Environmental controls on the terrestrial water cycle in forested mountain ecosystems.

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Guillén, Luis Andrés. 2021. Environmental controls on the terrestrial water cycle in forested mountain ecosystems. Morgantown: West Virginia University. 110 p. Ph.D. dissertation.


Water is a key resource to natural ecosystems and human societies alike, and the water cycle is fundamentally linked to the climate and the characteristics of catchments. However, the challenges posed by environmental change makes it imperative to understand how the water cycle is affected by biotic and abiotic factors, in particular, in areas that are crucial sources of water like forested headwater catchments. Therefore, this doctoral dissertation aims to advance the knowledge on the dynamics between climate, vegetation and landscape that determine the water balance of forested mountain ecosystems. This document presents five chapters, an introductory chapter, three standalone scientific manuscripts and a concluding chapter. The research follows the common theme of evaporation controls, going from longterm and large scales, to the study of daily variations and the forest stand scale, showing the critical importance of scale on studying the relationships between forests, climate and the water balance.

The first manuscript tests the assumption of stability in reference catchments of classic US experimental catchments by investigating stability in long-term hydroclimatology records. Two methods are used: trend and break-point analyses, and a Budyko-based energy model to quantify the sensitivity of partitioning to changes in precipitation, potential evaporation and catchment properties. Several catchments presented instability in the partitioning of precipitation, yet most were hydrologically stable. Lower stability was linked to larger changes in the catchment characteristics, than to the changes in long-term precipitation and potential evaporation. This research is relevant to improve paired catchment studies and for understanding fundamental questions about the dynamics between long-term climate variables, climate controls, seasonality, and vegetation dynamics. The second investigation studies the precipitation partitioning controls in the central Appalachian mountain regions (US). The Budyko framework was applied to study the relative importance of overall climate regimes, partial correlation analysis and multivariate regressions were used to find the principal partitioning controls. Mean annual temperature and fraction of precipitation falling in the form of snow exerted a higher influence on partitioning than landscape controls (e.g. forest cover, Normalized Difference Vegetation Index, slope). Moreover, the study found that partitioning controls are scale dependent and could differ between basins in the same climate region, especially in a complex, mountainous topography setting. The third investigation quantified the degree to which the sap velocities of two dominant broadleaved species (Acer saccharum L. (sugar maple) and Quercus velutina Lam. (black oak)) in the central Appalachian mountain region, responded to ambient and experimentally altered soil moisture conditions using a throughfall displacement experiment. Also, future climates under two emissions scenarios were used to predict hypothetical forest evapotranspiration rates. Sap velocity in maples was higher and had a more plastic response to vapor pressure deficit than sap velocity in oaks. Increased vapor pressure deficits could increase transpiration, and potentially reduce the water available to the heavily populated areas downstream. This dissertation highlights the importance of studying ecohydrological processes at different temporal and spatial scales, as they reveal the complexity of tree-soil-water-atmosphere

Keywords: Evaporation, water balance, precipitation partitioning, experimental catchments, Budyko framework, sapflow, Appalachia