Plants and the palaeoatmosphere

The effects of the development of terrestrial vegetation during Devonian and Carboniferous times is considered and it is concluded that the maximum possible global biomass in Carboniferous times was equivalent to about 1.148 × 10 15 g C, which is in excess of the current Earth's biomass (560–860 × 10 I5 g C). Global primary productivity, however, estimated at about 73 × 10 15 g C a −1 is of the same order as that of the present day. The amount of carbon actually becoming stored as 'fossil fuel' was a very small proportion of this productivity, estimated here at only 0.00002% over 65 × 10 6 a. Thus, the vegetation of Carboniferous times cannot be regarded as a significant carbon sink. The atmosphere of the Mesozoic is discussed in relation to the evolution of angiosperms. In particular, the photosynthetic modifications in so-called C 4 and CAM plants are considered in relation to atmospheric and other environmental stresses. Taxonomic evidence indicates that the C 3 system is primitive in the angiosperms and that the modified systems have a polyphyletic origin. Physiological considerations suggest that their development relates more clearly to the demands of water balance rather than to lowered atmospheric CO 2 or high atmospheric levels of oxygen. It is not considered likely that the evolution of these mechanisms, or of the angiosperms in general, has had any profound effect upon the carbon content of the atmosphere.