Seasonal variations in leaf δ¹³C and nitrogen associated with foliage turnover and carbon gain for a wet subalpine fir forest in the Gongga Mountains, eastern Tibetan Plateau
2011
Luo, Tianxiang | Li, Mingcai | Luo, Ji
It is still unclear to what extent variations in foliar δ¹³C and nitrogen can be used to detect seasonal changes in canopy productivity. We hypothesize that in a wet and cloudy fir forest, seasonally higher litterfall and lower leaf area index (LAI) are correlated with higher mass-based leaf nitrogen (N mass) and net primary productivity (NPP), while foliar δ¹³C may change with specific leaf area (SLA), area-based leaf nitrogen (N area), and/or starch concentration. In order to test our hypotheses, stand-level litterfall and the means of δ¹³C, N mass, N area, SLA, and starch concentration of canopy needles for a wet and cloudy Abies fabri forest in the Gongga Mountains were monthly measured during the growing season. Seasonal estimates of LAI were obtained from our previous work. A conceptual model was used to predict seasonal NPP of the fir forest. Seasonal mean δ¹³C and N mass and climatic variables were used as inputs. The δ¹³C across 1-7-year-old needles increased from May to September associated with decreasing SLA and increasing N area. There were no significant differences in seasonal starch concentration. With increasing litterfall and decreasing LAI, seasonal mean N mass increased, while the δ¹³C varied little. The simulated NPP increased with increasing litterfall and related traits of N mass and N area. Our data generally supported the hypotheses. The results also suggest that in the forest with relatively moist and cloudy environment, the largest fraction of annual carbon gain may occur in the early part of the growing season when higher litterfall results in higher N mass of canopy leaves.
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