Forest decline: endogenous dynamics, tree defenses, and the elimination of spurious correlation

Abstract

Two important unresolved problems in pollution impact research are early detection of changes in tree and forest growth and determination of the causes of observed growth declines. Consideration of tree defensive systems can shed light on dieback phenomena. Trees possess both chemical and structural defenses, with both types having both active and passive dimensions. When stress or pollution causes a reduction in vigor, defenses may become ineffective, resulting in insect pest and pathogen attack. This leads to a positive feedback cycle of declining vigor. Level of defenses differs by species, site, and life-stage. Consideration of these factors leads to better statistical methods for correlating pollution dose with tree growth response. It is shown that several recent studies are potentially flawed by inadequate consideration of species type, tree age, or stand density. It is shown that forest growth models can be used to correct for endogenous changes in tree growth rates as long as the trees used to derive parameters for the model were not themselves affected by pollution.