ISOLATION OF NATURAL FACTORS AFFECTING CROWN DENSITY AND CROWN COLOR IN CONIFEROUS FOREST - IMPLICATIONS FOR MONITORING OF FOREST DECLINE

This paper reports a link between crown density of coniferous trees an d natural factors in Norway. The findings are discussed in relation to monitoring of forest decline, potentially caused by air pollutants. P roductivity class, tree age and average temperatures in April and July accounted for 43% of the variance in crown density of spruce (Picea a bies). For crown color, productivity class, age and aspect of plot exp lained 11% of the variance. Variation in crown color and crown density in pine was low and results therefore more diffuse. By using these ob servations to stratify the sample plots, a higher frequency of plots w ith relatively low crown densities was found in the heaviest polluted region. Sample size was too small to verify a true regional pattern. T o avoid confounding the effects of air pollutants with natural factors , a prior stratification of sample plots is needed particularly in rel ation to tree age and climate. An investigation of the spatial distrib ution of the relatively poorest forest within each strata can thus be used to identify patterns less biased by natural factors, and may aid in recognizing otherwise undetected effects of air pollutants on the f orest ecosystem.