The recent expansion of western juniper (Juniperus occidentalis) 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 2015, the 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 (Centrocercus urophasianus) (GSG) habitat. Although the actions of the SGI were successful in preventing the listing of the GSG under the Endangered Species Act in the short term, it remains unclear whether and how recent changes in climate combined with rising CO2 levels have affected tree growth in this rapidly changing ecosystem The goal of our research is to address this issue by examining patterns of tree growth using dendrochronological records of dominant woody species across a typical sagebrush-steppe in eastern Oregon.
Results/Conclusions
Fifty-eight tree cores (old-growth western juniper, n=18; post-settlement western juniper, n= 20; ponderosa pine (Pinus ponderosa), n= 20) were collect from the Chewaucan River Basin, Lake County, OR, for the first field season of this experiment (Summer 2017). Annually resolved time series of standardized ring width show larger variation within than across species, suggesting a quasi-synchronic growth pattern for all three groups regardless of time of establishment. The data also indicate large decadal to centennial oscillations, with rapid transitions from highest to lowest decadal growth with no apparent CO2 fertilization effect. Interestingly, growth trends of juniper and pine trees were decoupled in recent decades, with increasing growth observed for the former and decreasing growth for the latter since the 2000s. Mixed effect models including climatic variables, landscape position, and disturbance history will be performed to determine conditions under which synchronic or divergent growth rates occur within and across groups. Additionally, we will use stable isotopes to illuminate specific mechanisms of tree growth shifts. Specifically, carbon and oxygen isotope ratios of tree ring cellulose will be used to examine changes in stomatal conductance, photosynthetic capacity, water-use efficiency, and depth of water uptake across the landscape. Ultimately, our multi-proxy approach will inform land managers and ecosystem modelers of past, present, and future responses of Sagebrush-Steppe to a wide range of environmental conditions.