Regarding time ranges of years, a rationale has been developed which is capable of explaining observed ‘spruce decline’ symptoms observed when spruce is exposed to air containing ambient levels of SO2. It integrates and interrelates (i) ecophysiological data (tree morphology, assimilate partitioning, canopy turnover, senescence physiology, stomatal conductance, canopy throughfall, sulphur metabolism, tonoplast symport), (ii) pedological data (soil leaching, cation recycling, litter decomposition, forest nutrition), and (iii) meteorological data (site elevation, length of the annual trunk growth period, SO2-pollution). Furthermore, it can explain field observations at numerous sites of spruce decline in central Europe where SO2 is implicated as a factor of forest decline: (i) thinning of the canopy structure; (ii) early needle senescence; (iii) cation deficiency; (iv) low SO2 tolerance at sites with depleted soils in the mountains; (v) synergism of SO2pollution and acidic precipitation; (vi) recovery after liming, fertilization and after decreasing SO2 pollution; and (vii) higher SO2 tolerances of deciduous angiosperms. Different SO2tolerance strategies are identified that are employed by more SO2-tolerant tree species. Ecophysiological SO2tolerance factors interact in a complex synergistic or antagonistic manner. It is concluded that chronic SO2 pollution at ambient concentrations predisposes mainly evergreen gymnosperms to suffer under synergistic environmental stresses (frost, drought, pathogens, etc.). Thinning of the crown structure is massive at extreme sites, where several stresses act simultaneously on the trees (depleted soils, high SO2 pollution, acidic rain, etc.). Mathematical formulations allow precise definitions of terms such as cooperativity, synergism, antagonism, vitality, predisposition, latency, etc. This universal rationale, which is applicable 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’.