Anthropogenic Edges and Effective Preserve Size in the Coast Redwood (<i>Sequoia sempervirens</i> (D.Don) Endl.) Forest

Abstract

Ecological edges created through human activities influence both biotic and abiotic factors within forest communities. The extent of this influence within a preserve is informed by the arrangement, location, and abruptness of edges, as well as the nature of the disturbances that created them. The purpose of this study was to evaluate the impacts of anthropogenic edges on soil and vegetation in the Sequoia sempervirens (D.Don) Endl. (coast redwood) forest, and to compare two methods for estimating the effective size of forest preserves. We used a combination of field data and remote image analysis collected in six forest preserves in the Santa Cruz Mountains of California. Analysis of field data collected on randomly distributed transects indicated an average depth of influence of 200 meters based on correlations between abiotic and biotic metrics and distance from the forest edge. Abiotic factors including soil temperature and compaction were negatively correlated to distance from the edge in the direction of the forest interior, while soil pH, moisture, and duff depth exhibited positive relationships. Positive correlations were also found for biotic variables including tree canopy cover, the dominance of coast redwood and Notholithocarpus densiflorus (Hook. and Arn.) Manos, C.H. Cannon, and S. Oh (tanoak), total understory plant cover, and the cover of coast redwood forest associated plant species. In contrast, the cover and richness of non-native species were highest for samples closest to the edge. To assess the effective size of preserves, high resolution digital areal images were accessed on an ArcGIS platform. Analysis indicated variation in abruptness between types of edges, with the greatest abruptness found on edges associated with urban development and roads and the lowest abruptness associated with agricultural, grazing, and commercial timber uses. Little variation was exhibited in sinuosity between land use types or in the relative influence of edges for exurban versus urban parks. The cumulative result of edges in the parks studied, based on the depth of influence assessed from field analysis, was a substantial reduction in the operative size of the preserves. Comparison of two methods of estimating this influence indicated a mean affected area of >26% using the “perimeter” method and >64% of the preserved area affected when using an “aggregated” method. These results suggest that including internal anthropogenic edges created by roads, developments, and management activities in estimates of effective preserve size is more accurate and significantly reduces the estimated size of the core area. With an increasing level of active management occurring within coast redwood preserves, it is important to consider the cumulative impact of internal anthropogenic edges and the subsequent potential decline in the effective size of preserves.