Decline of forest structural elements across forest–urban interfaces is stronger with high rather than low residential density

Abstract

Residential development creates a variety of wildland–urban interfaces, which in turn, can affect biodiversity to different extents. Yet, the paucity of biodiversity research at wildland–urban interfaces limits our ability to guide management and conservation planning. We assessed the effect of residential density on vegetation structure (measured along 50m transects) across urban–forest interfaces (at 0, 100 and 300m from the urban boundary into urban and forest cover). We compared five functional forms of the effect of distance to a boundary (no-effect, linear, quadratic, piecewise linear, and categorical distance), which can vary with residential density (high and low) and identified the best models describing the proportional cover of five vegetation variables across urban–forest interfaces. We found the proportional cover of most vegetation structures had a high magnitude of change across forest–urban interfaces of high residential density (towns); whereas smoother transitions were found across forest–urban interfaces of low residential density (rural residential). Town interiors exhibited the lowest estimated proportional cover of structural elements characteristic of forests (95% CI: litter=0–0.05, understory=0.01–0.03, projective foliage=0.04–0.18). In contrast, rural residential interiors retained structural elements typical of forests (95% CI: litter=0.89–0.99, understory=0.11–0.27, projective foliage=0.24–0.6). The proportion of understory in forest decreased with the proximity to an urban boundary, but the rate of decline was higher closer to towns than to rural residential areas. Because fauna heavily relies on vegetation structure, the loss of forest structural elements in towns and adjacent forests highlights the urgent need to plan for biodiversity conservation in these areas. Due to rapid urbanization of forest ecosystems worldwide, we discuss advances in land planning and fire risk management that may contribute to the conservation of vegetation structures at wildland–urban interfaces.