Repackaging precipitation into fewer, larger storms reduces ecosystem exchanges of CO2 and H2O in a semiarid steppe

Authors: LIU, W.J. - Chinese Academy Of Sciences; LI, L.F. - Chinese Academy Of Sciences; Biederman, Joel; HAO, Y.B. - Chinese Academy Of Sciences; KANG, X.M. - Chinese Academy Of Forestry; CUI, X.Y. - Chinese Academy Of Sciences; WANG, Y.F. - Chinese Academy Of Sciences; LI, M.W. - Chinese Academy Of Sciences; XU, Z.H. - Griffiths University; XU, C.Y. - Queensland University - Australia

Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/21/2017
Publication Date: 10/6/2017
Citation: Liu, W., Li, L., Biederman, J.A., Hao, Y., Kang, X., Cui, X., Wang, Y., Li, M., Xu, Z., Xu, C. 2017. Repackaging precipitation into fewer, larger storms reduces ecosystem exchanges of CO2 and H2O in a semiarid steppe. Agricultural and Forest Meteorology. 247:356-364. https://doi.org/10.1016/j.agrformet.2017.08.029.
DOI: https://doi.org/10.1016/j.agrformet.2017.08.029

Interpretive Summary: Current predictions suggest that future precipitation may be “packaged” into fewer, larger storms, with the total seasonal rainfall amount relatively unchanged. It is unknown how semiarid grasslands will respond to these larger storms with longer dry intervals. Therefore we conducted a four-years experiment in a semiarid grassland steppe in Northern China. Experimental plots all received the same amount of rainfall each summer, but the rainfall was packaged into treatments of 6, 10, 16, or 24 storms per summer (24 is a historically typical number of storms). We found that reduced rainfall frequency and longer dry intervals significantly reduced the CO2 uptake. Plant productivity did not decline significantly, and we found that increased productivity by drought-adapted plants compensated for producitivity declines in more water-loving plants. These results imply that semiarid grasslands may be resilient to the temporal repackaging of precipitation in agroecosystems that are biologically diverse.

Technical Abstract: Global circulation models predict that precipitation patterns will become more extreme, i.e. seasonal rainfall events tend to be larger in size, but fewer in number. Studies in North American grasslands have shown that above-ground net primary productivity (ANPP) was enhanced by such repackaging of precipitation into fewer, larger events. However, ANPP responses in other regions remain poorly understood, and responses of carbon and water exchanges with the atmosphere remain unknown. Here we manipulated rainfall inputs in a steppe ecosystem in northern China over 4 years to investigate how temporal packaging of precipitation impacts ANPP, evapotranspiration (ET), net ecosystem CO2 exchange (NEE) and the component fluxes of NEE: gross ecosystem productivity (GPP) and ecosystem respiration (RE). Experimental plots received effective precipitation equivalent to the 60-year growing season (May - September) average of 240 mm, variously packaged into 6, 10, 16, or 24 events representing extreme (6 events) to historical average (24 events) rainfall frequency. Reduced precipitation frequency negatively affected NEE, GPP, RE, ET and water use efficiency (WUE=|NEE|/ET). The average NEE, GPP and RE were depressed by 34.6%, 44.5% and 47.5% respectively in the treatment with 6 precipitation events (P6) as compared to P16, which showed maximum ET and CO2 exchange. RE was more sensitive to altered precipitation patterns than NEE and GPP. Lower CO2 fluxes associated with reduced precipitation frequency were partly directly influenced by soil water content, soil temperature and leaf area index, with ET amplifying their interacting effects. ANPP was highly variable from one year to the next, and we detected no significant ANPP effects from altered precipitation frequency. This stable ANPP was attributed to a compensatory increase in productivity by eurytopic xerophyte and xerophyte plants. Our results suggest that extreme temporal repackaging of precipitation into few events with correspondingly long dry intervals may reduce the capacity of steppe ecosystems to assimilate atmospheric CO2 whereas plant productivity may be resilient in diverse communities.