf or needle loss inventories and from investigations on growth trends in German forests have been found to contradict one another: while surveys of leaf loss indicate that tree vitality is declining since the 1980s, growth develop-ments of the main species show an upward tendency since the 1950s, even in forest stands with high leaf loss and lie, at present, considerably above yield table level. Remarkable growth trends are apparent in the network of long-term experimental plots, as evident from results from forest inventories in 1970/71 and 1987 and in growth data based on increment cores and from the stem analyses of some 10 000 trees. While the causes underlying growth trends have already been successfully in- vestigated and subsequently much discussed there is as yet no comprehensive rep- resentation of increment levels and increment trends for German forest stands. With the present chapter, an attempt is made to make up for this shortcoming. In spruce stands (Picea abies [L.] Karst.) diminishing height and volume growth at higher el- evations in the Middle Mountains and the Alps are in sharp contrast to improving growth trends in the lowlands, where annual height increment and volume growth reach about 250% of yield table levels. On pure soils, in particular, pine stands (Pinus sylvestris L.) show site improvement manifest in an increase in basal area and volume increment of, again, up to 250% of yield tables levels. Growth decline be- low the level of commonly used growth models occurs only when leaf loss exceeds 60%, but was found to be rather rare. Beech and oak (Fagus sylvestris L. and Quercus petrea Liebl.) show a similar increase in height growth, volume increment and standing volume since the 1950s, with maximum values far exceeding the range of yield tables. Both species seem to compensate for leaf loss by additional productivity in the remaining leaves and show no definite growth reactions even in heavily damaged stands (60-99 % leaf loss). Dieback in beech stands sets in only after long phases of disease, whereas it tends to occur rather abruptly in oak stands. For the past two decades repeated forest inventories in Bavaria have diagnosed an increase of 10 - 20% in standing volume and show that species-specific annual volume increment exceeds yield tables by 10-40%. Similar production levels and growth trends in other parts of Germany were recorded through the nationwide network of long-term experimental plots.