Abstract:

Aims Precipitation use efficiency (PUE) is a key to understanding the coupling between ecosystem carbon and water cycles. Our objective was to probe the spatial PUE pattern and its response to climate on the Qinghai- Xizang Plateau to better understand mechanisms of vegetation productivity and improve ecosystem process models. Methods GLOPEN-CEVSA model was applied to estimate net primary production (NPP) by using the Fraction of Photosynthetically Active Radiation Absorbed by Vegetation (MOD15A2), and spatially interpolated meteorological data in 2000-2008. The modeled NPP was significantly correlated with the observed above-ground net primary productivity (R ² = 0.49, p < 0.001, n = 97). The PUE was calculated as the ratio of NPP to the annual sum of precipitation. Important findings The spatial pattern of PUE showed large differences among vegetation types. Crops had the highest PUE, and alpine meadow had higher PUE than alpine steppe. These differences were related to the precipitation and temperature distribution on the plateau. The PUE was relatively stable and the lowest value of (0.026 ± 0.190) g C·m ^-2·mm ^-1 (mean ± standard deviation) with the highest coefficient of variance (CV) of 721% was where precipitation was < 90 mm. Where precipitation was 90-300 mm, PUE was relatively stable and also low ((0.029 ± 0.074) g C·m ^-2·mm ^-1) with relatively high CV (252%). Together precipitation and air temperature in this precipitation range explained 43.4% of the spatial variance of PUE, and the effect of precipitation was 1.7 times that of temperature (p < 0.001). The area with precipitation from 300-650 mm, mainly covered by alpine steppe (45%), had relatively high PUE ((0.123 ± 0.191) g C·m ^-2·mm ^-1) with a CV of 155%. The significant correlation of PUE with climate factors explained 97.8% spatial variance of PUE. Temperature had the dominant role, having 1.5 times the effect of precipitation. With increasing precipitation, PUE reached a peak of 0.26 g C·m ^-2·mm ^-1 at 650 mm of precipitation and then showed a decreasing trend. The precipitation of the mountainous Nyingchi region, Xizang, is >845 mm, and the region is mainly covered with evergreen needleleaf forest. It has relatively high PUE ((0.210 ± 0.246) g C·m ^-2·mm ^-1) with a minimum CV of 117%. Temperature and precipitation together explained 93.1% of the spatial variation of PUE for Nyingchi. Precipitation was negatively correlated with PUE and its effect was 3.5 times that of temperature.

Key words: GLOPEM-CEVSA model, precipitation use efficiency (PUE), Qinghai-Xizang Plateau