HOW MUCH HAVE CHANGES IN ATMOSPHERIC CO2 CONCENTRATION IMPACTED LAND PLANT EVOLUTION?

Land plant evolution has strongly influenced atmospheric CO₂ levels, but speculation has also proceeded in the reverse direction with CO₂ fluctuations considered an important driver of broad patterns in plant evolution. For example, the evolution of leaves, of flowering plants, and of specific flowering plant traits have all been interpreted as responses to declining CO₂ concentrations. These hypotheses are difficult to falsify: unique events in plant evolution are roughly matched up with features in a slowly fluctuating CO₂ curve, leaving no room for a priori testing. This is particularly problematic due to the focus on grand events in the traditional narrative of plant evolution near the beginning and end of the land plant fossil record--when CO₂ levels were at historically high and low levels for this time period. However, an extrinsic driver like atmospheric CO₂ should affect all plants, not just the main protagonists in the evolutionary narrative. Thus, for example, multiple individual Paleozoic clades can be investigated to determine whether CO₂ impacted the early radiation of vascular plant form and the place to investigate the impact of CO₂ on angiosperm evolution is not the angiosperms themselves, but the ferns and other lineages that also should have been affected by Cretaceous declines in CO₂. The results of such tests with Paleozoic and Mesozoic plants are not consistent with any significant role for CO₂ as a driver of morphological evolution (aside from the widely documented, but evolutionarily minor changes in stomatal density and index through time). Without such an extrinsic driver, intrinsic features of individual clades must be the focus of evolutionary studies. The role of evolutionary novelties that alter constraints on hydraulic physiology are likely candidates and can be subjected to similar tests as for CO₂ due to the repeated evolution of such features in different lineages.