YELLOW CEDAR GROWTH RESPONSE TO DECADAL CLIMATIC SHIFTS AT CEDAR LAKE, JUNEAU, ALASKA

Yellow cedar (Cupressus nootkatensis) are experiencing widespread mortality throughout the coastal regions of the Pacific Northwest. According to the leading hypothesis explaining the decline, warmer springs promote an earlier melting of the insulating snowpack covering shallow root systems, making them susceptible to frost damage. We travelled to Cedar Lake north of Juneau, Alaska and collected 28 cores to examine cedar’s response to climate. We used standard dendrochronological methods to create a well-replicated master chronology composed of 60 ring-width series including data from previous collections. The 250 year-long chronology shows a declining step function at about 1950 CE, which correlates to the yellow cedars’ changing relationship with precipitation, temperature, and Pacific Decadal Oscillation (PDO). Ring width correlations with monthly temperature and precipitation are positive before 1950 and change to negative after 1950. Correlations with temperature post-1950 are strongly negative for January through May. These results are consistent with the leading hypothesis for cedar decline. Our results also indicate a changing stress in the trees possibly linked to the inundation of the root systems during the high precipitation fall months. When comparing ring width to the PDO index before 1950 the correlations are generally positive, but after 1950 there are strong negative correlations suggesting the late 1940’s phase shift of the PDO is associated with unfavorable conditions. After 1950 the yellow cedar at Cedar Lake are not responding to the positive phase shifts of PDO (ie., the 1976-77 shift); instead they continue a downward trend. We found that Delta Blue Intensity (DBI), another parameter derived from the tree-rings, is consistent with and supports our findings from the tree ring width analyses. Overall, this study supports the leading hypothesis for cedar decline and provides a well replicated ring-width and BI record for further research. Environmental changes in the region are proceeding too rapidly for the yellow cedar to naturally adapt. This work provides a case study to understand how other species may respond to rapid climate change.