Incidences of oak decline have occurred repeatedly during the past three centuries as well as in the most recent decades. On the basis of historical records and dendrochronological measurements, oak decline in Central Europe has been attributed to the single or combined effects of climatic extremes (winter frost, summer drought), defoliating insects, and pathogenic fungi. Starting from a literature review, we discuss the possible roles of various abiotic (air pollution, nitrogen eutrophication, soil chemical stress, climatic extremes, site conditions) and biotic factors (insect defoliation, borer attack, infection by pathogenic fungi, microorganisms) that have been related to oak decline. On the basis of investigations on Quercus petraea and Quercus robur at three different levels (from experiments with young trees to monitoring on a supraregional scale), we suggest a conceptual model of the interaction of abiotic and biotic factors responsible for the onset of oak decline. This model should be valid for Central European oak stands at more acidic sites (soil pH (H 2 O) £ 4.2; on soils with higher pH, pathogenic Phytophthora species may contribute to oak decline). The combination of severe insect defoliation in at least two consecutive years with climatic extremes is the most significant complex of factors in the incidence of oak decline. Combined with defoliation, summer drought or winter ⁄ spring frost or both have to occur within the same year or in consecutive years to trigger major outbreaks of decline. Important additional stress factors are the following: (1) hydromorphic site conditions which, particularly in the case of Q. robur, render the trees more susceptible to drought stress as a result of an impairment of root growth in the subsoil; and (2), possibly, excess nitrogen which, in combination with drought stress, results in distinct decreases in the foliar concentrations of allelochemicals in Q. robur, thereby probably making the trees more susceptible to insect defoliation. Air pollution, soil chemical stress (including excess manganese), and nitrogen-induced nutritional imbalance do not seem to be important causal factors in the complex of oak decline. On the basis of the model, the appearance of the most recent oak decline in North-western Germany can be adequately explained.
