133 Is Nighttime Transpiration Enhanced after Fog Events? M.S. Alvarado-Barrientos 1 , H. Asbjornsen 1 and F. Holwerda 2 1 Department of Natural Resources and the Environment, University of New Hampshire, USA 2 Centro de Ciencias de la Atmósfera, Universidad Nacional Autónoma de México, Mexico Keywords: cloud forest, sap flow, inversion layer, leaf wetness, Pinus patula, visibility Abstract Fog occurrence has been shown to suppress transpiration (E t ). On the other hand foggy conditions during which leaf wetness does not block stomatal gas exchange may enhance stomatal conductance, and so E t immediately after fog. Furthermore, although nighttime E t has been found to be prevalent for a wide range of species from cloud-affected forests, its magnitude relative to daytime E t has been reported to be generally small. Here, we report considerable variability in nighttime E t rates of Pinus patula trees associated to rapidly changing meteorological conditions typical for the dry season in the tropical montane cloud belt of the Eastern Sierra Madre, Mexico. Stand level tree E t was derived from sapflow measurements with the Heat Ratio Method in the stem of P. patula trees growing in contrasting stands and at different elevations within the cloud belt: 10-year-old reforestation at 2180 m a.s.l. and mature forest at 2470 m a.s.l. The dry-season range of nighttime E t for the young and mature forest was 0-0.08 and 0-0.06 mm h -1 , respectively. Expressed as a proportion of dry-season daily totals, nighttime E t was high and variable (42±28 and 19±23% for the young and mature stand, respectively). This large variation was related to the wide range of air humidity, caused by the alternation of cold front intrusions bringing about fog events and high pressure weather characterized by dry nights with vapor pressure deficits up to 2 kPa. Shortly after the end of fog events without concurring rainfall, nighttime E t for the young stand was higher (although not significantly) and more variable than for fog-free nights. Climate change-related alterations in lifting condensation level that have been projected for tropical montane regions will also affect the dynamics of the inversion layer, and as shown here, nighttime E t may increase/decrease considerably depending on a lowering/rise of the cloud ceiling. INTRODUCTION Projected future drier and warmer regional climate in tropical montane regions worldwide is likely to cause a rise in the lifting condensation level, thereby reducing the frequency of fog occurrence in many cloud forests (Still et al., 1999). The reduction in cloud immersion may considerably affect the hydrological cycle in these environments, as well as many ecological functions. Frequent cloud immersion not only can introduce an additional source of water (via cloud water interception), but also can suppress transpiration (Ritter et al., 2009). On the other hand, foggy conditions may enhance stomatal conductance (Reinhardt and Smith, 2008), and so transpiration (E t ). E t may be enhanced as long as stomatal gas exchange is not blocked by a film of water formed by fog deposition, as in light and/or intermitted fog with relatively low liquid water content and/or because of hydrophobic leaf surfaces (Rosado and Holder, 2012). In many tropical montane regions, fog events occur more frequently during the late afternoon and nighttime due to the diurnal pattern of orographic-convective cloud development over the mountains. Furthermore, although nighttime E t has been found to be prevalent for a wide range of species from cloud-affected forests, its magnitude relative to daytime E t has been reported to be relatively small (Dawson et al., 2007). In order to better understand the net effect of fog on E t , we formulated the following Proc. 9 th International Workshop on Sap Flow Ed.: K. Steppe Acta Hort. 991, ISHS 2013