Site condition effects on beech leaf disease symptom severity in southwestern Ontario hardwood forests

Abstract

The health of American beech (Fagus grandifolia Ehrh.) is threatened in North America due to its susceptibility to various pathogens, including beech leaf disease. Little is known about beech leaf disease, and forest health specialists have failed to determine the causal agent responsible or how the disease is spread. The purpose of this study is to determine whether particular site conditions, which may negatively impact the health and vigour of American beech trees, affect the susceptibility of beech to infection by beech leaf disease, by assessing foliar symptom severity relative to environmental conditions occurring at 34 unique beech stands in southwestern Ontario. This study was conducted at various American beech stands occurring throughout the Ontario Ministry of Natural Resources and Forestry’s Guelph and Aylmer districts. Plots were constructed at each site to capture symptom severity data for overstorey, sapling, and understorey trees, by recording metrics such as percentage of the total canopy afflicted by dieback, chlorosis, undersized leaves and foliar banding at different severity levels. To assess whether a relationship exists between site conditions and symptom severity, the data collected in the field was run through a CART analysis to produce a decision tree that predicted the characteristics of the sites being studied and describe the stressor-response relationship that exists within the data between American beech trees and the environmental conditions occurring on the sites surveyed. The results revealed that slopes equal to or less than 32.8% are associated with an increased presence of beech leaf disease symptoms among seedlings under 1 m in height. Further, it was determined that shoulders, back slopes, and flat areas, and the occurrence of beech scale infestation intensities equal to or greater than the ‘trace’ classification are associated with the development of severe beech leaf disease symptoms among saplings with DBH under 10 cm. A stressor-response relationship exists between those site conditions that are less conducive to the growth of American beech seedlings and saplings and increased occurrence and severity of beech leaf disease symptoms. In doing so, the results of this study indicated that the causal agent or vectoring organism responsible may attack stressed trees opportunistically, thus providing insight as to how the spread of beech leaf disease can be controlled.