Long-term trends of Gross Primary Productivity (GPP) and its drivers: A Comprehensive Analysis Based on Flux Measurements

This study aims to investigate the interplay of various biotic and abiotic factors on Gross Primary Productivity (GPP). Meteorological and flux observations from 71 global sites (1991-2014) obtained from FLUXNET, along with corresponding remotely sensed Fraction of Absorbed Photosynthetically Active Radiation (FPAR) data, were analyzed. The data encompassed 11 vegetation types based on the IGBP classification, including evergreen needleleaf forests, evergreen broadleaf forests, and deciduous broadleaf forests. Using the Mann-Kendall trend analysis method, trends in various parameters were extracted. Among the 71 sites, 66 sites showed significant trends in GPP, and 36 sites exhibited significant increases in GPP, accompanied by mostly increasing trends in Light Use Efficiency (LUE), except for one grassland site with a declining LUE. The trends in GPP and LUE were significantly related with a correlation coefficient of approximately 0.44. FPAR increased in some sites but decreased in 40% of the cases, while only one evergreen needleleaf forest site showed a decreasing trend in CO2 concentration.  GPP and LUE were insignificantly correlated with meteorological factors (such as Air temperature (Ta), vapor pressure deficit (VPD), precipitation, and soil moisture (SM)), indicating that these environmental factors are not the main drivers. FPAR exhibited minimal changes, and the insignificant correlation between GPP and FPAR trends suggested that the increase in GPP is not solely driven by Leaf Area Index (LAI). The strong negative correlation between LUE and FPAR trends (R2 ≈ 0.31) implies that increasing LAI may decrease LUE. Additionally, a significant positive correlation between the trends of CO2 and LUE (R2 ≈ 0.19) suggests that the fertilization effect of CO2 promotes LUE, thereby promoting the increase in GPP. However, the trend in GPP shows a negative correlation with the trend in CO2 concentration, indicating that higher CO2 levels may limit the extent of GPP increase, possibly due to changes in the allocation of photosynthetic products. Further analysis is needed to understand the driving factors, especially for sites where GPP and LUE are decreasing without clear correlations with FPAR, CO2 concentration, and other meteorological factors.