Dendroecological perspectives on climate change on the southern Tibetan plateau

Abstract

Tree rings are indicators of historic environmental changes and plant response to past and current climate change. Summer temperature reconstructions from maximum latewood density on the southeastern Tibetan plateau (TP) for the past 600 years revealed cool summer temperatures between ca. 1580 and 1790 A.D., corresponding to the “Little Ice Age (LIA)." This period was characterized by several glacier advance periods, with a maximum glacier extent ending around 1740-1780 A.D. and smaller readvance phases during the early nineteenth to late nineteenth century. Stable carbon isotope analyses of tree-ring cellulose indicate species-specific ecophysiological response patterns of trees to environmental conditions related to enhanced atmospheric CO 2 levels and drier site conditions which might affect future forest composition. Spatial patterns of altitudinal changes of climate-growth relationships indicate the dominance of different growth-limiting factors in different regions of the TP. In semiarid regions along the western distribution limit of forests, moisture availability during the growing season is most relevant for growth of juniper tree species even in high altitudes. In contrast, warmer temperatures have a stimulating effect on radial growth close to the upper tree limit on the humid eastern TP. These findings are corroborated by first studies of cambial phenology, indicating a stimulating influence of early growing season temperatures on cell formation. In the dry northeastern TP, wet conditions during the main growing season in June are favorable for radial growth. Due to the low number of studies and a long history of human impact on forests, a climate-driven upward shift of the upper tree limit cannot yet clearly be stated. Tree-ring analyses on long-living dwarf shrubs may increase the potential for dendrochronological climate reconstructions beyond the upper limit of tree growth on the TP.