Variations in leaf functional traits of Stipa purpurea along a rainfall gradient in Xizang,China
西藏紫花针茅叶功能性状沿降水梯度的变化

Aims Stipa purpurea is the dominant species in alpine arid and semi-arid grasslands on the Tibetan Plateau. Our objectives are to determine if this species exhibits a strategy shift in its specific leaf area (SLA) to nitrogen (N) concentration relationship along a rainfall gradient and to detect possible effects of environmental factors on related leaf traits. Methods We investigated variations in leaf traits of S. purpurea associated with climatic and soil factors along an east-west transect with a rainfall gradient (69–479 mm) but similar altitudes (4 300–4 700 m). Five locations from east to west are Damxung, Namco, Gêrzê, Mount Qomolangma and Rutog. We measured SLA, mass- and area-based leaf N concentration (Nmass, Narea), leaf density and thickness and soil total N along the transect. Important findings In pooled data, SLA and Nmass varied little with the growing season mean temperature and precipitation and the soil total N concentration. The SLA-Nmass relationship in S. purpurea did not shift between the semi-humid areas (ratio of rainfall to evaporation > 0.11) and the arid and semi-arid areas (ratio < 0.11), although there was a positive correlation between SLA and Nmass across the five locations. Variation in SLA was mainly determined by leaf density in the semi-humid areas and by leaf thickness in the arid and semi-arid areas; both were negatively correlated with SLA. With increasing temperature or declining precipitation, leaf density decreased and leaf thickness increased, leading to non-significant relationships between SLA and climatic factors. The increase of leaf density in the semi-humid areas was correlated with the increase of Narea, but the increase of leaf thickness in the arid and semi-arid areas did not lead to change of Narea, resulting in unchanged Narea along the rainfall gradient. A positive correlation was detected between aboveground biomass and Narea in S. purpurea, indicating that increased Narea may increase plant productivity. Our findings suggest that alpine plants in arid and semi-arid areas may maintain a constant Narea by increased leaf thickness in order to achieve a similar photosynthetic productivity and water use efficiency compared to the relatively humid areas. The relative impacts of leaf density and leaf thickness on SLA shifted between the semi-humid areas and the arid and semi-arid areas, which may provide insight in detecting the threshold of water limitation in alpine grasslands.