Effect of Overstory Density and Beech Bark Diseaase Severity on Height Growth Patterns of American Beech and Associated Species
- Patton, Tracy Adair
University of Maine Graduate School
The beech bark disease (BBD) has had numerous detrimental effects on northern hardwood stands containing American beech (Fagus grandifolia Ehrh.). Reducing the component of susceptible beech in many stands would help to alleviate the impact of the disease on stand productivity, wood quality, species composition, and wildlife diversity. The objectives of this study were to investigate the effects of harvest intensity and disease severity on understory regeneration in the context of promoting hardwood species more desirable than beech. The effect of harvest intensity on the height development of beech regeneration, BBD severity and abundance, and beech crown position was assessed on 20 permanent inventory plots in eastern Maine, 10 to 30 years after a commercial timber harvest. Plots represented a continuum of harvest intensities which were grouped into four treatments: uncut, light partial harvests, heavy partial harvests, and clearcuts. Harvest intensity was determined by the overstory basal area (m2/ha) and volume (m3/ha) remaining after the harvest. Stem analysis was performed on sapling beech infected and non-infected by the beech bark disease. Two common associates and competitors of beech, yellow birch (Betula allegheniensis Britt.) and paper birch (Betula papyrifera Marsh), were also analyzed to determine situations where beech may be outcompeted by more desirable species. The effect of beech bark disease severity on sapling height development was assessed using a defect index developed for this study.
Results indicate that beech bark disease defect has a significant negative effect on beech sapling height growth. Residual basal area of the overstory also has a significant negative effect on beech sapling height growth rates. Beech sapling height growth rates for the first five years following harvest and for the most recent five years (1991-1996) were highest in the clearcuts and decreased linearly with increasing residual basal area (i.e., decreasing harvest intensity). However, direct comparisons of beech to birch (yellow and paper) height growth rates for each treatment showed that beech was outcompeted by the two competitor species in all harvest situations. In addition, harvest intensity has an effect on the presence and severity of BBD in the understory, 10-30 years after harvest. The relative proportion of understory beech infected by BBD (out of all understory beech) was positively related to residual overstory volume and the percentage of beech in the overstory while the mean level of BBD defect (percent of the bole covered in defect) on the defective saplings was negatively related to residual overstory volume only' The origin (seed versus sprout) of beech regeneration was also influenced by harvest intensity although origin was not found to have an effect on beech height growth rates. Beech advance regeneration grew significantly faster than post-harvest regeneration 10-30 years after the harvest. Drainage class did not have a significant effect on sapling height growth in any of the analyses.
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