Mobile carbon supply in trees and shrubs at the alpine treeline ecotone

Abstract

Although the growth limitation hypothesis (GLH) is the most accepted physiological explanation for alpine treeline formation, the debate about its formation mechanisms still remains controversial due to contradictory findings from different studies. The functional difference between trees and alpine low-stature shrubs may hold answers, as shrubs grow well at higher elevations. We investigated carbon (C) storage in deciduous treeline species Betula ermanii (Erman’s birch) and two dominant shrubs (deciduous Vaccinium uliginosum and evergreen Rhododendron aureum), which naturally grow next to each other at the treeline ecotone on Changbai Mountain, Northeast China. We determined growth and non-structural carbohydrate (NSC) concentrations in organs with increasing elevation at the mid-growing season. Results showed that in the treeline ecotone soil temperature was lower than tree canopy air temperature due to unobvious aerodynamic decoupling near the ground. Species growth consistently decreased with increasing elevation, while NSC concentrations responded differently to elevation between trees and shrubs. An elevational increase and decrease in NSC were observed in leaves and woody organs, respectively, of B. ermanii. NSC concentrations in each organ significantly increased with increasing elevation for R. aureum but decreased for V. uliginosum. At the treeline, shrubs had higher values than B. ermanii in NSC, ratios of soluble sugars to starch in leaves, and leaf mass per area. Organ dependence of NSC with increasing elevation in Betula trees provided partial support for the GLH, while R. aureum and V. uliginosum provided strong support for the GLH and carbon limitation hypothesis, respectively. These imply that alpine shrubs may have evolved to maintain more advantageous C balance and functional features than did trees as an adaptation to higher-elevation climates.