Spatial and seasonal variations in mobile carbohydrates in Pinus cembra in the timberline ecotone of the Central Austrian Alps

To test whether the altitudinal limit of tree growth is determined by carbon shortage or by a limitation in growth, we investigated nonstructural carbohydrates and their components starch and total soluble sugars in Pinus cembra trees along an elevational gradient in the timberline ecotone of the Central Austrian Alps. NSC contents in needles, branches, stems, and coarse roots were measured throughout an entire growing season. At the tissue level, NSC contents were not significantly more abundant in treeline trees when compared to trees at lower elevations. Along our 425-m elevational transect from the closed forest to the treeline, we failed to find a stable elevational trend in the total NSC pool of entire trees and observed within season increases in the tree's NSC pool that can be attributed to an altitudinal increase in leaf mass as needles contained the largest NSC fraction of the whole tree NSC pool. Furthermore, whole tree NSC contents were positively correlated with net photosynthetic capacity. Although our observed NSC characteristics do not support the hypothesis that treelife at their upper elevational limit is determined by an insufficient carbon balance, we found no consistent confirmation for the sink limitation hypothesis.