WATER NEEDS TO SUPPORT WETLAND, RIPARIAN AND HIGH MOUNTAIN FOREST ECOSYSTEMS IN THE AMERICAN CORDILLERA

Abstract

Mountain ecosystems are hydrologically and ecologically critical landscapes that help regulate climate, store freshwater, and sustain biodiversity and human populations worldwide. Their ecohydrology is shaped by steep elevational gradients that control temperature, precipitation, snowpack dynamics, vegetation, and soil processes. Climate change threatens these systems through reduced snowpack, glacier retreat, altered runoff seasonality, and increased extreme events. The Andes and Rocky Mountains illustrate both shared mountain functions and contrasting geological and climatic contexts. This dissertation examines mountain ecohydrology across three regions: the Rocky Mountains, the Verde River piedmont, and the Andean Altiplano and Atacama headwaters. Together, these studies highlight the vulnerability and global importance of mountain water systems under environmental change.In Chapter 2, I studied wetlands in the Rocky Mountains of Colorado and Wyoming. These ecosystems are sustained primarily by snowmelt, groundwater discharge, and summer monsoon rainfall. This study modeled the hydrologic regimes of 41 alpine and subalpine wetlands, including fens, wet meadows, marshes, riparian systems, alpine wetlands, and salt flats, to evaluate historical dynamics and projected responses to climate change. Using water balance approaches and generalized additive models (GAMs), historical depth-to-water (DTW) was linked to climatic and hydrological drivers, and future conditions were projected using downscaled CMIP6 climate data through 2100. Climate projections consistently show warming and increasing climatic water deficits across four of six wetland types. Under both climate scenarios, water tables are projected to decline, with the magnitude and timing of drying varying among sites. Fens and alpine wetlands are expected to experience the greatest proportional declines in water tables, whereas marshes and riparian wetlands appear relatively more resilient. Wet meadows exhibit complex and variable responses. These findings highlight the vulnerability of high-elevation wetlands to climate change and underscore the need for adaptive management strategies to sustain mountain water resources. In chapter 3, I analyzed three reaches in the Verde River in Arizona to examine river-floodplain groundwater interactions and riparian forest water demand. Groundwater fluctuations were used to estimate evapotranspiration (ETg), and a MODFLOW model quantified surface water–groundwater exchanges. Results show the Verde functions as a losing river in all three reaches, with substantial flow from the river to the floodplain. The 12.48 km² riparian forest has peak growing-season water demands up to 0.35 m3 s-1. However, low flows (<1 m3 s-1) can lower groundwater below tree rooting depths across much of the floodplain. These findings highlight the need to better quantify riparian water requirements to sustain river–floodplain ecosystems. In the fourth chapter, I investigated how climate and topography regulate sap flow in two high-Andean tree species, Polylepis rugulosa and P. tarapacana, across elevational gradients in southern Peru. Sap-flow sensors were installed on 26 trees at two sites (3,800–4,600 m), and climatic and morphometric variables were analyzed using PCA and clustering to define geomorphic groups. Results show that sap flow differs significantly by landform, season, and species. During the dry season, sap flow was higher and strongly driven by vapor pressure deficit (VPD), radiation, and soil moisture at 50 cm depth, whereas in the wet season soil moisture was less limiting and atmospheric demand dominated. Valley-bottom P. rugulosa has higher dry-season flows (mean ~72 L day-1), while P. tarapacana showed lower rates except on rock outcrops where fractured substrates enhanced water storage. These findings highlight the coupled influence of climate and geomorphology on plant water use in high-altitude drylands.