Recovering climate signals from tree ring color in the dry Sierra Nevada of California

Abstract:

The blue intensity tree ring proxy has gained increased attention because of its potential to capture much of the climate signal typically recovered by density proxies, but with lower costs and a shorter processing time. Studies of both blue intensity and latewood density proxies are strongly concentrated in temperature stressed regions, and we know little about how these proxies perform in moisture stressed environments. In this study, we investigate the climate signals recorded in blue intensity and tree ring width in Ponderosa and Jeffery Pines sampled at sites along a transect through the Sierra Nevada Range, with sites spaced across a gradient of hydrological stress regimes. A total of 5 sites were sampled, with 211 cores taken between 37.3 north and 39.6 north. At all 5 sites tree ring width was found to be positively correlated with annual mean (Sep*-Aug) rainfall, reflecting the large-scale arid climate of the region. The strength of this correlation decreases with elevation, ranging from R=0.54 at our lowest elevation site (~700 meters elevation on western side of Sierra Nevada), to R=0.15 at our highest elevation site (~2300 meters elevation also on western side). Despite the large-scale moisture-limitation regime, we find a positive relationship between blue intensity and growing season (Jun-Aug) temperature at 3 of our 5 analyzed sites. There is no obvious relationship between the strength of this relationship and elevation. We hypothesize that it will be possible to use blue intensity in conjunction with tree ring width across a gradient of moisture stress regimes to simultaneously estimate paleotemperature and paleomoisture in the California Sierra Nevada.