Fog reduces transpiration in tree species of the Canarian relict heath-laurel cloud forest (Garajonay National Park, Spain)

The ecophysiologic role of fog in the ever-green heath-laurel 'laurisilva' cloud forests of the Canary Islands has not been unequivocally domonstrated, 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 degrees C under foggy conditions whereas higher values, ranging from 9 to 21 degrees C. were registered fog-free periods. Additionally, during periods when fog water was collected, global solar radiation values were linearly related (r(2) = 0.831) to those under fog-free conditions. such that there was 75 +/- 1% reduction in median radiation in response to fog. Fog events greatly reducted median diurnal tree transpiration, with rates about 30 times lower than that during fog-free conditions and approximating the nightime rates in both species studies (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 microelectrological status, both when expressed on a media 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 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'.