The rise and evolution of large vascular land plants (mainly trees) during the mid-to-late Palaeozoic resulted in increased consumption of atmospheric CO2 by accelerated silicate rock weathering followed by further consumption due to increased terrestrially-derived organic matter burial in sediments, with the latter accompanied by increased production of O2. Changes in CO2 and O2 have been treated theoretically in terms of carbon- and sulphur-cycle modelling (Berner and Kothavala, 2001; Berner, 2006a,b). The CO2 modelling rests on the palaeobotanical record, field observations of the effect of trees on the rate of modern silicate weathering, and calculation of global organic matter burial based on the carbon isotopic record derived from carbonate fossils.