FOREST DECLINE AND INCREASING INCREMENTS - A SIMULATION STUDY

Forest decline, identified from crown transparency inventories of coni fers, seems to contradict increasing increments observed from long-ter m thinning experiments and from national forest inventories in Central Europe. In order to resolve this contradiction, a simplified version of Monserud's growth simulator FOREST was adapted to account for chang ing site quality, comparing 100 recent stem analyses of Norway spruce (Picea abies L. Karst.) and Scots pine (Pinus silvestris L.) with thos e performed by Guttenberg 100 years ago. Observed height growth has de viated from the expectations of Guttenberg's growth curves since 1960. Part of these deviations can be explained by changes in monthly tempe rature and precipitation. Addition effects of nitrogen deposition, inc reasing mineralization of raw humus layers through acidic deposition a nd increasing carbon dioxide concentration can be hypothesized. For on e 30-year-old stand, different scenarios of height increment change we re simulated. The simulation suggested that the accelerated height gro wth led to higher final volumes with fewer stem numbers through higher mortality. The mortality increases are larger for the intolerant Scot s pine than for Norway spruce. Thus the increasing mortality observed by the Austrian national inventory since 1961 and by the Austrian Fore st Condition Inventory since 1970 may at least be partly explained by the recent reports of higher volume increments per hectare whatever th e reason for these may be.