Exploring complex water stress–gross primary production relationships: Impact of climatic drivers, main effects, and interactive effects

Abstract

The dominance of vapor pressure deficit (VPD) and soil water content (SWC) for plant water stress is still under debate. These two variables are strongly coupled and influenced by climatic drivers. The impacts of climatic drivers on the relationships between gross primary production (GPP) and water stress from VPD/SWC and the interaction between VPD and SWC are not fully understood. Here, applying statistical methods and extreme gradient boosting models—Shapley additive explanations framework to eddy‐covariance observations from the global FLUXNET2015 data set, we found that the VPD‐GPP relationship was strongly influenced by climatic interactions and that VPD was more important for plant water stress than SWC across most plant functional types when we removed the effect of main climatic drivers, e.g. air temperature, incoming shortwave radiation and wind speed. However, we found no evidence for a significant influence of elevated CO2 on stress alleviation, possibly because of the short duration of the records (approximately one decade). Additionally, the interactive effect between VPD and SWC differed from their individual effect. When SWC was high, the SHAP interaction value of SWC and VPD on GPP was decreased with increasing VPD, but when SWC was low, the trend was the opposite. Additionally, we revealed a threshold effect for VPD stress on GPP loss; above the threshold value, the stress on GPP was flattened off. Our results have important implications for independently identifying VPD and SWC limitations on plant productivity, which is meaningful for capturing the magnitude of ecosystem responses to water stress in dynamic global vegetation models.

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

“…This method directly regulates the effects of each feature on model losses or gains using the differences between predicted and expected values (Fryer et al 2021). Recently, the XGB model was applied as a foundation for the SHAP-based interpretations (Lundberg et al 2020), and was used to address problems in public health (Nohara et al 2022), socioeconomic sciences (Mao et al 2021), and ecosystems (Wang et al 2022b). In this study, we combine the SHAP method with the XGB model to quantify the contributions of individual climatic factors and their interactive effects to GPP variability across different timescales and PFTs at global FLUXNET sites.…”

“…For example, once there are two features with 99% of correlation, decision tree will choose only one of them as statistic feature within the processes of deciding upon a split. Recently, the XGB method was used to address problems in atmospheric environment (Park et al 2021) and terrestrial ecosystems (Wang et al 2022b) and showed better potentials than traditional methods. Here, we apply the XGB model to reproduce site-level GPP based on eight climatic drivers including TA, diffuse radiation (SWdif), direct radiation (SWdir), VPD, CO 2 , WS, air pressure (P) and PRE.…”

“…At the short-term timescales, atmospheric vapor pressure deficit (VPD) usually controls GPP variability as an important factor via instantaneous modulations of plant stoma and photosynthesis (Fu et al 2022). For example, Wang et al (2022b) found that VPD contributes the highest importance to GPP variability at the growing seasons using datadriven models. At the longer timescales, soil moisture instead of VPD dominates plant carbon assimilations by altering total water availability in plant roots (Green et al 2019).…”

Satellite-Detected Contrasting Responses of Canopy Structure and Leaf Physiology to Drought

“…Main impacts of drought on plant photosynthesis are mediated through changes in canopy structure and leaf physiology [4], such as increases in leaf abscission and senescence that reduce the area of transpiration and associated water demand and increase the risks of xylem embolism and plant desiccation [5], and closure of stomata and inhibition of photosynthetic enzyme activity [6,7], respectively. Thus, changes in photosynthetically active radiation (PAR) and carbon dioxide (CO2) assimilation rates due to canopy structure and leaf physiology responses to drought conditions co-determine rates of ecosystem photosynthesis [8].…”

Interactive effects of UV radiation and water deficit on production characteristics in upland grassland and their estimation by proximity sensing

“…However, additive or multiplicative effects are often observed at high UV doses and under severe drought conditions (Bandurska et al, 2013; Jansen et al, 2022). The direction of interactive effects usually depends on the intensity of the dominant driver, and there is often a marked nonlinearity in plant responses, sometimes with unexpected “tipping points” (Jansen et al, 2022; Wang et al, 2022). Complex responses, including hermetic effects, may be involved even at low doses of stress factors (Erofeeva, 2022).…”