Lyssette E. Muñoz-Villers

Runoff generation in a steep, tropical montane cloud forest catchment on permeable volcanic substrate

[1] Most studies to date in the humid tropics have described a similar pattern of rapid translation of rainfall to runoff via overland flow and shallow subsurface stormflow. However, study sites have been few overall, and one particular system has received very little attention so far: tropical montane cloud forests (TMCF) on volcanic substrate. While TMCFs provide critical ecosystem services, our understanding of runoff generation processes in these environments is limited. Here, we present a study aimed at identifying the dominant water sources and pathways and mean residence times of soil water and streamflow for a first-order, TMCF catchment on volcanic substrate in central eastern Mexico. During a 6-week wetting-up cycle in the 2009 wet season, total rainfall was 1200 mm and storm event runoff ratios increased progressively from 11 to 54%. With the increasing antecedent wetness conditions, our isotope and chemical-based hydrograph separation analysis showed increases of pre-event water contributions to the storm hydrograph, from 35 to 99%. Stable isotope-based mean residence times estimates showed that soil water aged only vertically through the soil profile from 5 weeks at 30 cm depth to 6 months at 120 cm depth. A preliminary estimate of 3 years was obtained for base flow residence time. These findings all suggest that shallow lateral pathways are not the controlling processes in this tropical forest catchment; rather, the high permeability of soils and substrate lead to vertical rainfall percolation and recharge of deeper layers, and rainfall-runoff responses appeared to be dominated by groundwater discharge from within the hillslope.

Interactions between payments for hydrologic services, landowner decisions, and ecohydrological consequences: synergies and disconnection in the cloud forest zone of central Veracruz, Mexico

Payments for Hydrologic Services (PHS) programs are increasingly used as a policy tool to provide incentives for upstream landowners to adopt land use activities that favor sustainable provision of high-quality water to downstream areas. However, the effectiveness of PHS programs in achieving their objectives and the potential for unintended (often undesirable) consequences remain poorly understood. We integrate results from ecohydrological and socioeconomic research to explore the impact of Mexico’s PHS program on the target hydrologic services and people’s decisions, behavior, and knowledge regarding forest conservation and water. Using central Veracruz as our case study, we identify areas of both synchrony and disconnection between PHS goals and outcomes. Mature and regenerating cloud forests (targeted by PHS) were found to produce enhanced hydrologic services relative to areas converted to pasture, including reduced peak flows during large rain events and maintenance of dry-season base flows. However, unexpectedly, these hydrologic benefits from cloud forests were not necessarily greater than those from other vegetation types. Consequently, the location of forests in strategic watershed positions (e.g., where deforestation risk or hydrologic recharge are high) may be more critical than forest type in promoting hydrologic functions within watersheds and should be considered when targeting PHS payments. While our results suggest that participation in PHS improved the level of knowledge among watershed inhabitants about forest–water relationships, a mismatch existed between payment amounts and landowner opportunity costs, which may contribute to the modest success in targeting priority areas within watersheds. Combined, these findings underscore the complexity of factors that influence motivations for PHS participation and land use decisions and behavior, and the importance of integrating understanding of both ecohydrological and socioeconomic dynamics into PHS design and implementation. We conclude by identifying opportunities for improving the design of PHS programs and recommending priority areas for future research and monitoring, both in Mexico and globally.

Efectos hidrológicos de la conversión del bosque de niebla en el centro de Veracruz, México

La provision y regulacion de flujos de agua en cuencas es probablemente el servicio ecosistemico mas importante de los bosques de niebla, sin embargo, su funcionamiento hidrologico y como este es alterado por el cambio en el uso de suelo es aun muy poco entendido. Este estudio evaluo los efectos hidrologicos causados por la conversion del bosque mesofilo de montana sobre suelo volcanico a otros tipos de vegetacion en Veracruz, Mexico. Para ello, se realizaron mediciones micrometeorologicas, ecofisiologicas e hidrologicas combinadas con informacion isotopica. Los resultados mostraron mayores rendimientos hidricos anuales en el pastizal y plantaciones de Pinus patula joven y maduro debido a menores tasas de evapotranspiracion comparados con el bosque maduro y secundario. El caudal total anual y estacional fue similar en los bosques, sugiriendo que con 20 anos de regeneracion natural es posible restaurar la funcionalidad hidrologica de microcuencas. En contraste, la microcuenca de pastizal reporto un mayor caudal anual (10 %), pero flujos 50 % en promedio mas bajos al final de la epoca de estiaje, asociado probablemente a una topografia mas suave y una menor capacidad de infiltracion. Aun en sustratos volcanicos altamente permeables, se observo que la conversion de bosque a pastizal puede conducir a incrementos importantes en los flujos superficiales en respuesta a eventos maximos de precipitacion. El efecto de la reforestacion con P. patula a escala de cuenca se desconoce, pero tasas de infiltracion mas altas que el pastizal sugieren una probable recuperacion hidrica del suelo en el corto a mediano plazo.

Reduced dry season transpiration is coupled with shallow soil water use in tropical montane forest trees

Tropical montane cloud forests (TMCF) are ecosystems particularly sensitive to climate change; however, the effects of warmer and drier conditions on TMCF ecohydrology remain poorly understood. To investigate functional responses of TMCF trees to reduced water availability, we conducted a study during the 2014 dry season in the lower altitudinal limit of TMCF in central Veracruz, Mexico. Temporal variations of transpiration, depth of water uptake and tree water sources were examined for three dominant, brevi-deciduous species using micrometeorological, sap flow and soil moisture measurements, in combination with oxygen and hydrogen stable isotope composition of rainfall, tree xylem, soil and stream water. Over the course of the dry season, reductions in crown conductance and transpiration were observed in canopy species (43 and 34%, respectively) and mid-story trees (23 and 8%), as atmospheric demand increased and soil moisture decreased. Canopy species consistently showed more depleted isotope values compared to mid-story trees. However, MixSIAR Bayesian model results showed that the evaporated (enriched) soil water pool was the main source for trees despite reduced soil moisture. Additionally, while increases in tree water uptake from deeper to shallower soil water sources occurred, concomitant decreases in transpiration were observed as the dry season progressed. A larger reduction in deep soil water use was observed for canopy species (from 79 ± 19 to 24 ± 20%) compared to mid-story trees (from 12 ± 17 to 10 ± 12%). The increase in shallower soil water sources may reflect a trade-off between water and nutrient requirements in this forest.