Ecosystem CO2 flux responses to extreme droughts depend on interaction of seasonal timing and plant community composition

Authors: ZHENG, ZHENZHEN - University Of Chinese Academy Of Sciences; LI, LINFENG - Fujian Agricultural & Forestry University; Biederman, Joel; WANG, YANFEN - University Of Chinese Academy Of Sciences; GUAN, SHUNTIAN - University Of Chinese Academy Of Sciences; LI, CONGJIA - University Of Chinese Academy Of Sciences; WEN, FUQI - University Of Chinese Academy Of Sciences; LIU, YUAN - University Of Chinese Academy Of Sciences; XIONG, YUNQI - University Of Chinese Academy Of Sciences; QIAN, RUYAN - University Of Chinese Academy Of Sciences; DU, JIANQING - University Of Chinese Academy Of Sciences; XUE, KAI - University Of Chinese Academy Of Sciences; CUI, XIAOYONG - University Of Chinese Academy Of Sciences; HAO, YANBIN - University Of Chinese Academy Of Sciences

Submitted to: Journal of Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/24/2024
Publication Date: 7/23/2024
Citation: Zheng, Z., Li, L., Biederman, J.A., Wang, Y., Guan, S., Li, C., Wen, F., Liu, Y., Xiong, Y., Qian, R., Du, J., Xue, K., Cui, X., Hao, Y. 2024. Ecosystem CO2 flux responses to extreme droughts depend on interaction of seasonal timing and plant community composition. Journal of Ecology. 112(10):2198-2211. https://doi.org/10.1111/1365-2745.14374.
DOI: https://doi.org/10.1111/1365-2745.14374

Interpretive Summary: Long seasonal droughts (consecutive days without rain) are becoming increasingly common. While the impacts of long droughts are usually negative for plant production and carbon sequestration, the severity of impacts depends to an unknown degree on the timing of the drought within the growing season and the plant community composition. Here we imposed experimental droughts at different periods of the growing season on plots with grass only, shrubs only, and a mixed grass-shrub community. We found that drought early or late in the growing season decreased plant carbon uptake and productivity mainly by shortening the growing season duration by 5-10 days. Drought in the mid growing season, however, had the most negative impacts, due to occurring at a time of greatest heat stress and impacting plants at their peak activity level. Our results highlight the importance of drought timing in addition to the commonly-reported seasonal rainfall total.

Technical Abstract: Droughts can affect ecosystem CO2 fluxes directly or indirectly by changing plant community composition. However, it is unknown whether shifts in plant community composition buffer or amplify the response of ecosystem CO2 fluxes to droughts with different seasonal timing, as plant phenology and physiology of the different plant functional types respond differently to droughts. To identify the interaction of drought timing and plant community composition in regulating ecosystem CO2 fluxes, we conducted a three-year manipulative experiment in which extreme droughts occurring in the early, mid and late growing seasons were separately imposed on experimental plot communities comprising graminoids, shrubs and their combination in a semi-arid grassland of Inner Mongolia, China. Overall, mid-season drought caused the largest negative effects regardless of plant community composition. In addition to decreasing aboveground biomass, mid-season drought suppressed fluxes by reducing leaf photosynthetic rate, while early-season and late-season drought reduced fluxes mainly by shortening growing season length. All three community compositions had consistent responses to early-season and mid-season droughts. However, ecosystem CO2 fluxes in the combination community were less negatively affected by late-season drought than in either shrub or graminoid communities because the growing season length was shortened less. Synthesis. Our results highlight that it is important to account for interactions of seasonal timing and plant community composition when predicting magnitude and pathways of drought effects on ecosystem carbon cycling.