Century-Scale Trends in Climatic Variability for the Pacific Northwest from Western Juniper (Juniperus occidentalis Hook. ssp. occidentalis) Tree-Ring Data

The history of the Earth’s climate shows many fluctuations, and there is reason to believe that climatic fluctuations will continue to occur in the future. Climatic shifts related to processes such as the Pacific Decadal Oscillation (PDO) initiate many environmental anomalies, including interruptions in traditional storm tracks, average precipitation, fire regimes, streamflow, and animal behavior. Our historical climatic records are too short to fully illustrate climatic fluctuations over long time scales. Proxy records are often used to extend these historical climatic records. In this dendroclimatological study, I used wester juniper (Juniperus occidentalis Hook. ssp. occidentalis) from central Oregon as a climatic proxy.

Western juniper is the tree species most suitable for this project because of its extensive range, longevity, high sensitivity to moisture, and low sensitivity to temperature. Twenty-five western juniper cross sections were prepared and measured using standard dendrochronological techniques. A master chronology that extends from A.D. 797-2000 was established using the 25 western juniper cross sections. Pearson’s product-moment correlation analyses were conducted using regional precipitation data, the PDO index, and the Niño 3 index. The correlation between regional precipitation and radial tree growth was the strongest, suggesting that moisture is the most influential limiting factor of growth in central Oregon. Significant positive relationships, however, were also discovered between radial tree growth and the other climatic variables. Regression analyses were performed and climatic variables were reconstructed for the entire length of the tree-ring chronology.

Many climatic episodes are evident in the reconstructions. The most prominent features of the tree-ring chronology include sustained periods of suppressed radial tree growth during A.D. 1351-1390 and 1917-1947, as well as a long-term period of above-average growth during A.D. 1948-1999. These new data provide a better understanding of the strength and periodicity of past climatic change in the Pacific Northwest, and strengthens our predictive capabilities of potential future climatic shifts.