Dendroclimatology and Divergence: a Case Study From Glacier Bay National Park and Preserve, Alaska

Two mountain hemlock (Tsuga mertensiana) tree-ring chronologies from Glacier Bay National Park and Preserve were built to study the effect of divergence on the temperate rainforest of coastal Alaska. The Bear Track chronology, spanning from 1569 to 2006, shows significant correlations with previous spring and summer temperatures from Juneau, as well as a proxyextended temperature record for the Gulf of Alaska. The Excursion Ridge chronology, spanning from 1517 to 2006 displays a bimodal growth trend in recent decades. The chronology was therefore divided into two subchronologies; one displaying an overall increase in annual growth in recent decades, and one with an overall decreasing trend in annual growth. The former chronology shows significant correlations with previous spring and summer, as well as current spring temperatures from Juneau, and a Gulf of Alaska proxyextended temperature record. The downward trending chronology shows generally weak correlations with temperature. This recent downward trend in tree growth, called divergence, has been observed in several locations in the Northern Hemisphere. Divergence free dendroclimatic reconstructions can be created, but caution must be exercised in areas of divergent tree growth