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Seasonal not annual rainfall determines grassland biomass response to carbon dioxide

Abstract

The rising atmospheric concentration of carbon dioxide (CO2) should stimulate ecosystem productivity, but to what extent is highly uncertain, particularly when combined with changing temperature and precipitation1. Ecosystem response to CO2 is complicated by biogeochemical feedbacks2 but must be understood if carbon storage and associated dampening of climate warming are to be predicted3. Feedbacks through the hydrological cycle are particularly important4 and the physiology is well known; elevated CO2 reduces stomatal conductance and increases plant water use efficiency (the amount of water required to produce a unit of plant dry matter)5. The CO2 response should consequently be strongest when water is limiting6; although this has been shown in some experiments7, it is absent from many8,9,10,11. Here we show that large annual variation in the stimulation of above-ground biomass by elevated CO2 in a mixed C3/C4 temperate grassland can be predicted accurately using seasonal rainfall totals; summer rainfall had a positive effect but autumn and spring rainfall had negative effects on the CO2 response. Thus, the elevated CO2 effect mainly depended upon the balance between summer and autumn/spring rainfall. This is partly because high rainfall during cool, moist seasons leads to nitrogen limitation, reducing or even preventing biomass stimulation by elevated CO2. Importantly, the prediction held whether plots were warmed by 2 °C or left unwarmed, and was similar for C3 plants and total biomass, allowing us to make a powerful generalization about ecosystem responses to elevated CO2. This new insight is particularly valuable because climate projections predict large changes in the timing of rainfall, even where annual totals remain static12. Our findings will help resolve apparent differences in the outcomes of CO2 experiments and improve the formulation and interpretation of models that are insensitive to differences in the seasonal effects of rainfall on the CO2 response7,13,14.

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Figure 1: Annual elevated CO2 effect, seasonal soil water potential and rainfall in the TasFACE experiment.
Figure 2: The influence of seasonal rainfall variation on the elevated CO2 effect.
Figure 3: The impact of seasonal rainfall balance on the elevated CO2 effect.
Figure 4: The impact of spring rainfall on biomass nitrogen and soil mineral nitrogen content.

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Acknowledgements

This work was supported by grants from the Australian Research Council Discovery Projects scheme. We thank the Australian Department of Defence for access to the Pontville Small Arms Range Complex. J. Vander Schoor, A. Williams, J. Janes, J. Buettel and M. Porter provided technical assistance. R. Mallett assisted with statistical analyses and L. Barmuta provided advice on multimodel inference.

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Authors and Affiliations

Authors

Contributions

M.J.H. and P.C.D.N. conceived the TasFACE experiment, M.J.H. maintained and managed the experiment. K.E.W. assisted with the design and implementation of the biomass estimates and did preliminary analyses. M.J.H. and P.C.D.N. analysed the results and wrote the paper.

Corresponding author

Correspondence to Mark J. Hovenden.

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Competing interests

The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Seasonal rainfall during the study period compared with seasonal rainfall for the previous 100 years.

The seasonal rainfall during the study period is shown by filled data points, with the mean and interquartile values shown in the box plot. Error bars indicate the fifth and ninety-fifth percentile ranges; crosses indicate maximum and minimum recorded values from the previous 100 years. Historical values were obtained from the nearby Australian Bureau of Meteorology weather station at Bagdad.

Extended Data Figure 2 The impact of seasonal rainfall balance on the elevated CO2 effect on biomass of C3 vegetation only.

The mean (n = 6 replicate plots) annual elevated CO2 effect on above-ground biomass of C3 plants only as a function of the seasonal rainfall balance, which is defined as the difference between summer rainfall and the sum of autumn and spring rainfall. Spring rainfall totals were halved in determining the seasonal rainfall balance as the multimodel estimates indicated that the effect of spring rainfall was approximately half that of the other seasons. Relatively more rainfall in summer gives a positive rainfall balance value, whereas a negative rainfall balance occurs when more rain falls in autumn and spring. The solid line and associated r2 value are the result of a linear regression analysis (F1,6 = 18.1, P < 0.006).

Extended Data Table 1 Model coefficients and performance for CO2 effect on above-ground biomass

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Hovenden, M., Newton, P. & Wills, K. Seasonal not annual rainfall determines grassland biomass response to carbon dioxide. Nature 511, 583–586 (2014). https://doi.org/10.1038/nature13281

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