SPATIAL PATTERNS OF SPRUCE DEFOLIATION - RELATION TO ACID DEPOSITION,CRITICAL LOADS, AND NATURAL GROWTH-CONDITIONS IN NORWAY

Spatial patterns of defoliation of Norway spruce (Picea abies) in sout hern Norway were investigated by stratifying plots according to tree a ge, elevation and relative degree of defoliation considering the gener al forest condition in the study area. We found an agglomeration of 75 % of the plots with relatively high defoliation. The defoliation in th e area could not be explained from historic patterns of forest site pr oductivity, tree age, topography, distribution of vegetation types, di stribution of soil types or soil depth, distance to coast or altitude. The area with poor forest receives the highest acidic deposition in s outhern Norway, and is consistent with other investigations of fish di e-back and soil and lake acidification. Forest soils are clearly acid with a lower buffering capacity and up to 50% lower status of exchange able base cations compared to the remaining study area. Sulfate deposi tion has exceeded the MAGIC-calculated critical loads for this area, a nd the pollution excess was well correlated to amount of defoliation. To our knowledge, this is the first time that forest damage in terms o f defoliation has been correlated to excess of modelled critical loads . On sites with positive excess of critical loads, defoliation exceede d an absolute value of 30%. We report a pattern of defoliation consist ent with potential cumulative effects of long-range transported air po llution. If the pattern of defoliation is caused by air pollution as r esults indicate, then northern forest ecosystems may, in spite of thei r moderate pollution loads, be far more vulnerable than previously sus pected.