The recent expansion of western juniper (Juniperus occidentalis) woodlands into sagebrush-dominated shrub-lands of the Sagebrush Steppe has largely been attributed to favorable climatic conditions in the mid-nineteenth century, grazing by domestic livestock, and active fire suppression. From 2010 to present, the Sage Grouse Initiative (SGI) mechanically removed western juniper from over 160,000 hectares of public and private land across the western United States in order to conserve Greater Sage Grouse (GSG) habitat. However, it is apparent that there is a paucity of studies that aim to predict how future climate scenarios will affect the range and composition of the dominant plant communities within this rapidly changing ecosystem. The goal of our research is to demonstrate how ecological data sets, newly acquired field data, and remote sensing techniques can be combined to cultivate inference and elucidate patterns of responses of individual trees and tree stands to climate variables that would be otherwise impossible to examine with a single methodological approach.
Results/Conclusions
We leverage previous western juniper tree-rings studies stored in the International Tree-Ring Data Bank (n=26) and examine the growth responses of western juniper trees (n=492) to monthly resolute climatic variables across the geographic range of western juniper woodlands. By examining the coefficients and significance of tree-ring growth response to monthly climatic variables since 1895 we are then able to ordinate individual plots and individual years across growth response and climate space. These exploratory analyses show groupings of sites by growth response that form geographic patterns distinct from those formed when climate alone is ordinated; indicating that edaphic properties may play a role in tree growth response to climate. Additionally, at all sites there is a temporal period (ranging from approximately 1950-1980) of increased heteroscedasticity, where seasonally aggregated precipitation and temperature variables either lose their predictive power or their associated coefficient switches its sign (+,-). We hypothesize that this could be an indication of a CO2 induced temporary increase in water use efficiency. Additionally we explore the methods we are undertaking to extrapolate our findings across the entire range of western juniper woodlands and how we plan to forecast their responses to future climate scenarios.