Drought and heat wave impacts on grassland carbon cycling across hierarchical levels

Authors: LI, L.F. - Chinese Academy Of Sciences; ZHENG, Z.Z. - Chinese Academy Of Sciences; Biederman, Joel; QIAN, R. - Chinese Academy Of Sciences; ZHANG, B. - Chinese Academy Of Sciences; CHE, R.X. - Chinese Academy Of Sciences; WANG, F. - Chinese Academy Of Sciences; XU, Z.H. - Chinese Academy Of Sciences; CUI, X.Y. - Chinese Academy Of Sciences; HAO, Y.B. - Chinese Academy Of Sciences; WANG, Y. - Chinese Academy Of Sciences

Submitted to: Plant, Cell & Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/10/2020
Publication Date: 4/10/2020
Citation: Li, L., Zheng, Z., Biederman, J.A., Qian, R., Zhang, B., Che, R., Wang, F., Xu, Z., Cui, X., Hao, Y., Wang, Y. 2020. Drought and heat wave impacts on grassland carbon cycling across hierarchical levels. Plant, Cell & Environment. 1-12. https://doi.org/10.1111/pce.13767.
DOI: https://doi.org/10.1111/pce.13767

Interpretive Summary: Semiarid grasslands provide key ecosystem services including biomass for grazing and habitat. Climate predictions suggest increasingly frequent extreme events, such as drought or heat waves. While these events have been studied individually, we lack understanding of grassland response to combined drought and heat waves (hot drought). We conducted a four-year manipulative field experiment to test the effects of drought, heat wave, and hot drought on grassland plots in Inner Mongolia, China. While above-ground grass productivity was not altered, heat wave or hot drought decreased root production, reducing future grass resilience. Carbon dioxide uptake from the atmosphere was curtailed by drought, whether or not a heat wave occurred. Drought alone reduced the four-year net carbon sink from the atmosphere by 30%, while hot drought reduced the carbon sink by 47%. These results demonstrate the importance of considering how drought and heat wave combine to impact grassland structure and function.

Technical Abstract: Climate extremes including heat waves and drought are occurring with increasing magnitude and frequency, altering the carbon cycle from ecosystem to continental scales. However, neither the magnitude of carbon cycle impacts nor the underlying mechanisms is currently clear, especially when different types of extreme events co-occur. Here, we conducted a 4-year field experiment in which we imposed extreme drought and heat waves (~60-year recurrence) in a full factorial design to examine both their individual and interactive effects on carbon cycling of a fenced semiarid grassland of Inner Mongolia, China. We measured response parameters across multiple hierarchical levels including individual- (leaf photosynthetic physiology), community- (leaf area index (LAI), aboveground and belowground net primary productivity (ANPP and BNPP) and community structure) and ecosystem-level (ecosystem CO2 fluxes). Surprisingly, BNPP dominated responses at the community level, consisting of large reductions following heat wave but non-significant increases from drought, while ANPP and community structure remained stable through four years of repeated extreme events. Drought largely suppressed CO2 fluxes at both individual- and ecosystem-levels, while heat wave had little effects. Additive negative but not interactive effects of drought and heat wave were found on leaf photosynthesis, ANPP, LAI and ecosystem CO2 fluxes. Drought alone reduced the total four-year net carbon sink (NEE) by 30%, while co-occurrence of a heat wave increased the magnitude of sink reduction to 47%. Structural equation model analysis across hierarchical levels showed that climate extremes-induced changes in ecosystem-scale CO2 uptake are regulated primarily by individual-level photosynthesis and secondarily by community-level leaf area index and productivity. Overall, our study quantifies how the negative impacts of drought on carbon cycle are exacerbated when combined with a heat wave and identifies that community composition as well as plant physiological and structural responses moderate grassland carbon sink strength under climate extremes.