Water Codependency of the Chilean Mediterranean Climate‐Type Coastal Forest on Marine Fog

ABSTRACT In the coastal Mediterranean climate–type forests of Chile, precipitation is lower than evapotranspiration, resulting in an apparent water imbalance that contradicts the level of forest greening. As precipitation alone does not meet evapotranspiration requirements, fog may act as a complementary water input that helps offset this apparent imbalance. This input appears to come from fog intercepted by the forest, generated by marine stratocumulus (Sc) clouds trapped by ocean‐facing mountain slopes. Because the Sc cloud has a well‐defined vertical liquid structure and the forest is distributed along a steep slope that intersects it, the influence of fog varies with elevation. Likewise, as fog interacts with a heterogeneous topography that modifies the prevailing wind direction, its influence is strongly controlled by slope orientation. To address this complexity, we combine surface observations of fog‐water collection, tree fog dripping, satellite information and a fog‐harvesting model to analyse the water codependency of a coastal forest on the vertical and horizontal distribution of marine fog. Fog occurs with a frequency of 15% throughout the year, peaking in March (12 mm of fog‐drip collection) and September (16 mm). On the vertical scale, tree fog dripping increases linearly with elevation, as does fog influx ( R = 0.6), and the progressive increase in epiphytic lichens (Usnea sp. and Heterodermia sp.) along the same gradient reflects the enhanced influence of fog interception. On the horizontal scale, forest NDVI reaches its highest values (0.7) in areas where the model indicates the greatest collection. Considering modelled fog influx and tree fog dripping, we estimate a forest passive collection efficiency of ~12%. Finally, using a simplified and idealised water balance for fog‐facing slopes, we estimate that fog collection contributes 27% (127 mm year −1 ) of the annual water inputs, supporting the role of fog as a complement to precipitation in the forest water balance.