Nighttime transpiration in a seasonally dry tropical montane cloud forest environment

Highly variable nighttime transpiration, with higher rates generally observed after preceding fog, is prevalent in dominant tree species of the nutrient-poor tropical montane cloud forest environment of central Veracruz, Mexico. Although nighttime transpiration (E (n)) is prevalent in a wide range of species from cloud-affected forests, its magnitude relative to total daily transpiration (E (d)) as reported in the literature is generally small (E (n)/E (d) is 0.12 on average). In the present study, we observed high dry-season E (n)/E (d) ratios with large night-to-night variation in dominant species from the tropical montane cloud forest (TMCF) zone of central Veracruz, Mexico: 0.22 +/- A 0.18 for Quercus lancifolia (old-growth TMCF); 0.26 +/- A 0.14 and 0.16 +/- A 0.16 for Alchornea latifolia and Alnus jorullensis, respectively (regenerating post-fire TMCF); and 0.30 +/- A 0.20 to 0.12 +/- A 0.21 for Pinus patula (young and mature pine plantations). E (n) was determined as the difference between observed nocturnal sap flow and estimated refilling of stem water storage, the latter of which was on average: 21-25 % of nocturnal sap flow for Q. lancifolia; 6 and 5 % for A. latifolia and A. jorullensis, respectively; and 21-23 % for P. patula. Night-to-night variation in E (n) was mostly due to large variation in vapor pressure deficit (VPD) related in turn to the alternation of cold fronts (producing fog events) and high pressure weather (producing nights with VPD up to 2 kPa). Moreover, in the hours following fog events without concurring rainfall, E (n) was often higher as compared to fog-free nights with similar VPD across all species examined. Low-nutrient availability and high water content of the soils in the study area suggest a nutrient uptake benefit associated with the relatively high E (n) rates observed.