Dynamics of Defoliation, Biotic and Abiotic Damage During 1986–1998

Abstract

The estimation of tree defoliation, widely used to depict forest condition, has, despite its practicality, several disadvantages. Needle or leaf biomass in the crown is strongly affected by tree age, climatic and genetic factors, shading and a variety of abiotic or biotic stresses. Very little empirical information is available about the natural variation in leaf biomass (Westman and Lesinski 1986, McKay 1988). The reasons for an increase in defoliation must therefore be elucidated separately, e.g. with the help of statistical analysis. According to the ICP Forests recommendations, abiotic and biotic damage (especially that caused by snow, wind, ice, game, defoliating insects, needle cast and decay fungi) should be recorded in the forest health surveys (Manual on... 1994). In the reports from some countries, a rapid deterioration in forest vitality has been attributed to abiotic or biotic damage (Strelezki 1985, Roloff 1985, Hütte 1986, Innes et al. 1986, Keane et al. 1989, Innes and Schwyzer 1994). It has even been suggested that, apart from some special years when the growing conditions have been extremely poor, the results of forest health surveys merely reflect the effects of abiotic and biotic damage (Kandier 1989).