Forest patch symmetry depends on direction of limiting resource delivery

Abstract

Edge effects are a major concern in the study and conservation of forest patches. The traditional perspective, derived from patches formed by fragmentation, considers forest edges as intermediates in a gradient between interior and exterior conditions, symmetrically distributed around the core of the patch. We present a more general conceptual model that shows that this perspective is only one of several possible environmental gradients across forest patches. When resources are delivered horizontally (e.g., fog, surface runoff ), environmental parameters and species composition are expected to have very different, asymmetric, distributions within forest patches. We conducted transect surveys characterizing environmental conditions (light, soil moisture, soil nutrients), vegetation structure and species composition in fog-fed patches of relict temperate forest in northern Chile. Windward edges differed most from the surrounding scrubland, whereas the core merely represented an intermediate between windward and leeward edges. Community composition changed drastically from temperate forest specialists on the windward edge to Mediterranean shrub species leeward. The simple edge-core model is shown to be inadequate for describing spatial patterns in fog-influenced forests: a more universal model including the directionality of external resource inputs and internal dynamics must be considered when evaluating forest patch dynamics.