The study of functional traits and physiological mechanisms determining species' drought tolerance is important for the prediction of their responses to climatic change. Fog-dependent forest patches in semiarid regions are a good study system with which to gain an understanding of species' responses to increasing aridity and patch fragmentation. Here we measured leaf and hydraulic traits for three dominant species with contrasting distributions within patches in relict, fog-dependent forests in semiarid Chile. In addition, we assessed pressure-volume curve parameters in trees growing at a dry leeward edge and wet patch core. We predicted species would display contrasting suites of traits according to local water availability: from one end favoring water conservation and reducing cavitation risk, and from the opposite end favoring photosynthetic and hydraulic efficiency. Consistent with our hypothesis, we identified a continuum of water use strategies explaining species distribution along a small-scale moisture gradient. Drimys winteri, a tree restricted to the humid core, showed traits allowing efficient water transport and high carbon gain; in contrast, Myrceugenia correifolia, a tree that occurs in the drier patch edges, exhibited traits promoting water conservation and lower gas exchange rates, as well low water potential at turgor loss point. The most widespread species, Aextoxicon punctatum, showed intermediate trait values. Osmotic compensatory mechanism was detected in M. correifolia, but not in A. punctatum. We show that partitioning of the pronounced soil moisture gradients from patch cores to leeward edges among tree species is driven by differential drought tolerance. Such differences indicate that trees have contrasting abilities to cope with future reductions in soil moisture.

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.