Fog precipitation has long been assumed as an additional water source in the relic laurel ecosystems of the Canary Islands, located at 500–1400 m MSL. However, to what extent fog water can contribute to the laurel forest water balance is not yet clear. Combining data from artificial fog catchers and a physically based impaction model, the authors evaluated the potential contribution of fog water captured by needle-leafed Erica arborea L. trees in a selected watershed of the Garajonay National Park (La Gomera Island) for a 2-yr period (February 2003–January 2005). Fog water collection was measured with artificial catchers at four micrometeorological stations placed at 1145, 1185, 1230, and 1270 m MSL. Average fog water collection was only significant at the highest measurement site (one order of magnitude greater than at lower altitudes), totaling 496 L m−2 yr−1 during the 2-yr period. The average fog water yield in the first and second annual periods ranged between 0.2–5.0 and 0.1–2.1 L m−2 day−1, respectively. Rainfall exhibited seasonality, distinguishing between rainy and dry seasons, while fog water collection was distributed more evenly throughout the year. Regarding fog water captured by the vegetation, the impaction model predicted a significant amount of fog water potentially collected by a single E. arborea tree, on the order of 1810–2090 L yr−1. Taking tree population density into account, the yearly average water contribution to the soil surface by wind-driven fogs was 251–281 mm, whereas annual rainfall was 635 and 1088 mm, respectively. The hourly course of micrometeorological variables shows a 58% reduction in global radiation under foggy conditions and a concomitant 3°–6°C mean temperature decrease compared to fog-free periods. Thus, limiting evapotranspiration may also be a relevant effect of fog in this subtropical elfin cloud forest.

The ecophysiologic role of fog in the evergreen heath-laurel ‘laurisilva’ cloud forests of the Canary Islands has not been unequivocally demonstrated, although it is generally assumed that fog water is important for the survival and the distribution of this relict paleoecosystem of the North Atlantic Macaronesian archipelagos. To determine the role of fog in this ecosystem, we combined direct transpiration measurements of heath-laurel tree species, obtained with Granier’s heat dissipation probes, with micrometeorological and artificial fog collection measurements carried out in a 43.7-ha watershed located in the Garajonay National Park (La Gomera, Canary Islands, Spain) over a 10-month period. Median ambient temperature spanned from 7 to 15 °C under foggy conditions whereas higher values, ranging from 9 to 21 °C, were registered during fog-free periods. Additionally, during the periods when fog water was collected, global solar radiation values were linearly related (r² = 0.831) to those under fog-free conditions, such that there was a 75 ± 1% reduction in median radiation in response to fog. Fog events greatly reduced median diurnal tree transpiration, with rates about 30 times lower than that during fog-free conditions and approximating the nighttime rates in both species studied (the needle-like leaf Erica arborea L. and the broadleaf Myrica faya Ait.). This large decrease in transpiration in response to fog was independent of the time of the day, tree size and species and micrometeorological status, both when expressed on a median basis and in cumulative terms for the entire 10-month measuring period. We conclude that, in contrast to the turbulent deposition of fog water droplets on the heath-laurel species, which may be regarded as a localized hydrological phenomenon that is important for high-altitude wind-exposed E. arborea trees, the cooler, wetter and shaded microenvironment provided by the cloud immersion belt represents a large-scale effect that is crucial for reducing the transpirational water loss of trees that have profligate water use, such as those of the ‘laurisilva’.