Long-term fluctuations in atmospheric CO<inf>2</inf> concentration influence plant speciation rates

© The Systematics Association 2011. The influence of vascular land-plant evolution on long-term fluctuations in atmospheric carbon dioxide (CO2) concentration has been much discussed. However, the direct evolutionary consequences of changes in past atmospheric CO2 concentration have not been widely explored despite experimental evidence showing that elevated CO2 can intensify interplant competition, alter plant reproductive biology, and thus potentially influence plant speciation. In this chapter, we put forward the hypothesis that changes in atmospheric CO2 may have directly enhanced vascular plant speciation rates in the course of land-plant evolution, particularly in the late Palaeozoic when numerous pteridophyte and gymnosperm lineages were radiating rapidly (between approximately 400 and 240 million years ago – mya). Our hypothesis is based on the observation of significant correlations between land-plant speciation rates and long-term records on fluctuations in atmospheric CO2 levels over the past 410 million years. The relationship is complex, however, with a strong positive correlation between gymnosperm and pteridophyte speciation rates and CO2 concentration above 1000 ppmv, but a weak negative correlation between angiosperm speciation rates and atmospheric CO2 levels. These results suggest that fundamental plant evolutionary responses to atmospheric CO2 may be dependent on evolutionary grade. Model estimates of palaeo-atmospheric CO2 values are subject to considerable uncertainties, as are large-scale literature compilations of fossil plant speciation rates through geological time. Future collections of plant speciation data and palaeo-CO2 estimates are therefore required to test this hypothesis further.