End of season carbon supply status of woody species near the treeline in western China

As trees and shrubs approach the high elevation tree limit, it is often assumed that they fall short in photosynthate (source [imitation). Alternatively, tow temperature may restrict carbon investment (growth, sink Limitation). The content of mobile nonstructural. carbohydrates (NSC) in tissues is considered a measure of the carbon source-sink balance. To test the source vs. sink Limitation hypothesis, we compared tate-season NSC concentrations of various woody taxa. across attitudinal gradients from the subalpine forest to the treeline at the eastern edge of the Tibetan Plateau. Since we were interested in the generality of trends, we present "community" trends across four taxa, namely Quercus aquifolioides, Abies faxoniana, Rhododendron fabri subsp. prattii and Sorbus rufopilosa. NSC concentrations increased significantly with attitude in branch wood, current-year and last-year leaves, while there were no significant trends in stem sapwood and root xylem. The sugar to starch ratio was roughly 1:1 in branches and evergreen leaves, white stems and roots showed a higher starch fraction. Analyses of total nitrogen in leaves and wood tissues indicated no change in the trees' nitrogen supply with elevation. The overall attitudinal. trends of NSC in this group of woody plant species revealed no depletion of carbon reserves near the tree Limit, suggesting that sink limitation predominates woody plant Life across this treeline ecotone community.. (c) 2005 Published by Elsevier GmbH on behalf of Gesellschaft fur Okologie.