BLUE INTENSITY TREE RING PROXIES BREATHE NEW LIFE INTO NORTHEAST PACIFIC CLIMATE STUDIES

The advent of blue intensity records (BI) from conifer trees, a relatively new proxy for summer temperature, is yielding remarkable results and insights into climate variability from along the Gulf of Alaska. Mountain hemlock records have been the mainstay for chronology-development and climate reconstruction for the past few decades, gleaned from a network of ring-width records used to reconstruct thermal histories. These chronologies span millennia and are composed of living trees and dead wood recovered from retreating glaciers. Sitka spruce, yellow-cedar and western hemlock have been largely ignored as records of past climate variability. This is, in part, due to the lower elevation locations of these species, decline and dieback in some populations (cedar), the lack of occurrence in the subfossil records and/or that the ring-width data shows divergence (inability to track recent warming).

A new look at low-elevation Sitka Spruce using BI shows a remarkable sensitivity to summer temperature that is weak at best in ring-width data from the same wood. The divergence issue seen in the western hemlock ring-width records from the Pacific Northwest is not as evident in the BI records. Our recent BI data of long-lived cedar show promise for climate reconstruction and to evaluate populations in decline. These discoveries are prompting us to reevaluate and reprocess archives, as well as target these overlooked species in new collections. In addition to the living tree-ring BI in progress, recent discoveries of ancient subfossil forests of cedar and western hemlock along the outer coasts of the Gulf of Alaska will allow us to extend these BI records back in time.

Of course, challenges exist in the development and processing of these BI time series. Discoloration in the living and dead wood is a challenge as well as the transition of heartwood to sapwood in samples. These factors need to be considered as the BI measurements are color sensitive. Maintaining low frequency variability in the long series is also under discussion and experimentation. As the community works to overcome these challenges, BI records are providing new information on climate, seasonality and forest health, and allow us to extend the network of temperature sensitive tree-ring records from the Pacific Northwest coastal sector.