On Western Juniper Climate Relations
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Date
2022-10-26
Authors
Reis, Schyler
Journal Title
Journal ISSN
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Publisher
University of Oregon
Abstract
Western juniper woodlands are highly sensitive to climate in terms of tree-ring growth, seedling establishment and range distribution. Understanding the dynamics of western juniper woodlands to changes in precipitation, temperature, and atmospheric CO2 levels is an important component in the development of the next generation of ecological models, natural resource policies, and land management actions. Increased atmospheric CO2 has been hypothesized to reduce the impact of drought through an increase in intrinsic water use efficiency. However, whether this increase in drought tolerance will mitigate predicted increases in temperature and decreases in precipitation in poorly understood. Additionally, potential geospatial patterns of changes in sensitivity to climate, and differential responses of competing plant species warrants further investigation.Recent projection models focused on the rangelands of Oregon retain a high level of uncertainty regarding the dynamics of western juniper woodlands. My dissertation reduces this uncertainty by quantifying the impacts of increased atmospheric CO2 on the sensitivity of western juniper tree-ring growth to precipitation and temperature. In Chapter II, I applied a method for quantifying changes in tree-ring sensitivity to climate variables under changing CO2 values to thirteen previous dendrochronological studies. Idiscovered that climate sensitivity dynamics of western juniper woodlands follow a pattern of increasing baseline sensitivity, and greater recent reductions in sensitivity, as site aridity increase across climate-space. Additionally, I developed a permutation model to assess the coverage of site locations across western juniper climate-space. In Chapter III, I applied the same analytical method on western juniper and ponderosa pine trees I sampled in the Chewaucan river basin. I discovered that western juniper are more sensitive to precipitation, and ponderosa pine are more sensitive to temperature. Also, including a long-term precipitation variable in tree-ring growth models improved model fit. In Chapter IV, I compared sensitivity trends from Chapter II with trends from bootstrapped moving window correlation and response functions and found strong agreement between model types. Throughout these chapters I infer how changes in climates sensitivity of western juniper trees may impact the future range and distribution of western juniper woodlands along with the potential impacts on policy and land management actions.
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Keywords
Biogeography, Climate Sensitivity, Dendrochronology, Semi-arid Ecosystems, Western Juniper