EXPLORING THE IMPACT OF CLIMATE CHANGE ON WESTERN JUNIPER EXPANSION IN A SOUTHWESTERN IDAHO WATERSHED

Since the mid-1800s, western juniper (Juniperus occidentalis) has expanded into sagebrush steppe communities across Idaho, Washington, and Oregon, causing major land cover change. Although vegetation expansion may not typically be thought of as a hazard, ongoing studies reveal that western juniper expansion can disrupt hydrologic cycles, plant regimes, and wildlife habitat. These disruptions have significant implications for rangeland management. Prior studies address the complex relationships between juniper establishment, expansion, and climate. We examine correlations among western juniper expansion and recent (~last 100 years) changes in temperature and precipitation within a southwestern Idaho watershed. By identifying critical climatic thresholds on western juniper expansion, land managers can better understand the role of contemporary climate change on expansion of western juniper and subsequent changes in land cover.

To evaluate the impact of contemporary climate change on western juniper expansion, we first determined the establishment dates of western juniper within the study area using a multiple linear regression model. This model uses tree height and canopy diameter as the independent variables to estimate tree age. We created a Canopy Height Model (CHM) for our study area using LiDAR and validated the model with ~70 tree cores used to directly measure tree age. Once we confirmed the accuracy of our LiDAR-derived measurements, we utilized the CHM to obtain tree height and canopy diameter for all western juniper trees within the study area. These measurements were then input into the multiple linear regression model to estimate the date of establishment of western juniper within the study area; establishment ages are then compared with WorldClim temperature and precipitation during periods of significant western juniper expansion across the landscape.

This study enables us to identify past critical climatic thresholds that influenced the expansion of western juniper. Future work will assess the influence of non-climatic factors (e.g., changes in grazing and fire) on the expansion of western juniper. Identifying the primary disturbance factor driving western juniper expansion will inform land managers for future rangeland management practices aimed at minimizing land cover change.