Patterns of chronic wind mortality in a small, old-growth Pseudotsuga menziesii forest in the western Cascades, Oregon Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/76537342x

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  • Windthrow has been studied extensively as a cause of mortality and as a landscape disturbance agent in temperate forests throughout the United States. The effects of windthrow mortality on stand species composition and structure, forest regeneration and seral development have been well described at the site (e.g. single gap) and landscape (e.g. thousands of hectares) scales, predominantly by using field and remote sensing techniques, respectively. Few studies of windthrow have been conducted in small (e.g. hundreds of hectares), topographically confined and ecologically similar forests. This study addresses the spatial and temporal patterns of windthrow mortality in an old-growth Douglas-fir (Pseudotsuga menziesii) forest of this understudied spatial extent (ca. 150 ha). Coarse woody debris (CWD) data were collected as proxy for mortality along transects in the field and were spatially distributed in a GIS. The causes of mortality and the spatial and temporal distributions of CWD were assessed. Density and volumetric distributions of CWD were compared to those of the live tree population in the study area. Correlations between landscape attributes known to affect mortality susceptibility at the landscape scale, such as topographic exposure and slope steepness, were tested by comparing CWD data to field-collected and GIS-derived landscape attributes. Wind was found to be the dominant cause of mortality of sampled CWD. The spatial distribution of CWD, and thus mortality susceptibility, was found to be uniform across classes of landscape attributes, and subtle distributional variations that did exist were not related to landscape attributes in a statistically significant manner. These findings suggest that chronic wind disturbance is the dominant cause of mortality in the study area, and that episodic wind mortality is not an active process affecting the study area forest. These findings differ from those of studies conducted at the landscape scale, which have shown a predictable relationship between landscape pattern and chronic and episodic windderived tree mortality. The chronic wind disturbance affecting the study area predominantly generates small, single tree gaps that contribute to seral progression rather than alter it.
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