Inventories of leaf losses resp. needle losses and investigations of the growth trends in German forests show opposite results: on the one hand the survey of leaf losses indicates a decreasing vitality since the 80s. On the other hand since the 50s the growth of the main species is even in many forest stands with high leaf losses rising and lies at present considerably above the level rated by the yield tables. Environmental conditions like air pollution, frost and draught causing a decrease of growth are superposed by effects of rising carbon dioxide concentration and nitrogen input, both increasing the growth, so that growth decline below the level of yield tables is merely observed in the high elevations of the German Middle Mountains with heavy influx of air pollution.
The network of long term experimental plots, the results of forest inventories in 1970/71 and 1987 and the growth information from increment cores and stem analysis of about 10,000 trees show remarkable growth trends. Pine stands (Pinus sylvestris L.) show especially on pure soils site improvement connected with an increase of basal area and volume increment up to 250 % of the yield tables level. A growth decline below the level of commonly used growth models occurs when leaf losses exceed 60 % and is rather seldom. Beech and oak (Fagus sylvatica L. resp. Quercus petrea Liebl.) show similar increase in height growth, volume increment and standing volume since the 50s, with maximum values far above the frame of the yield tables. Both species seem to compensate leaf losses by additional productivity of the remaining leaves and show even in heavy damaged stands (6099 % leaf losses) no definite growth reactions. Die off occurs in beach stands only after long disease phases, in oak stands it often happens abruptly. In spruce stands (Picea abies Karst.) decreasing height and volume growth in higher elevations of the Middle Mountains and Alps stands opposite to increasing growth trends in the lowlands, where increment of annual height and volume growth reach about 250 % of the yield tables level.
Repeated forest inventories in Bavaria diagnose an increase of 20 % in the standing volume and a species specific superiority of the annual volume increment in relation to the yield table of 50100 %. The nationwide network of long term experimental plots show similar growth trends in other regions of Germany.
Key words: growth acceleration, increment loss, leaf loss, carbon dioxide concentration, nitrogen input, air pollution.
Correspondence: Hans Pretzsch, Lehrstuhl für Waldwachstumskunde, Ludwig-Maximillians-Universität München, Hohenbachernstraße 22, D-85354 Freising, Germany
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