Abstract
Models constitute the primary approaches for predicting terrestrial ecosystem gross primary production (GPP) at regional and global scales. Many satellite-based GPP models have been developed due to the simple algorithms and the low requirements of model inputs. The performances of these models are well documented at the biome level. However, their performances among vegetation subtypes limited by different environmental stresses within a biome remains largely unexplored. Taking grasslands in northern China as an example, we compared the performance of eight satellite-based GPP models, including three light-use efficiency (LUE) models (vegetation photosynthesis model (VPM), modified VPM (MVPM), and moderate resolution imaging spectroradiometer GPP algorithm (MODIS-GPP)) and five statistical models (temperature and greenness model (TG), greenness and radiation model (GR), vegetation index model (VI), alpine vegetation model (AVM), and photosynthetic capacity model (PCM)), between the water-limited temperate steppe and the temperature-limited alpine meadow based on 16 site-year GPP estimates at four eddy covariance (EC) flux towers. The results showed that all the GPP models performed better in the alpine meadow, particularly in the alpine shrub meadow (R2 ≥ 0.84), than in the temperate steppe (R2 ≤ 0.68). The performance varied greatly among the models in the temperate steppe, while slight intermodel differences existed in the alpine meadow. Overall, MVPM (of the LUE models) and VI (of the statistical models) were the two best-performing models in the temperate steppe due to their better representation of the effect of water stress on vegetation productivity. Additionally, we found that the relatively worse model performances in the temperate steppe were seriously exaggerated by drought events, which may occur more frequently in the future. This study highlights the varying performances of satellite-based GPP models among vegetation subtypes of a biome in different precipitation years and suggests priorities for improving the water stress variables of these models in future efforts.
Highlights
Gross primary production (GPP), defined as the sum of photosynthetic carbon uptake by vegetation, is the first step in the input of atmospheric CO2 to terrestrial ecosystems [1,2]
The temperate steppe sites were characterized by a dry-warm climate, with a growing season precipitation and mean temperature of 219 mm and 14 ◦C, respectively, whereas the alpine meadow sites were dominated by a humid-cold climate, with precipitation and temperature of 465 mm and 6 ◦C, respectively
The drier climate in the temperate steppe compared with that in the alpine meadow can be viewed in terms of soil water content (SWC) (0.12 vs. 0.26 m3 m−3, respectively) and vapor pressure deficit (VPD) (9.75 vs. 3.13 hPa, respectively)
Summary
Gross primary production (GPP), defined as the sum of photosynthetic carbon uptake by vegetation, is the first step in the input of atmospheric CO2 to terrestrial ecosystems [1,2]. The first simulates the full biogeochemical fluxes (e.g., carbon, water, and nitrogen cycles) on the basis of calculating key ecological processes, such as canopy interception and evaporation, transpiration, photosynthesis, growth and maintenance respiration, carbon allocation above and below-ground, litterfall, decomposition, and nitrogen mineralization [12] They are usually driven by data, such as climate, soil, and land cover/use or vegetation specific parameters [13]. They are mainly driven by satellite observations, which are only available historically and cannot predict future GPP [14,17] Such models require few model inputs and parameters and can effectively address the spatial and temporal dynamics of GPP over large areas [17,18]. With advancements in remote sensing and spatial science, satellite-based models have been increasingly used to estimate GPP at various spatial scales in recent decades [9,17,18]
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