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

Abstract

Tree rings form an important part of our observing network for surface climate before the introduction of widespread instrumental observation. Tree rings produce annual climate reconstructions and outnumber other paleoclimate proxies. In most regions of California tree ring width correlates positively with seasonal precipitation amount. The blue intensity tree ring proxy has gained increased attention because of the potential to capture much of the climate signal typically recovered by density proxies, 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 little is known 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 for 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 8 sites were sampled, with 304 cores taken between 37.3 north and 39.6 north. With 5 sites currently analyzed, all 5 show tree ring width 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 apparent relationship between the strength of this relationship and elevation. We find limitations in reconstructing temperature at our 2 northern study sites, where we found a positive relationship between blue intensity and annual mean (Sep*-Aug) rainfall. 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.