EXAMINING THE EFFECTS OF A CHANGING CLIMATE ON ALASKA YELLOW CEDAR: ANALYSES OF RING WIDTH AND BLUE INTENSITY TREE RING CHRONOLOGIES

The decline of Alaska Yellow Cedar (Callitropsis nootkatensis) over the past 100 years is considered to be a leading example of the effect of a changing climate on a forest species. We have generated two new sets of ring-width and blue intensity (BI) tree-ring chronologies from old growth yellow-cedar sites on Pleasant Island and Excursion Ridge adjacent to and in Glacier Bay National Park and Preserve, Southeast Alaska. These chronologies are compared with monthly climate records from along the Gulf of Alaska to explore further the climate response of cedar. The objectives here are to better understand the response of yellow cedar to a changing climate, to continue testing the leading hypothesis of the decline of this species, and to evaluate the use yellow-cedar’s potential use in climate reconstruction.

Comparisons with climate records over the past 100 years show that ring-width has a strong negative correlation with temperatures during most months and is positively correlated with snow cover. Both of these responses are consistent with the accepted hypothesis that decline of yellow-cedar is related to warmer temperatures, reduced snow pack, and resulting damage to fine roots with cold events. Comparisons with BI chronologies (a measure of lignin content) suggests a shift in climate response after 1950 CE. Earlywood BI series show a strong positive relationship with summer temperature especially for the 50 years prior to 1950, after this time the relationship is more negative. This reversal in response is also suggested in the ring-width data, however, for earlywood BI it is much more pronounced and may be due to early season root damage experienced as snowpack is diminished just prior to or early in the growing season. Comparisons with the latewood BI chronologies also show a negative shift in climate response after 1950, suggesting that the negative effects early in the growth season may persist through the year. In general, the response of the lower elevation of the two study sites is more dramatic than the higher elevation site where snowpack would survive longer insulating the fine roots. Climatic analyses of these newly generated BI records provides a new tool for forest decline studies and may help dendroclimatic studies to more fully assess the climate response in tree growth.