RESPONSE OF TREE-LINE POPULATIONS OF FOXTAIL PINE (PINUS-BALFOURIANA)TO CLIMATE VARIATION OVER THE LAST 1000 YEARS

Stand density, recruitment rate, and mortality rate in tree-line fores ts of foxtail pine (Pinus balfouriana Grev. & Balf.) were reconstructe d for the last 1000 years. Mortality and recruitment were uncorrelated over the 1000-year period of the reconstruction. At the majority of s ites, variance in recruitment was not significantly greater than varia nce in mortality. Recruitment and stand density were significantly and inversely correlated with summer temperature during the last 1000 yea rs. Mortality rates were uncorrelated with climate. The inverse correl ation between recruitment and climate suggests that water balance may mediate the effects of temperature on tree-line forests, a hypothesis that is supported by a significant positive correlation between seedli ng establishment and winter snowpack during the last 50 years. Despite large changes in the elevation of tree line at these sites during the time period of the reconstruction, populations near tree line were la rgely unaffected by climate variation, suggesting that steep gradients in vulnerability to climate change may exist at tree line in the Sier ra Nevada.