Date of Graduation

1999

Document Type

Dissertation/Thesis

Abstract

Temperate forest canopy gaps are generally characterized as spatially discrete patches whose biophysical properties are a function of gap size and age acting in conjunction with subcanopy composition and structure. The surrounding gap environment is rarely considered, although there are reasons to suggest that gaps may be influenced by adjacent canopy openings such as agricultural fields, roads, and timber harvests. Experimental canopy gaps (∼75 m 2) were created adjacent to silvicultural clearcuts (∼5 ha.) in second-growth mixed-mesophytic forest to better understand the effect of clearcutting on adjacent canopy gap understones. Gaps were created on west- and south-facing edges at 15 m, 30 m, and 45 m from clearcut boundaries to mimic natural gaps in shape and size. Sampling occurred in gaps and non-gap controls. Herb-layer (0.5 m) characteristics of total abundance, species richness, and diversity all declined significantly over distance from clearcuts and were higher on south-facing edges, but only abundance demonstrated a significant effect of gap formation. Flower production in white snakeroot (Eupatorium rugosum [Houtt]), a native perennial composite herb common in edges, was analyzed as an indicator of edge effect and as a phytometer of the interaction between clearcuts and adjacent gaps. Flower biomass declined significantly over distance from clearcuts, and was significantly higher in gaps than in closed canopy controls. Small canopy gaps apparently intensify the edge effect, extending the effects of clearcutting into the adjacent interior forest. Clearcut and edge seed banks were also analyzed to determine whether seed density, richness, and diversity are associated with edge aspect and distance from clearcuts. Seed bank richness and diversity were unaffected by distance, but seed density declined significantly on west-facing edges. The most dramatic edge effect was found in wind dispersed, disturbance-associated species whose seed density declined significantly over distance from clearcuts. Seed dispersal may be an important process in altering edge understory composition and structure, as disturbance-associated species disperse from openings into hospitable edges and canopy gaps. Taken in composite, these findings have important ramifications for forest reserve design and management, particularly in fragmented mid to late successional forests where edge effects and canopy gaps interact.

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