The shift of autumnal colouring of leaves to later in the year is due to high ambient atmospheric carbon dioxide concentrations and is independent of temperature.
Over the last 30 years, leaves have started to change colour and fall later in the year, a phenomenon that scientists can now attribute directly to rising levels of atmospheric carbon dioxide.
Gail Taylor from the University of Southampton and colleagues1 studied the growth and leaf fall of Populus trees — a genus which includes aspen and poplars — growing in Tuscania and Wisconsin from 2003 to 2004. The trees were grown in plots under either current or elevated atmospheric carbon dioxide levels and their colour was monitored using remotely sensed images of canopy greenness. Leaves turned yellow later in the year under higher carbon dioxide concentrations, even when exposed to the same temperatures. The researchers think that the change in leaf colour is probably due to the effect of carbon dioxide on plant physiology.
Whereas earlier springtime leaf growth is strongly related to temperature, the belated autumn leaf fall — previously inexplicably — is not. Deciduous trees are staying greener for longer than they were 30 years ago owing to the earlier arrival of new leaves and later leaf falls.
References
Taylor, G. et al. Future atmospheric CO2 leads to delayed autumnal senescence. Global Change Biol. doi:10.1111/j.1365-2486.2007.01473.x (2007).
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Thompson, A. Late leaf fall. Nature Geosci (2007). https://doi.org/10.1038/ngeo.2007.61
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DOI: https://doi.org/10.1038/ngeo.2007.61
