Divergent Response of Dryland Primary Production to Intensified Rainfall

Amplification of hydrological cycle under warming climate is anticipated to result in intensified precipitation characterized by fewer, more intense events and correspondingly longer dry intervals between events, even without major changes in annual total precipitation. Vegetation gross primary production (GPP) in drylands is highly responsive to intensified precipitation, however, how intensified precipitation influences GPP in global drylands is not well under- stood. Based on multiple satellite datasets from 2001 to 2020 and in-situ measurements, we investigated the effects of intensified precipitation on global drylands GPP under diverse annual total precipitation along the bioclimate gradient. We further evaluated how extreme precipitation events with antecedent dry and wet water conditions influence carbon exchange along gradient of arid, semi-arid, and sub-humid ecosystems based on eddy covariance datasets. Dry, normal, and wet years were identified as years with annual precipitation anomalies below, within, and above the range of one standard deviation. Intensified precipitation led to increases or decreases of GPP during dry or normal years, respectively. However, such effects were largely weakened during wet years. The responses of GPP to intensified precipitation were mirrored by soil water availability, as intensified precipitation enhanced root zone soil moisture, and thus vegetation transpiration and precipitation use efficiency during dry years. During wet years, root zone soil moisture showed less response to changed precipitation intensity. Land cover types and soil texture regulated the magnitude of the effects along the bioclimate gradient. Under intensified precipitation, shrubland and grassland distributed in drier region with coarse soil texture showed greater increases of GPP during dry years. As global precipitation will likely further intensify, the impacts of intensified precipitation on dryland carbon uptake capacity will be highly diverse along the bioclimate gradients.

We further evaluated how extreme precipitation events (EPEs) with antecedent dry (dry-EPEs) and wet (wet-EPEs) water conditions influence carbon exchange along gradient of arid, semi-arid, and sub-humid ecosystems based on eddy covariance datasets. After EPEs, ecosystem respiration and gross primary productivity (GPP) were stimulated by pulses of soil moisture in arid and semi-arid regions, but suppressed by decreased soil temperature in sub-humid region. Antecedent water condition determined asynchronous response of ecosystem respiration and GPP to EPEs, and therefore fluctuations in net carbon balance. Net carbon uptake capacity was enhanced immediately following wet-EPEs because of more rapid and greater response of GPP than respiration. However, after dry-EPEs, net carbon uptake capacity decreased immediately and increased thereafter because the response of GPP to dry-EPEs lagged behind ecosystem respiration. More antecedent precipitation further stimulated accumulative net carbon uptake. In general, the accumulated net carbon uptake within 3 weeks after wet-EPEs was two times and seven times of that after dry-EPEs in arid and semi-arid region, respectively. In sub-humid region, ecosystems acted as a carbon sink after wet-EPEs, but as a carbon source after dry-EPEs. We conclude that antecedent water conditions and local climate regimes need to be considered when interpreting the response of carbon exchange to EPEs in dryland ecosystems.

It was demonstrated that major impacts of intensified and shifting rainfall occurred in dryland ecosystems along the farming-pastoral ecotones over Mongolian plateau and Africa drylands where rainfed agriculture and livestock grazing are especially vulnerable. The changing rainfall patterns have led to unexpected loss of cropland yields and compromised local food security. It is also linked to recent changes of landuse and management, especially conversion and shifting between rangelands and croplands along the ecotones.

Zhao, H., G. Jia*, X. Xu, A. Zhang, H. Wang, 2023: Divergent effects of intensified precipitation on primary production in global drylands, Science of the Total Environment, 892, 164736, doi: 10.1016/j.scitotenv.2023.164736.

Zhao, H., X. Xu, G. Jia*, A. Zhang, H. Wang, 2022: Antecedent water condition determines carbon exchange response to extreme precipitation events across global drylands, Theoretical and Applied Climatology, 149, 1705-1715, doi: 10.1007/s00704-022-04134-0.