SOLAR INFLUENCES ON HOLOCENE TREELINE ALTITUDE VARIABILITY IN THE SIERRA NEVADA

Abstract

Studies utilizing treeline altitude and growth variability over time contribute to our understanding of the timing and magnitude of Holocene climate change and the response of alpine treelines to this change. Analysis of treeline annual growth at a site in the southern Sierra Nevada, California, indicates that much of the past climate variability on timescales of centuries to millennia during the Holocene may be attributed to periodic changes in solar irradiance. Analysis of treeline-altitude variation in the Sierra Nevada and White Mountains of California suggests that this periodic change is superimposed on a generally decreasing temperature trend over the past 5000 years. This has led to a general stepped decrease in treeline altitude in both areas with treeline falling significantly following downturns in reconstructed solar activity levels and stabilizing rather than rising during upturns in solar activity. Treeline-altitude decrease generally lags changes in solar radiation by 400 to 500 years, suggesting that the change in climatic conditions that produces downward movement of treeline is sufficient to terminate most reproduction at the existing treeline but not severe enough to terminate growth. As such, this suggests that recovery of climate-change information from altitude change at slow-growing upper treeline sites may be complicated because of growth inertia. [Key words: alpine-treeline ecotone, climate change, Holocene, Sierra Nevada.]