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Manaras, Katherine W.

Forestry Program

2006

M.S.

Stand structure is an important component of diversity in forests, and provides critical ecosystem functions including the provision of wildlife habitat. The range of structural variability in the northern-hardwood conifer forests has not been comprehensively described, however, and existing models of forest structural development describe a single, predictable pathway of successional development initiated by stand-replacing disturbances. They do not adequately include the influences of low to intermediate intensity disturbances prior to the old-growth or shifting mosaic stage of development. We hypothesize that variability in disturbance history results in multiple pathways of structural development. We sampled 45 young to mature forest stands in central Vermont, and used a series of multivariate analyses to characterize the structural variability among sites. We created a structural classification system, in which classes are generally characterized by the degree to which structural processes associated with gap phase dynamics are evident. Our findings suggest that small-scale disturbances are playing an important role in structurally diversifying the young to mature forests that currently dominate northern New England landscapes and in the regional shift to late-successional matrix forests.
Relationships between ecosystem structure and function were explored by evaluating the role of spatial scale of forest sampling in models of avian occurrence. Avian species are thought to select habitat based on both landscape pattern and sitespecific vegetation. The later is often judged to be less predictive of avian community composition, while the importance of the former has been stressed extensively in recent studies. Avian habitat relationships at the scale of individual forest stands may not have been accurately assessed in previous research, however, due to a mismatch of spatial scale between forest sampling and bird sampling (point count radius). Two neotropical migrant species, black-throated blue warbler (Dendroica caerulescens) and ovenbird Seiurus aurocapillus), were used as indicators of biodiversity and ecosystem function because of their reliance on different components of stand level structure for reproduction and foraging. We estimated site occupancy for 20 sites, and characterized the forest structure at three spatial extents, (0.2 ha, 3.0 ha and 12.0 ha). We hypothesized that habitat models would better explain the observed patterns in site occupancy as the scale of local vegetation sampling increased. Informationtheoretic approaches indicated that the weight of evidence was greatest for habitat models using forest stand structure at the 12.0 ha scale, a scale which is slightly larger than the average territory size for both species. Habitat models characterized at the 0.2 ha scale had low support, yet are the closest in design to those used in many of the habitat studies we reviewed. These results suggest that the role of stand-level vegetation may have been underestimated in the past, which will be of interest to land managers who use habitat models to judge the suitability of habitat for species of concern.