Recent advances in cause-effect research on oak decline in Europe.
2008
Thomas, F. M.
This review attempts to assess the causal factors and their interrelations that cause decline of widespread European oak (<i>Quercus</i>) species. In particular, it provides an overview of recent advances in cause-effect research since 2000. The first recorded outbreak of oak decline took place in 1739-1748, and was followed by several other decline episodes, most of them occurring in the twentieth century. The mortality rates differ substantially among the European regions and decline episodes; however, the decline of entire stands is a rare event. The principal causes of oak decline are the following site-unspecific factors: severe (in particular, repeated) defoliation by insect larvae, especially in combination with infection by powdery mildew and weather extremes (summer drought, winter frost and spring frost); and the following site-specific factors: the root pathogen <i>Phytophthora quercina</i> (which is virulent in soils with a pH[CaCl<sub>2</sub>] value >=3.5 and certain fractions of exchangeable calcium), and a distinct decrease in the groundwater level. At hydromorphic sites, intermittent soil moisture can intensify the impact of drought. At least in the temperate regions, two or more of these factors have to act in combination to cause major outbreaks of decline. Other biotic factors such as the root pathogen <i>Collybia fusipes</i> and the borers <i>Agrilus biguttatus</i> or <i>Platypus cylindrus</i> can contribute to the damage in later stages of decline. In the Mediterranean region, the aggressive root pathogen <i>Phytophthora cinnamomi</i> as well as prolonged periods of intense drought probably are capable of causing oak decline without any contribution of additional factors. Other possible causal factors, whose importance still has to be corroborated, are the decrease in chemical defence compounds against leaf herbivores due to eutrophication with nitrogen, and increased ozone concentrations.
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