Globally consistent influences of seasonal precipitation limit grassland biomass response to elevated CO2

Authors: HOVENDEN, MARK - University Of Tasmania; LEUZINGER, SEBASTIAN - Auckland University Of Technology; NEWTON, PAUL - Agresearch; FLETCHER, ANDREW - Csiro European Laboratory; FATICHI, SIMONE - Eth Zurich; HOFMOCKEL, KIRSTEN - Iowa State University; REICH, PETER - University Of Minnesota; ANDRESEN, LOUISE - Desiderio Finamore Veterinary Research Institute (FEPAGRO); BEIER, CLAUS - University Of Copenhagen; Blumenthal, Dana

Submitted to: Nature
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/21/2018
Publication Date: 2/8/2019
Citation: Hovenden, M., Leuzinger, S., Newton, P., Fletcher, A., Fatichi, S., Hofmockel, K., Reich, P.B., Andresen, L.S., Beier, C., Blumenthal, D.M. 2019. Globally consistent influences of seasonal precipitation limit grassland biomass response to elevated CO2. Nature. 5:167-173. https://doi.org/10.1038/s41477-018-0356-x.
DOI: https://doi.org/10.1038/s41477-018-0356-x

Interpretive Summary: The rising atmospheric carbon dioxide concentration ([CO2]) should stimulate plant growth by two main mechanisms: direct biochemical stimulation of photosynthesis and indirectly via water savings caused by increased plant water use efficiency. Although water-use effects should strengthen with increasing site dryness, this relationship has only been apparent in some cases and absent from many, preventing useful generalisations. Here we show that the carbon fertilization effect (CFE) in eleven globally-distributed temperate grassland experiments does increase with increasing site dryness but that this effect is offset by an opposing influence of mean spring precipitation. We propose that there is little chance for indirect stimulation of ecosystem productivity in wet sites, but sites where spring rainfall is high relative to rainfall at other times of the year have plant communities biologically equipped to take maximum advantage of the direct effects of increased [CO2]. Using this relationship to project the CFE across temperate grasslands demonstrates substantial geographic variation in the potential for rising [CO2] to stimulate plant growth including negative responses. Observed greening patterns are consistent with our model but further experimentation is desirable and can now be targeted to particularly critical regions and locations using our CFE projections. Alterations in precipitation timing and amounts could determine the strength of the future CFE.

Technical Abstract: The rising atmospheric carbon dioxide concentration ([CO2]) should stimulate ecosystem productivity by two main mechanisms: direct biochemical stimulation of carbon assimilation and indirectly via water savings caused by increased plant water use efficiency. The direct stimulation can be limited by scarcity of other resources such as soil nutrients and water, reducing the total CO2 fertilisation effect (CFE). Contrastingly, the water-use effects should strengthen with increasing site dryness. This relationship between CFE and dryness has only been apparent in some cases and absent from many, preventing useful generalisations. Here we show that the CFE in eleven globally-distributed temperate grassland experiments does increase with increasing site dryness but that this effect is offset by an opposing influence of mean spring precipitation (r2=0.826, P<0.005). Mean CFE across all experiments was 9.1%, which increased by two-thirds with every 100 mm increase in site spring rainfall but was reduced by one-third with each 100 mm increase in rainfall at other times of the year. We propose that there is little chance for indirect stimulation of ecosystem productivity in wet sites, but sites where spring rainfall is high relative to rainfall at other times of the year have plant communities biologically equipped to take maximum advantage of the direct effects of increased [CO2]. These competing effects explain why previous analyses were unable to discern a reliable trend between site dryness and the CFE. Using this relationship to project the CFE across temperate grasslands demonstrates substantial geographic variation in the potential for rising CO2 to stimulate ecosystem productivity including negative responses. Observed greening patterns7 are consistent with our model but further experimentation is desirable and can now be targeted to particularly critical biomes and locations using our CFE projections. Alterations in precipitation timing and amounts could determine the strength of the future CFE.