Abstract
Vegetation light use efficiency (LUE) is a key parameter of Production Efficiency Models (PEMs) for simulating gross primary production (GPP) of vegetation, from regional to global scales. Previous studies suggest that grasslands have the largest inter-site variation of LUE and controlling factors of grassland LUE differ from those of other biomes, since grasslands are usually water-limited ecosystems. Combining eddy covariance flux data with the fraction of photosynthetically active radiation absorbed by the plant canopy from MODIS, we report LUE on a typical steppe and a desert steppe in Inner Mongolia, northern China. Results show that both annual average LUE and maximum LUE were higher on the desert steppe (0.51 and 1.13 g C MJ−1) than on the typical steppe (0.34 and 0.88 g C MJ−1), despite the higher GPP of the latter. Water availability was the primary limiting factor of LUE at both sites. Evaporative fraction (EF) or the ratio of actual evapotranspiration to potential evapotranspiration (AET/PET) can explain 50–70% of seasonal LUE variations at both sites. However, the slope of linear regression between LUE and EF (or AET/PET) differed significantly between the two sites. LUE increased with the diffuse radiation ratio on the typical steppe; however, such a trend was not found for the desert steppe. Our results suggest that a biome-dependent LUEmax is inappropriate, because of the large inter-site difference of LUEmax within the biome. EF could be a promising down-regulator on grassland LUE for PEMs, but there may be a site-specific relationship between LUE and EF.
Highlights
Grasslands constitute approximately 40% of earth’s terrestrial land area, excluding areas of permanent ice cover [1]
For the majority of Production Efficiency Models (PEMs) developed in the past, light use efficiency (LUE) is calculated by multiplying a potential value (LUEmax) by modifiers representing the effects of environmental stressors [7,12]
We aim to test the following hypotheses: a) LUE should be higher on the typical steppe than on the desert steppe; b) water availability is more important than temperature in regulating LUE dynamics on the two steppes, since grasslands are usually characterized as water-limited ecosystems; c) responses of LUE to environmental factors vary significantly between the two sites
Summary
Grasslands constitute approximately 40% of earth’s terrestrial land area, excluding areas of permanent ice cover [1]. Production Efficiency Models (PEMs) that use the light use efficiency (LUE) concept have the most potential to address spatiotemporal dynamics of GPP, because of their theoretical basis and practicality [6,7,8]. With this method, GPP is defined as the product of photosynthetically active radiation absorbed by the plant canopy (PARa) and a conversion factor, LUE [9,10,11]. The LUEmax value and modifiers differ greatly in PEMs, as listed in Yuan et al [7] and Xiao [12] This indicates that spatiotemporal variation and controlling factors of LUE remain poorly understood. Relatively few studies have been conducted in grasslands of China, half the area of temperate grassland on the Eurasian continent is within this region [16]
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