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Effect of Elevated CO2 on Leaf Senescence of Populus Trees
Taylor, G., Tallis, M.J., Giardina, C.P., Percy, K.E., Miglietta, F., Gupta, P.S., Gioli, B., Calfapietra, C., Gielen, B., Kubiske, M.E., Scarascia-Mugnozza, G.E., Kets, K., Long, S.P. and Karnosky, D.F. 2008. Future atmospheric CO2 leads to delayed autumnal senescence. Global Change Biology 14: 264-275.

What was done
The authors "present evidence from two continents and over two years that increasing atmospheric CO2 acts directly to delay autumnal leaf coloration and leaf fall," based on data obtained from freely-rooted, field-grown closed-canopy Populus trees in two Free-Air CO2 Enrichment (FACE) Experiments: AspenFACE near Rhinelander, Wisconsin, USA, and PopFACE in Tuscania, Italy.

What was learned
In response to an approximate 45% increase in atmospheric CO2 concentration, Taylor et al. report that "across the two FACE experiments, elevated CO2 enhanced late season leaf area index by 20-50% compared with ambient CO2." They also report that in Wisconsin, "elevated CO2 extended autumn leaf retention by 10-40% in pure stands of tembling aspen (P. tremuloides), by 8-48% in mixed stands of aspen and birch (Betula papyrifera), and by 17-32% in mixed stands of aspen and maple (Acer saccharum)." In addition, they "found no relationship between end-of-season leaf area index and temperature summed as growing degree days and precipitation amount." Last of all, the fourteen researchers state that "late season carbon uptake was stimulated in elevated CO2 at both sites for all species," making specific note of the fact that late autumn stimulation of photosynthesis was between 30% (aspen clone 42E) and 86% (aspen clone 271) on 6 October 2004 due to "extended canopy greenness," and approximately 300% in clone 271 on 12 October 2004.

What it means
In discussing their results, Taylor et al. say they indicate that "delayed autumnal senescence may occur in forests as a direct response to elevated CO2, independent of temperature," and that their "late-season measurements of photosynthesis indicate that function of the canopy was retained and carbon uptake maintained" throughout the CO2-induced extension of the growing season. Most importantly, they add that "an extension in the growing season through increased canopy longevity and carbon gain may provide an increased sink for atmospheric carbon," citing the work of Keeling et al. (1996) and Lucht et al. (2002), while additionally noting that "the need to incorporate a dynamic growing season length in predictive models of forest productivity has previously been identified," citing the work of White et al. (1999). Taken together, these several studies reveal another important negative feedback -- which is currently not included in state-of-the-art climate models -- that can temper the rate of rise of the air's CO2 content.

Keeling, C.D., Chin, J.F.S. and Whorf, T.P. 1996. Increased activity of northern vegetation inferred from atmospheric CO2 measurements. Nature 382: 146-149.

Lucht, W., Prentice, C.I., Myneni, R.B. et al. 2002. Climatic control of the high-latitude vegetation greening trend and Pinatubo effect. Science 296: 1687-1688.

White, M.A., Running, S.W. and Thornton, P.E. 1999. The impact of growing-season length on carbon assimilation and evapotranspiration over 88 years in the eastern U.S. deciduous forest. International Journal of Biometeorology 42: 139-145.

Reviewed 21 May 2008