THE RISE OF TREES IN THE PALEOZOIC AND THEIR EFFECTS ON ATMOSPHERIC CO2 AND O2

Large vascular plants, with deep and extensive rooting systems, arose and spread over the continents starting about 380 mA during the Devonian Period. The effects on atmospheric composition of the occupation of the land by the plants was twofold. First, the uptake of nutrients from rocks resulted in the enhanced weathering of Ca-Mg silicate minerals. This resulted in accelerated removal of atmospheric CO2 via conversion to dissolved HCO3-, transport of HCO3- by rivers to the oceans, and deposition of HCO3- as Ca-Mg carbonates in marine sediments. The tree-accelerated weathering was balanced by greenhouse and CO2/plant induced deceleration of weathering due to falling CO2, and this resulted in the stabilization of CO2 at lower levels.

The other effect of the rise of trees was the production of large amounts of microbially resistant organic matter, in the form of lignin. This resulted in the increased burial of organic matter in sediments. Organic burial represents an excess of global photosynthesis over respiration so that increased burial rate brought about further CO2 removal and the excessive production of atmospheric O2. These changes in the carbon cycle and atmospheric composition led to massive long-term glaciation, the formation of vast coal deposits, and the rise of giant insects over the period 350-270 mA.

Computer models of the long term carbon cycle, based partly on field studies of the effects of plants on modern weathering and on laboratory studies of the effect of O2 on carbon-isotope fractionation during photosynthesis, have been employed to calculate atmospheric CO2 and O2 levels over this time period and resulting values agree with independent estimates.