Observations of a longer growing season through earlier plant growth in temperate to polar regions have been thought to be a response to climate warming1,2,3,4,5. However, data from experimental warming studies indicate that many species that initiate leaf growth and flowering earlier also reach seed maturation and senesce earlier, shortening their active and reproductive periods6,7,8,9,10. A conceptual model to explain this apparent contradiction11, and an analysis of the effect of elevated CO2—which can delay annual life cycle events12,13,14—on changing season length, have not been tested. Here we show that experimental warming in a temperate grassland led to a longer growing season through earlier leaf emergence by the first species to leaf, often a grass, and constant or delayed senescence by other species that were the last to senesce, supporting the conceptual model. Elevated CO2 further extended growing, but not reproductive, season length in the warmed grassland by conserving water, which enabled most species to remain active longer. Our results suggest that a longer growing season, especially in years or biomes where water is a limiting factor, is not due to warming alone, but also to higher atmospheric CO2 concentrations that extend the active period of plant annual life cycles.
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The following individuals contributed to the installation and maintenance of the Prairie Heating and CO2 Enrichment (PHACE) project: D. Smith, D. Blumenthal, E. Pendall, E. Hardy, L. Griffith, A. Hansen, K. Corp, V. Banuelos, G. Tinnin, M. West, C. Brooks, M. Busick, D. Milchunas, G. Dunn and L. Ahuja. Funding for this work was supported by the US Department of Agriculture Agricultural Research Center (USDA-ARS) Climate Change, Soils & Emissions Program, by the US Department of Energy’s Office of Science through the Terrestrial Ecosystem Science Program, by the National Science Foundation (DEB no. 1021559) and by Colorado State University. D. Inouye provided comments that improved the manuscript.
The authors declare no competing financial interests.
Extended data figures and tables
a, b, Seasonal variation in precipitation and air temperature for the study site (a) and cumulative growing degree days (GDD) in control and warmed plots, averaged across CO2 levels (b) (means, n = 10 plots). Mean annual temperature (MAT) and total annual precipitation (TAP) are listed each year.
Values are means ± 1 s.e.m. for soil depth 5–25 cm (n = 5 plots). Mean annual soil water content (SWC) for control is represented by the horizontal grey line; vertical dashed lines show reproductive season timing for control.
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Reyes-Fox, M., Steltzer, H., Trlica, M. et al. Elevated CO2 further lengthens growing season under warming conditions. Nature 510, 259–262 (2014). https://doi.org/10.1038/nature13207
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