Oak decline around the world

Oak (Quercus spp.) decline is a malady related to the consequences of stress and successful attack of stressed trees by opportunistic (secondary) organisms (Wargo et al. 1983). It is a progressive process where trees decline in health for several years before they die. Houston (1981) developed a model of declines that is presented in Figure 1. So what is stress? It is pressure that brings ahout changes in a tree's physiology, form, or structure that predispose it to invasion by organisms that it ordinarily can resist. Stress can be biotic (defoliation by insects or fungi), abiotic (frost damage, defoliation by frost, drought, excess moisture, air pollution), or stand dynamics/life stage induced (low vigor due to competition or age). So what are secondary organisms? They are biotic agents, usually insects or fungi, that normally attack weakened trees and kill them but rarely can successfully attack healthy trees. These organisms play an important ecological role by killing trees that are weakened due to competition or other natural processes. Due to the numerous stress agents that can affect oak forests, oak decline can occur simultaneously in many different geographic areas, be triggered by entirely different or identical stress agents, and result in death of oaks from a wide variety of organisms.