Patterns and processes of treeline forest response to late Holocene climate change in the Sierra Nevada, California.

摘要

Climate variation that occurred during the late Holocene left a visible imprint on the Sierra Nevadan landscape. Dead trees rooted above the current treeline testify to the dynamic history of subalpine forests, while seedlings established above treeline in recent decades hint at ongoing change. I used a combination of modern studies of tree growth and retrospective studies of forest population dynamics and treeline history to investigate the patterns and processes of subalpine forest response to late Holocene climate variation in the Sierra Nevada. Seedling apical growth increased with elevation, suggesting that local-scale microclimatic gradients, perhaps in combination with changes in resource availability, were a more important control over growth than are coarse-scale features of climate. The retrospective analysis of past population dynamics indicated that recruitment was more sensitive to climate variation than mortality. Recruitment was inversely correlated with temperature; mortality exceeded recruitment during warm, dry episodes, whereas recruitment exceeded mortality during cold, wet episodes. Mortality was uncorrelated with climate, except in the most marginal locations. Life history traits (extreme longevity, long reproductive life span) may impart inertia to treeline forests: population demise has not occurred in the past 3,500 years without increased adult mortality. Treeline elevation was higher than present for most of the last 3,500 years. Tree abundance and treeline elevation declined between A.D./B.C. and A.D. 400, A.D. 1000 and A.D. 1400, and after A.D. 1500. Treeline elevation and tree abundance increased at 2 sites from A.D. 400 to A.D. 700. Both moisture and precipitation appear to be important controls over population processes at treeline. Contraction of subalpine forests was associated with cold and warm/dry conditions, and the expansion of subalpine forests was associated with warm conditions. Treeline response to temperature can apparently be modified by water availability, which can invert treeline response to temperature, causing treeline to decline during warm periods. The most marginal populations, within 30 m of treeline, are sensitive to decadal to centennial-scale climate conditions. Non-marginal populations are generally insensitive to climate, and are likely buffered both by life history and by their position relative to the forest border.