Twentieth-century warming and the dendroclimatology of declining yellow-cedar forests in southeastern Alaska

Decline of yellow-ceder (Chamaecyparis nootkatensis ((D. Don) Spach) has occured on 200 000 ha of temperate rainforests across southeastern Alaska. Because declining forests appeared soon after the Little lee Age and are limited mostly to low elevations (whereas higher elevation forests remain healthy), recent studies have hypothesized a climatic mechanism involving early dehardening, reduced snowpack, and freezing injury. This hypothesis assumes that a specific suite of microclimatic conditions occurs during late winter and declining cedar populations across the region have responded similarly to these conditions. Based on the first geographically extensive tree ring chronologies constructed for southeastern Alaska, we tested these assumptions by investigating regional climatic trends and the growth responses of declining cedar populations to this climatic variations. Warming winter trends were observed for southeastern Alaska, resulting in potentially injurious conditions for yellow-cedar due to reduced snowfall and frequent occurrence of severe thaw-freeze events. Declining cedar forests shared a common regional chronology for which late-winter weather was the best predictor of annual growth of surviving trees. Overall, our findings verify the influence of elevational gradients of temperature and snow cover on exposure to climatic stressors, support the climatic hypothesis across large spatical and temporal scales, and suggest cedar decline may expand with continued warming.