Dry season foliar fog uptake and reverse sapflow in the tropical montane cloud forests of Mexico

Background/Question/Methods Dry season fog is a ubiquitous feature of seasonal tropical cloud forests. Although cloud forests receive generous inputs of yearly precipitation, rainfall occurs primarily in the wet season. In the tropical montane forests of Veracruz Mexico, 80% of rainfall occurs in the wet season while fog occurs primarily in the dry season. Since dry-season fog occurs during months when precipitation is low or absent, this meteorological phenomenon may be important in alleviating dry-season water stress either directly through foliar fog uptake, or indirectly through a reduction in transpiration causing relaxation in xylem water tension. We determined the importance of fog on the water relations of tropical montane forest species in La Cortadura Reserve in Veracruz, Mexico by using micrometeorological data and stable isotope analysis and by measuring sap flow, leaf water potential and stomatal conductance. Throughout the dry season of 2010, we continuously measured sap flow (using the heat-ratio technique), leaf wetness, air temperature and relative humidity in the canopies of three large oak trees in a mature forest site. Using this technique we were able to determine whether reverse sap flow was occurring during fog events which would indicate foliar fog uptake. Furthermore, using stable isotopes, we determined the potential for several additional species to take up fog water into leaves. Finally, we measured the leaf water potential and stomatal conductance of seedlings of dominant cloud forest species in a common garden environment in the presence and absence of fog. Results/Conclusions Stable isotope analysis indicated that these cloud forest species exhibit the capacity to take up fog water, and that this capacity is higher than that found in other systems. We also found reverse sap flow in the canopy of a dominant oak species, indicating foliar fog uptake. Although the addition of fog to whole-tree water balance may be minimal, we found a reduction in midday leaf water potential on foggy versus sunny days which demonstrates the importance of fog in reducing the negative physiological impact of dry periods. Fog appears to play an important role in the amelioration of dry season water stress in this tropical montane cloud forest. Climate change scenarios often predict a reduction of fog in montane habitats; our results indicate that this may have important repercussions on the physiology of montane forest trees and persistence of montane cloud forest vegetation in this region.