Distinguishing the main climatic drivers to the variability of gross primary productivity at global FLUXNET sites

Abstract

Climate exerts both short-term and long-term impacts on the ecosystem carbon assimilation. However, the main climatic drivers for the variability of gross primary productivity (GPP) remain unclear across various timescales and vegetation types. Here, we combine the state-of-the-art machine learning algorithms with a well-established explanatory method to explore the impacts of climatic factors on long-term GPP variability at global FLUXNET sites across four timescales and six plant functional types. Results showed that diffuse shortwave radiation (SWdif) dominates GPP variability at the sub-daily (half-hourly to 3-hourly) timescales especially for the tree species, and acts as the secondary contributor after air temperature at the daily or longer timescales. Attribution analyses further showed that the main effects of SWdif are much higher than their interactive effects with other climatic factors in regulating the GPP variability. By identifying the main climatic drivers, this study improves the understanding of the climate-driven GPP variability and provides important implications for the future projection of ecosystem carbon assimilation under global climate change.

Impact of diffuse radiation on the coupling of carbon and water fluxes in the grassland of northeastern China

“…Our analyses revealed larger increasing rate of GPP than ET in response to diffuse radiation, consistent with previous studies. Wang et al (2023) also found that the GPP of grassland was 1.15 ± 0.16 times higher under diffuse radiation compared to direct radiation, based on flux measurements from global FLUXNET sites. Meanwhile, they observed no significant differences in ET between the two radiation conditions, with a ratio of only 1.00 ± 0.13.…”

“…Instead, our analyses suggested that ET responses to K d were nonlinear due to the joint modulation of total PAR. At low to medium PAR levels, ET increased with K d due to the increased plant photosynthesis, which promoted ecosystem transpiration through carbon-water coupling (Strada et al 2015, Wang et al 2023. However, at the high PAR levels, ET decreased with the increase of K d at both sites.…”

“…For each radiation category, we aggregated the data points into PAR bins of 40 W m −2 s −1 at the site level, ensuring that comparisons between diffuse and direct conditions were made at equivalent PAR levels. The differences in carbon/water fluxes between the two groups were mainly attributed to the effects of diffuse radiation at hourly timescales (Zhou et al 2023).…”

“…This may be attributed to leaf photosynthesis nearing saturation with increased light, transitioning from light limitation to influences from other factors such as temperature content. Meanwhile, ET tended to decrease with K d at high PAR levels by −0.004 ± 0.002 mm h −1 (−2.2 ± 1.2%) per 0.1 increase in K d , possibly reflecting changes in intercepted water and soil evaporation due to reduced direct radiation (Wang et al 2023). The WUE showed nonlinear responses to K d with initial increase of WUE by K d but then decreasing WUE with the further increase of K d .…”

“…The ET consists of not only transpiration from plants but also evaporation from the soil and water surface (Zhang et al 2022). The latter components are not directly linked to GPP and thus can lead to a decoupling between ET and GPP, affecting the ecosystem's water-use efficiency (WUE) under varying environmental conditions (Wang et al 2023). The regulation of both ET and GPP involves a complex interplay of environmental factors such as radiation, temperature, water vapor pressure deficit and so on (Zhang et al 2014, Grossiord et al 2020, Wan et al 2023.…”