Relationships of pinon juniper woodland expansion and climate trends in the Walker Basin, Nevada

Landscapes are in constant flux. Vegetation distributions have changed in conjunction with climate, driven by factors such as Milankovitch cycles and atmospheric composition. Until recently, these changes have occurred gradually. Human populations are altering Earth's systems, including atmospheric composition and land use. This is altering vegetation distributions at landscape scales due to changes in species potential niche, as well as current and historical alteration of their realized niche. Vegetation shifts have the potential to be more pronounced in arid and mountainous environments as resources available to plants such as soil moisture are more limiting.;In the Great Basin physiographic region of the western United States, woody encroachment of pinon juniper (Pinus monophylla & Juniperus osteosperma) woodlands is well known, but the drivers of its expansion are not well understood across elevational gradients. Predominant theories of future vegetation distribution change due to a changing climate, predict that montane species will move upslope in response to increasing temperatures. In pinon juniper woodlands, the focus has been on downslope movement of woodlands into other ecosystem types. The drivers for this are typically thought to be historical land uses such as grazing and fire exclusion. However, infilling and establishment is occurring throughout its distribution and relatively little attention has been paid to woodland movement uphill. This study focuses on two mountain ranges within the Walker Lake Basin, so as to understand changes occurring along the full gradient of pinon juniper woodlands, from lower to upper treeline, on both the western and eastern side of the ranges.;The overall goal of this study was to understand trends of change (increasing, decreasing canopy density) in pinon juniper woodlands and determine if these trends were related to climate change trends. Trends in both vegetation and climate were analyzed for the entire distribution of pinon juniper within the study area, and aggregated by ecologically pertinent zones of woodlands. Climate is highly variable and difficult to accurately represent at fine spatial scales, so aggregation to pertinent zones such as lower ecotones, upper ecotones, and main distribution of woodlands, allowed for meaningful inferences of how the amount of change among climatic variables over time were related to densification of pinon juniper canopy.;The Mann-Kendall test of trend is able to detect trends in time series stacks of spectral vegetation indexes and discern between both large and small magnitude trends. Within the study area, pinon juniper woodlands showed the greatest increases in canopy density at upper ecotones, followed by lower ecotones. The amount of change corresponds to an increase of about 25% in canopy density over the 30 year time period in the upper ecotones, and about 18% increase at lower ecotones. Larger change at the upper ecotone suggests that pinon juniper is responding to climatic change in line with theories of vegetation response to climate. This is best explained by small increases in winter precipitation and larger increases in minimum temperature. These conditions would favor increased canopy by promoting seedling establishment. Weaker correlations with changes in climatic variables at the lower ecotone suggest that other factors, especially historic land use effects, are likely influencing pinon juniper to a greater degree. This follows others findings concerning encroachment, but suggests that the amount of change among climatic variables directly relates to overall rates of establishment and increasing canopy density. Encroachment at the lower bound of pinon juniper distribution is likely a response to artificial reduction of the realized niche.