Conifers dominated the lowland tropics 100 million years ago, but have now been largely replaced by angiosperms, coincident with a large decline in atmospheric CO2. Using greenhouse treatments of pre-industrial (280 ppm), ambient (400 ppm) and Eocene (800 ppm) CO2 levels, Dalling et al. demonstrate that seedlings of tropical conifers show stronger growth enhancement with increasing CO2 than angiosperms.
The conifers also exhibit greater water-use efficiency (WUE), reflecting both increased photosynthetic rate and reduced stomatal conductance under elevated CO2. High plasticity in photosynthetic and WUE traits may help account for the continued persistence of conifers despite dramatic changes in CO2 levels since the Eocene epoch.
James W. Dalling, Lucas A. Cernusak, Klaus Winter, Jorge Aranda, Milton Garcia, Aurelio Virgo, Alexander W. Cheesman, Andres Baresch, Carlos Jaramillo, Benjamin L. Turner, 2016, 'Two tropical conifers show strong growth and water-use efficiency responses to altered CO2 concentration', Annals of Botany, vol. 118, no. 6, pp. 1113-1125 http://dx.doi.org/10.1093/aob/mcw162