Stable isotopes reveal linkages among ecohydrological processes in a seasonally dry tropical montane cloud forest

Abstract

Despite their critical role as freshwater resources and their vulnerability to anthropogenic pressures, our knowledge of the ecohydrology of tropical montane cloud forests remains limited. Here, we use a dual stable isotope approach (δ 2 H and δ 18 O) to trace water inputs, fluxes, and pools through a seasonally dry tropical montane cloud forest in central Veracruz, Mexico. We found strong seasonal variation in the isotope composition of precipitation inputs (δ 2 H ~130‰ and δ 18 O ~18‰), with significantly more depleted wet‐season values as compared with dry‐season values. These seasonal differences are subject to inter‐annual variation, as reflected by shifts in the local meteoric water line. Even at the peak of the dry season, stable isotope values of plant xylem water from six different deciduous and evergreen species occurring in the understory and canopy of mature and secondary forests were all consistent with the use of shallow soil water (20–60 cm). Alternative limiting factors, such as nutrient distribution along the soil profile, may be strongly contributing to plant rooting patterns. Stable isotope values of soil water were evaporatively enriched compared with precipitation inputs, whereas stream values plotted with precipitation on the local meteoric water line. In contrast to current hydrologic models, this indicates the presence of two distinct soil water pools, one highly mobile pool of precipitation that contributes to streams and a second less mobile pool of soil water that contributes to plant fluxes. This comprehensive perspective can provide the basis for generating process‐based models of ecohydrology in the future. Copyright © 2011 John Wiley & Sons, Ltd.