Mobile carbohydrates in Himalayan treeline trees I. Evidence for carbon gain limitation but not for growth limitation
Mai-He Li (1, 2, 3), Wen-Fa Xiao (4), San-Gen Wang (5), Gen-Wei Cheng (2), Paolo Cherubini (1), Xaio-Hu Cai (6), Xing-Liang Liu (6), Xiao-Dan Wang (2) and Wan-Ze Zhu (2)
1. Swiss Federal Institute for Forest, Snow and Landscape Research WSL, Zuercherstrasse 111, CH-8903 Birmensdorf, Switzerland / 2. Institute of Mountain Hazards and Environment, Chinese Academy of Sciences, Chengdu 610041, P.R. China / 3. Corresponding author () / 4. Key Laboratory of Forest Ecology and Environment, State Forestry Administration, Chinese Academy of Forestry, Beijing 100091,
P.R. China / 5. Southwest University, College of Agronomy and Biotechnology, Chong-Qing 400716, P.R. China / 6. Institute of Forest Ecology, Sichuan Academy of Forestry, Chengdu 610081, P.R. China / Received June 1, 2007; accepted January 13, 2008; published online June 2, 2008
Summary
To test whether the altitudinal distribution of trees is determined by a carbon shortage or an insufficient sugar fraction
(sugar:starch ratio) in treeline trees, we studied the status of nonstructural carbohydrates (NSC) and their components (total
soluble sugars and starch) in Abies fabri (Mast.) Craib and Picea balfouriana var. hirtella Rehd. et Wils. trees along three elevational gradients, ranging from lower elevations to the alpine treeline, on the eastern
edge of the Tibetan Plateau. For comparison, we investigated a low-altitude species (Tsuga yunnanensis (Franch.) Pritz.) which served as a warm-climate reference because it is distributed in closed montane forests below 3100
m a.s.l. in the study area. The carbon status of T. yunnanensis responded to altitude differently from that of the treeline species. At the species level, total NSC was not consistently
more abundant in treeline trees than in trees of the same species growing at lower elevations. Thus there was no consistent
evidence for carbon limitation of growth in treeline trees. For the three treeline species studied (P. balfouriana and A. fabri in the Kang-Ding Valley and A. fabri in the Mo-Xi Valley), winter NSC concentrations in treeline trees were significantly lower than in lower-elevation trees
of the same species, suggesting that, in winter, carbon is limited in treeline trees. However, in no case was there total
overwinter depletion of NSC or its components in treeline trees. Treeline and low-altitude species had similar sugar:starch
ratios of about three at their upper-elevational limits in April. We conclude that survival and growth of trees at the elevational
or latitudinal climate limit depend not only on NSC concentration in perennial tissues, but also on the maintenance of an
overwintering sugar:starch ratio greater than three.
