Year of Award

2018

Document Type

Dissertation - Campus Access Only

Degree Type

Doctor of Philosophy (PhD)

Degree Name

Forestry

Department or School/College

W.A. Franke College of Forestry and Conservation

Committee Chair

Kelsey Jencso

Commitee Members

Jia Hu, Zachary Holden, Brady Allred, Solomon Dobrowski

Keywords

climate sensitivity, ecohydrology, hydrology, hydrometeorology, remote sensing, subsurface flow

Abstract

Our understanding of how climate and topographic gradients interact to determine patterns of subsurface moisture availability, microclimates and the occurrence of shallow subsurface flow remains limited. These hydrologic and atmospheric processes are not only first order drivers of watershed function (i.e. streamflow generation), but may also have significant impacts on ecosystem productivity, structure and sensitivity to climate. The major objective of my dissertation was to understand how climate and topography influence landscape ecohydrology across plot, watershed, and regional scales. I pursued the following questions as part of this dissertation: 1) How does the superposition of landscape topography and the climatic water balance influence hillslope scale hydrometeorology and the resultant spatial and temporal patterns of shallow subsurface flow? 2) How do the spatial and temporal persistence of these processes influence conifer growth, ecosystem biomass accumulation and photosynthetic activity as watersheds transition from energy to water limited conditions? and 3) How do topographically driven hydrologic processes contribute to differences in ecosystem sensitivity to climate across the western United States. In order to address these questions, I used a mixture of field-based hydrologic and ecological observations and remotely sensed datasets in conjunction with detailed landscape terrain analysis and climatic water balance modeling. The combination of these techniques allowed me to transfer plot and watershed scale observations of hydrological processes and vegetation response to regional assays of productivity at the scale of the entire western U.S. I determined that hillslope topography and the climatic water balance control spatiotemporal distributions of soil and atmospheric moisture which in turn contribute to the spatial persistence of shallow subsurface flow and downslope subsidies to vegetation. Ecosystems responded to these hydrologic and atmospheric conditions with enhanced productivity, greater total ecosystem biomass and longer durations of photosynthetic activity in convergent hillslope positions. My work suggests that the distribution of landscape topography is a critical consideration for quantifying ecosystem sensitivity to the climatic water balance. My work also provides a framework for identifying ecosystems with enhanced resilience to climate change across time and space and provides the necessary hydrological underpinnings for predicting changes in the terrestrial carbon cycle.

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© Copyright 2018 Zachary Harwood Hoylman