Ecotones as biological outcomes: spatial variation in tree water use across a boundary of a fog forest.

Semi-arid coastal basins where fog sustains fragmented forest patches provide a powerful natural laboratory for examining how vegetation–microclimate feedbacks shape ecotone position and stability. Yet most studies of woodland–open vegetation transitions treat ecotones as passive boundaries imposed by climate or soil conditions, rather than as zones where plant water-use strategies may actively reinforce or relax those boundaries. Here, a coastal Chilean fog forest–shrub ecotone is used to evaluate whether tree water can biologically promote ecotone persistence.

We studied fog-fed relict forests of the endemic temperate tree species Aextoxicon punctatum found on mountain tips of the semiarid coast of central Chile. We quantified sap flux and microclimatic conditions along a transect spanning forest edge to interior, using long-term sap flow measurements from 13 trees of A. punctatum, the dominant tree species in these forest patches, combined with continuous records of temperature, humidity, and vapor pressure deficit (VPD). This design allowed us to assess how tree water use responds to contrasting microclimatic environments across the ecotone.

We found strong edge-to-interior gradients in microclimate, with forest edges experiencing higher temperatures, higher VPD, and greater microclimatic variability than the forest interior. Correspondingly, tree water use differed systematically with tree position along the edge-to-interior gradient. Edge trees exhibited distinct seasonal dynamics and greater sensitivity to atmospheric conditions compared to interior individuals, particularly during periods of higher water availability. Contrary to expectations for a strictly water-limited temperate system, tree water use peaked during cool, foggy autumn and winter months, and contrasts between edge and interior trees were strongest during periods of high water availability, when trees used water most liberally. These patterns indicate that trees occupying different positions within the ecotone persist under contrasting physiological constraints and capacities.

Together, these results support the idea that forest–shrub ecotones are not merely passive boundaries imposed by climate, but may be biologically reinforced by spatial variation in tree water-use strategies. We further suggest that tolerance to edge microclimates, potentially coupled with the ability to exploit non-rain water inputs, may contribute to the persistence and resilience of fog-inundated forest patches. This perspective highlights ecotones as dynamic zones where individual-level physiological performance shapes vegetation boundaries, with implications for predicting coastal dry–humid transitions under climate change.