Causes for the unimodal pattern of leaf carbon isotope composition in Abies faxoniana trees growing in a natural forest along an altitudinal gradient.

Leaf morphological and physiological traits of Abies faxoniana growing in a natural forest along an altitudinal gradient were measured with the aim to identify the central mechanism for the marked variation in foliar δC determined by an isotope ratio mass spectrometer. There is a unimodal pattern of plant functional traits in these temperate and semi-humid areas. Stomatal parameters, specific leaf area, and C/N ratio increased, whereas C, N and δC values decreased with increasing altitude below 3000 m a.s.l.. In contrast, they exhibited opposite trends above 3000 m a.s.l.. Our results demonstrated that high-altitude plants achieve higher water use efficiency (WUE) at the expense of decreasing nitrogen use efficiency (NUE), whereas plants at 3000 m can maintain a relatively higher NUE but a lower WUE. Such intra-specific differences in the trade-off between NUE and WUE may partially explain the altitudinal distribution of the plants in relation to moisture and nutrient availability. Our results clearly indicate that the functional relations between nutritional status and the structure of leaves are responsible for the altitudinal variations associated with δC. The pivotal role of specific leaf area in regulating plant adaptive responses provides a potential physiological mechanism for the observed growth advantage of populations occupying the medium altitude. These adaptive responses to altitudinal gradients showed that an altitude of approximately 3000 m a.s.l. is the optimum distribution zone for A. faxoniana, allowing the most vigorous growth and metabolism. These results improve our understanding of the various roles of environmental and biotic variables upon δC dynamics and provide useful information for subalpine coniferous forest management. [ABSTRACT FROM AUTHOR]