Title: Exploring the Impact of Climate Change on Western Juniper Establishment Rates in a Southwestern Idaho Watershed Using a LiDAR Derived Tree Age Prediction Model

Program: Geoscience MS

Committee Chair: Jen Pierce

Committee Co-Chair: Megan Cattau

Committee: Jen Pierce, Megan Cattau, Dave Huber, David Wilkins

Abstract: The expansion of western juniper (Juniperus occidentalis) from upper slope rocky ridges into sagebrush steppe communities in lower valleys across the Pacific Northwest impacts ecosystem dynamics in rangelands. The transition of these formerly shrub-dominated landscapes to closed canopy woodlands alters hydrologic processes and reduces available habitat and forage. Western juniper expansion is partially a result of increases in establishment rates, which has been linked to the rapid change in disturbance regimes throughout rangelands, including fire suppression, intensified grazing, and changing climate. This study focuses solely on the influence of climate in order to provide a framework to better understand climate change implications on western juniper establishment rates. The study area is Johnston Draw, a small parcel of land located in Reynolds Creek Experimental Watershed in southwestern Idaho, where both researchers and landowners have noted western juniper expansion over the last century.
Using a combination of remote sensing and field sampling, we evaluate western juniper establishment rates and provide insights into the impact of seasonal climatic conditions as well as the overall annual climate impacts on the rate of establishment. First, we created a tree age prediction model that bridges the traditional field-sampling method for vegetation sampling with an updated remote sensing approach. By utilizing 77 field-samples of western juniper tree cores, heights, and canopy diameters to train a model, and obtaining the tree height and canopy diameter metrics for the non-sampled western juniper from Light Detection and Ranging (LiDAR), we developed a tree age model that estimated the age of >3,600 western juniper within the study area. Second, we chose the seasonal climate variables for this study a priori and identified that cooler spring and summer temperatures in the current year, as well as increased winter precipitation in the previous year and spring precipitation in the current year, were influential climate factors on western juniper expansion. We also assessed the relative importance of annual temperature versus annual precipitation on western juniper establishment rates.
Our results show that increased current year spring precipitation and overall annual precipitation promote western juniper expansion in the study area. Using future climate model predictions, we determined spring precipitation in the Pacific Northwest to increase, which can suggest continued expansion of western juniper in rangelands in the future. By refining the ability to measure western juniper structural metrics, determine western juniper age, and understand the climatic conditions that influence the establishment of western juniper, we provide a simplified, improved means for land managers to assess western juniper dynamics on the landscape.