A decline-model interpretation of genetic and habitat structure in oak populations and its implications for silviculture

Oak decline is attributable to predisposing conditions of both biotic and abiotic origin, and to numerous insects, pathogens, and abiotic events that act as intermediate or ultimate agents of mortality. The precise causal factors vary with circumstances, but the fundamental role of the tree genotype in the process appears to be one of predisposition. Trees are predisposed to oak decline when they are genetically incapable of surviving the stresses natural to the habitats they occupy. Quercus rubra is discussed as a case study for which there is a considerable amount of genetic and ecological information. This species is characterized by (1) unusual sensitivity in growth to small-scale environmental heterogeneity and (2) unusually high levels of within-population variation in growth rate. Using growth rate as the best-available surrogate measure of ecological fitness, it is clear that high levels of genetic variability are important, even necessary, for the ecological success of populations of this species. Differential adaptation of genotypes to microsites within even very small areas is likely. To minimize susceptibility to oak decline, forest practices should favour (1) the maintenance of high levels of genetic variation, (2) the use of local provenances in artificial regeneration, and (3) large seedling populations. A further implication is that high rates of seedling mortality in plantations may be normal and even desirable for long-term forest health.