EARLY NEEDLE SENESCENCE AND THINNING OF THE CROWN STRUCTURE OF PICEA-ABIES AS INDUCED BY CHRONIC SO2 POLLUTION .1. MODEL DEDUCTION AND ANALYSIS

Regarding time ranges of years, a rationale has been developed which i s capable of explaining observed 'spruce decline' symptoms observed wh en spruce is exposed to air containing ambient levels of SO2. It integ rates and interrelates (i) ecophysiological data (tree morphology, ass imilate partitioning, canopy turnover, senescence physiology, stomatal conductance, canopy throughfall, sulphur metabolism, tonoplast sympor t), (ii) pedological data (soil leaching, cation recycling, litter dec omposition, forest nutrition), and (iii) meteorological data (site ele vation, length of the annual trunk growth period, SO2-pollution). Furt hermore, it can explain field observations at numerous sites of spruce decline in central Europe where SO2 is implicated as a factor of fore st decline: (i) thinning of the canopy structure; (ii) early needle se nescence; (iii) cation deficiency; (iv) low SO2 tolerance at sites wit h depleted soils in the mountains; (v) synergism of SO2 pollution and acidic precipitation; (vi) recovery after liming, fertilization and af ter decreasing SO2 pollution; and (vii) higher SO2 tolerances of decid uous angiosperms. Different SO2 tolerance strategies are identified th at are employed by more SO2-tolerant tree species. Ecophysiological SO 2 tolerance factors interact in a complex synergistic or antagonistic manner. It is concluded that chronic SO2 pollution at ambient concentr ations predisposes mainly evergreen gymnosperms to suffer under synerg istic environmental stresses (frost, drought, pathogens, etc.). Thinni ng of the crown structure is massive at extreme sites, where several s tresses act simultaneously on the trees (depleted soils, high SO2 poll ution, acidic rain, etc.). Mathematical formulations allow precise def initions of terms such as cooperativity, synergism, antagonism, vitali ty, predisposition, latency, etc. This universal rationale, which is a pplicable to all tree species, is exemplified here for Norway spruce ( Picea abies [L.] Karst.). Integration of parameters yields an ordinary differential equation, which can be solved analytically. It predicts reversible dynamics of crown structures and gives an ecophysiological background to 'damage'.