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14 páginas.- 6 figuras.- 1 tabla.- referencias.- Additional supporting information may be found online in theSupporting Information section at the end of this article.https://onlinelibrary.wiley.com/action/downloadSupplement?doi=10.1002%2Feco.2238&file=eco2238-sup-0001-Data_S1.docx, Many tropical planted forests are being subjected to drier conditions due to climate change, but the interaction between drought and species diversity on species' response to drought is poorly understood. We took advantage of a historic El Niño drought in Central Panama to test drought responses of two species—Terminalia amazonia and Dalbergia retusa—in simplified, planted forests. We asked whether and how species adjust strategies when grown in monocultures and mixtures with reduced precipitation. We collected sap flux density, volumetric water content, litterfall, leaf physiology traits and leaf water potential before and during the drought. The main drivers of sap flux density (Js) in monocultures and mixtures changed from radiation, vapour pressure deficit (VPD) and air temperature to volumetric water content or precipitation towards the end of the drought. The exception was for D. retusa in the mixtures, where VPD was the main driver (0.73–0.77) of Js during the normal year and switched to precipitation during the drought year (0.47–0.72). Increasing VPD led to increases in Js until air temperature exceeded 32°C, at which point Js declined for D. retusa and stabilized for T. amazonia. T. amazonia's litterfall doubled during the drought, whereas D. retusa's litterfall peaked at the start of the dry seasons and was unaffected by the drought. D. retusa, however, delayed leaf flush until the drought ended, whereas T. amazonia transitioned through a series of water‐conserving strategies until prematurely shedding leaves prior to the peak of the drought.

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 (δ2H and δ18O) 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 (δ2H ~130‰ and δ18O ~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.

Montane ecosystems are known for their high numbers of endemic species, unique climate conditions, and wide variety of ecosystem services such as water supply and carbon storage. Although many ecohydrological and climatic studies of montane environments have been carried out in temperate and boreal regions, few have been done in Neotropical regions. Hence, the objective of this review is to synthesize the existing literature on the main factors (biotic and abiotic) that influence vegetation distribution, functional traits, and ecohydrological processes and feedbacks in tropical montane ecosystems and to identify key knowledge gaps. Most of the literature used includes work conducted in Neotropical montane rainforests, cloud forests, and grass/scrublands (e.g., páramos, punas, and campos de altitude/rupestres). Fog is a major climatic attribute in tropical montane habitats. We found that fog regimes (frequency and intensity of fog events) influence both water inputs (i.e., canopy interception and foliar water uptake) and outputs (evapotranspiration) and represent an important driver of local species composition, dominance of plant functional types, and ecological functioning. The stability and conservation of tropical montane ecosystems depends on such ecohydrological fluxes, which are sensitive to increases in air temperature and changing precipitation and fog regimes. Furthermore, to better inform effective conservation and restoration strategies, more work is needed to elucidate how key ecohydrological processes are affected by land use conversion to agriculture and pasture lands, as human activities influence the water budgets in Neotropical montane watersheds not only at regional‐scales but also globally.

Recent studies using water isotopes have shown that trees and streams appear to return distinct water pools to the hydrosphere. Cryogenically extracted plant and soil water isotopic signatures diverge from the meteoric water lines, suggesting that plants would preferentially use bound soil water, while mobile soil water that infiltrates the soil recharges groundwater and feeds streamflow all plots on meteoric water lines. These findings have been described under the “two water worlds” (TWW) hypothesis. In spite of growing evidence for the TWW hypothesis, several questions remain unsolved within the scope of this framework. Here, we address the TWW as a null hypothesis and further assess the following: (a) the theoretical biophysical feasi- bility for two distinct water pools to exist, (b) plant and soil processes that could explain the dif- ferent isotopic composition between the two water pools, and (c) methodological issues that could explain the divergent isotopic signatures. Moreover, we propose a way forward under the framework of the TWW hypothesis, proposing alternative perspectives and explanations, experiments to further test them, and methodological advances that could help illuminate this quest. We further highlight the need to improve our sampling resolution of plants and soils across time and space. We ultimately propose a set of key priorities for future research to improve our understanding of the ecohydrological processes controlling water flows through the soil–plant‐ atmosphere continuum.