Non-stationarity in western juniper growth and climate relationships

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Schyler Reis, Environmental Science, University of Oregon, Eugene, OR and Lucas Silva, Institute of Ecology and Evolution and Department of Geography, University of Oregon, Eugene, OR

Background/Question/Methods
Most conceptual models that attempt to describe the recent expansion of western juniper woodlands into lower elevation and more arid sagebrush scrublands consider the primary drivers to be overgrazing and fire suppression since the late 19th and mid-20th century, respectively. A lack of recognition of the roll changing levels of atmospheric CO2 and changing patterns of precipitation and temperature could have played in this recent expansion has resulted in a paucity of studies examining such relationships, and therefore has resulted in a lack of empirically derived parameters describing such phenomena. Such knowledge is necessary for the implementation of climate-informed vegetation models aimed at predicting likely shifts in vegetation ranges, compositions, and dynamics. The goal of our research is to systematically explore and select the best mixed effects model that describes how climate variables and increasing atmospheric CO2 levels interact and influence the growth of western juniper trees, and how these relationships have changed over time.
Results/Conclusions
Our study leverages tree-ring data from 13 previous studies stored in the International Tree-Ring Data Bank and examines the relationship between tree-ring growth, climate variables at a several various multi-monthly temporal resolutions, and their interactions with increases in atmospheric CO2 levels. Results from our model indicate that precipitation is more influential on western juniper tree-ring growth than temperature and that intra-annual timing of precipitation follows a hierarchical pattern of importance. With previous October to current January precipitation being the most influential predictor of current year tree-ring growth, then February to May precipitation, then June to September precipitation. Additionally, a negative interaction between CO2 and previous October to January precipitation, and a positive interaction between CO2 and February to May precipitation indicates that sensitivity to seasonal precipitation patterns in non-stationary. Additionally, we explore a novel method for quantifying how well our 13 study sites represent the entire range of western juniper woodlands, by conducting a permutation test that compares the distribution of our sites in western juniper climate space to 1000 iterations of 13 randomly selected potential sites based on a rubric that incorporates: 1) the area of coverage in climate space, 2) the evenness of site distributions, and 3) the proximity of sites to high density potential site location within the climate space occupied by western juniper woodlands.