Relationships of piñon juniper woodland expansion and climate trends in the Walker Basin, Nevada
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Authors
Donald, Jonathon J
Issue Date
2014
Type
Thesis
Language
Keywords
Ecotone , Geographically Weighted Regression , Great Basin , Pinon Juniper , Trend Analysis , Woody Encroachment
Alternative Title
Abstract
Landscapes are in constant flux. Vegetation distributions have changed inconjunction with climate, driven by factors such as Milankovitch cycles and atmosphericcomposition. Until recently, these changes have occurred gradually. Human populationsare altering Earth's systems, including atmospheric composition and land use. This isaltering vegetation distributions at landscape scales due to changes in species potentialniche, as well as current and historical alteration of their realized niche. Vegetation shiftshave the potential to be more pronounced in arid and mountainous environments asresources available to plants such as soil moisture are more limiting.In the Great Basin physiographic region of the western United States, woodyencroachment of piñon juniper (Pinus monophylla & Juniperus osteosperma) woodlandsis well known, but the drivers of its expansion are not well understood across elevationalgradients. Predominant theories of future vegetation distribution change due to achanging climate, predict that montane species will move upslope in response toincreasing temperatures. In piñon juniper woodlands, the focus has been on downslopemovement of woodlands into other ecosystem types. The drivers for this are typicallythought to be historical land uses such as grazing and fire exclusion. However, infillingand establishment is occurring throughout its distribution and relatively little attentionhas been paid to woodland movement uphill. This study focuses on two mountain rangeswithin the Walker Lake Basin, so as to understand changes occurring along the fullgradient of piñon juniper woodlands, from lower to upper treeline, on both the westernand eastern side of the ranges.The overall goal of this study was to understand trends of change (increasing,decreasing canopy density) in piñon juniper woodlands and determine if these trendswere related to climate change trends. Trends in both vegetation and climate wereanalyzed for the entire distribution of piñon juniper within the study area, and aggregatedby ecologically pertinent zones of woodlands. Climate is highly variable and difficult toaccurately represent at fine spatial scales, so aggregation to pertinent zones such as lowerecotones, upper ecotones, and main distribution of woodlands, allowed for meaningfulinferences of how the amount of change among climatic variables over time were relatedto densification of piñon juniper canopy.The Mann-Kendall test of trend is able to detect trends in time series stacks ofspectral vegetation indexes and discern between both large and small magnitude trends.Within the study area, piñon juniper woodlands showed the greatest increases in canopydensity at upper ecotones, followed by lower ecotones. The amount of changecorresponds to an increase of about 25% in canopy density over the 30 year time periodin the upper ecotones, and about 18% increase at lower ecotones. Larger change at theupper ecotone suggests that piñon juniper is responding to climatic change in line withtheories of vegetation response to climate. This is best explained by small increases inwinter precipitation and larger increases in minimum temperature. These conditionswould favor increased canopy by promoting seedling establishment. Weaker correlationswith changes in climatic variables at the lower ecotone suggest that other factors,especially historic land use effects, are likely influencing piñon juniper to a greaterdegree. This follows others findings concerning encroachment, but suggests that theamount of change among climatic variables directly relates to overall rates ofestablishment and increasing canopy density. Encroachment at the lower bound of piñonjuniper distribution is likely a response to artificial reduction of the realized niche.
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In Copyright(All Rights Reserved)