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Estimation of Global Grassland Net Ecosystem Carbon Exchange Using a Model Tree Ensemble Approach
Author(s) -
Liang Wei,
Zhang Weibin,
Jin Zhao,
Yan Jianwu,
Lü Yihe,
Wang Shuai,
Fu Bojie,
Li Shuai,
Ji Qiulei,
Gou Fen,
Fu Shuyi,
An Shantao,
Wang Fengjiao
Publication year - 2020
Publication title -
journal of geophysical research: biogeosciences
Language(s) - English
Resource type - Journals
eISSN - 2169-8961
pISSN - 2169-8953
DOI - 10.1029/2019jg005034
Subject(s) - grassland , environmental science , carbon sink , atmospheric sciences , ecosystem , shortwave radiation , photosynthetically active radiation , leaf area index , carbon cycle , terrestrial ecosystem , sink (geography) , grazing , photosynthesis , ecology , geography , radiation , botany , biology , physics , cartography , quantum mechanics
Understanding the net ecosystem CO 2 exchange (NEE) between terrestrial ecosystems and the atmosphere is crucial for accurate estimation of carbon budgets, which remains unclear for grassland ecosystems. Here we upscaled site‐level NEE from 44 grassland flux towers (1,457 site‐months) to the global scale by using a model tree ensemble approach that considers the management activities (grazing and cutting) (MTE‐GM). Cross‐validation showed that MTE‐GM performs reasonably well in terms of among‐site variability and seasonal variation, with a Nash‐Sutcliffe efficiency of 0.90 and 0.86 and an R 2 of 0.91 and 0.86, respectively. Radiation (shortwave and longwave), temperature, leaf area index, and fraction of absorbed photosynthetic active radiation had the highest relative explanatory power in predicting NEE. Based on MTE‐GM, mean annual NEE of global grassland was 72 ± 4 g C m −2 year −1 (1.9 ± 0.11 Pg C year −1 ) during 1982–2011, suggesting that the grassland ecosystems have been acting as a small carbon source during the past three decades. However, grasslands in temperate and continental regions had the largest carbon sink of −61.9 ± 5.7 and −51.8 ± 7.9 g C m −2 year −1 , respectively. Moreover, we found that elimination of grassland management effect resulted in an extra emission of 1.7% CO 2 to the atmosphere (CO 2 sink from the management is 0.03 Pg C year −1 ). From the 1980s to the 2000s, 38% (22%) and 17% (18%) of pixels showed an increased (decreased) carbon uptake and decreased (increased) carbon release, respectively. Uncertainty assessment suggested that there would be higher confidence in NEE estimates in most parts of middle‐ to high‐latitude regions in the Northern Hemisphere.