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

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.

Restoration of Coast Redwood (<i>Sequoia sempervirens</i>) Forests through Natural Recovery

The management of second-growth Sequoia sempervirens (coast redwood) forests for the purpose of restoration and ecological conservation is a growing trend. However, little is known about the long-term regenerative potential of this forest type in the absence of post-harvest management techniques such as thinning and planting. Data on forest composition and structure were collected on a chronosequence (80 - 160 years) of mature recovering stands in the southern coast redwood range using a replicated, randomized, plot design. Results indicated that many stand characteristics including tree density, canopy cover, redwood dominance, species richness, herbaceous cover, and shrub cover reached levels statistically equivalent with old-growth reference sites in recovering stands within the time frame of this chronosequence. The recovery of individual herbaceous understory species was inconsistent however. While the cover of redwood-associated species (Oxalis oregana, Trientalis latifolia, and Disporum hookeri) reached levels statistically equivalent to old-growth reference sites, others (Trillium ovatum and Viola sempervirens) did not. Total basal area and species evenness also trended toward, but did not reach, old-growth conditions. The arboreal aspects of coast redwood forests appear to be remarkably resilient following a single logging event, and recover rapidly in the absence of active restoration techniques. The protracted recovery of certain redwood associated herbaceous understory species will require further study.

Herbaceous Understory Indicators of Post-Harvest Recovery in Coast Redwood (<i>Sequoia sempervirens</i>) Forests

Assessment of forest recovery following disturbance is enhanced by the use of biological indicators. One such indicator, the abundance of understory species, was examined in coast redwood (Sequoia sempervirens) forests using non-metric multiple dimensional scaling (NMDS) and indicator species analysis (ISA). Randomly distributed 10 m diameter circular plots were employed to record the abundance of all understory species across three treatments: actively managed (0 - 45 years since harvest);mature second-growth (~80 - 120 years since harvest);and unharvested old-growth stands. NMDS with perMANOVA analysis signified separation between treatments with the shade tolerant herbaceous species Trillium ovatum, Viola sempervirens, and Oxalis oregana positively correlated with mature second-growth and old-growth treatments. ISA supported the inclusion of T. ovatum, with the addition of Prosartes hookerii, as indicators of mature second-growth and old-growth. Both NMDS and ISA specified associations for Ceanothus thyrsiflorus and Stachys bullata with actively-managed stands. Occurrence of non-natives was low across treatments, though significantly higher on actively managed stands, with the shade intolerant invasive plants, Cortaderia sp. and Myosotis latifolia, occurring exclusively in actively-managed sites.