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.
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