Altitudinal differences in the leaf fitness of juvenile and mature alpine spruce trees (Picea crassifolia)
Changming Zhao (1, 2), Litong Chen (1, 2), Fei Ma (1), Buqing Yao (1) and Jianquan Liu (1, 3, 4)
1. Key Laboratory of Arid and Grassland Ecology, Ministry of Education, School of Life Sciences, Lanzhou University, Lanzhou 730000, Gansu, P.R. China / 2. These authors contributed equally to this work / 3. Key Laboratory of Adaptation and Evolution of Plateau Biota, Northwest Plateau Institute of Biology, Chinese Academy of Sciences, Xining 810001, Qinghai, P.R. China / 4.
Corresponding author (liujq@nwipb.ac.cn) / Received January 22, 2007; accepted May 7, 2007; published online October 15, 2007
Summary : In many plant species, leaf morphology varies with altitude, an effect that has been attributed to temperature. It remains uncertain whether such a trend applies equally to juvenile and mature trees across altitudinal gradients in semi-arid mountain regions. We examined altitude-related differences in a variety of needle characteristics of juvenile (2-m tall) and mature (5-m tall) alpine spruce (Picea crassifolia Kom.) trees growing at altitudes between 2501 and 3450 m in the Qilian Mountains of northwest China. We found that stable carbon isotope composition (δ13C), area- and mass-based leaf nitrogen concentration (N a, N m), number of stomata per gram of nitrogen (St/N), number of stomata per unit leaf mass (St/LM), projected leaf area per 100 needles (LA) and leaf mass per unit area (LMA) varied nonlinearly with altitude for both juvenile and mature trees, with a relationship reversal point at about 3100 m. Stomatal density (SD) of juvenile trees remained unchanged with altitude, whereas SD and stomatal number per unit length (SNL) of mature spruce initially increased with altitude, but subsequently decreased. Although several measured indices were generally found to be higher in mature trees than in juvenile trees, N m, leaf carbon concentration (C m), leaf water concentration (LWC), St/N, LA and St/LM showed inconsistent differences between trees of different ages along the altitudinal gradient. In both juvenile and mature trees, δ13C correlated significantly with LMA, N m, N a, SNL, St/LM and St/N. Stomatal density, LWC and LA were only significantly correlated with δ13C in mature trees. These findings suggest that there are distinct ecophysiological differences between the needles of juvenile and mature trees that determine their response to changes in altitude in semi-arid mountainous regions. Variations in the fitness of forests of different ages may have important implications for modeling forest responses to changes in environmental conditions, such as predicted future temperature increases in high altitude areas associated with climate change. Keywords: altitude, carbon isotope, ecophysiology, morphology, tree age.