Paleobotanical and sedimentary evidence indicates that PFLs a-b-c-d-e-f-g and a-b-c-d-m-n were most strongly expressed as CO2 levels began to fall during the extraordinary early evolution and diversification of land plants in the Paleozoic. Early land plants experiencing very high Early Devonian CO2 concentrations (Fig. 2) were small and typically leafless with naked simple or branched axial stems (4). As atmospheric CO2 levels subsequently declined (3) over the next 40 million years, stomatal densities rose, and maximum leaf width in several independent groups increased by a factor of up to 25 (6, 7, 20), with a diminished requirement for convective heat loss (Fig. 2). Larger leaves signaled the parallel evolution of trees, whose maximum height quickly increased, leading to the formation of stratified forests (4). By the end of the Devonian, when leaf size had reached 80% of its maximal enlargement (7), stem diameter increased logarithmically (18, 21) from 3 mm to 1.5 m, implying a rise in height from a just a few centimeters to 30 m (Fig. 2). Trees entrained the evolution of more complex rooting systems to provide anchorage and exploit larger volumes of soil for water and nutrients (16, 18). Along with root symbionts such as mycorrhizae, this increased nutrient removal and the surface area of the soil–root interface, both features enhancing rates of chemical weathering of silicates and increasing the rate of removal of CO2 from the atmosphere (3, 9, 16, 18). The expanding terrestrial biomass further promoted CO2 removal because of an enormous increase in organic burial of plant-derived organic matter (mostly on land but also in the sea), which tracked the rise in leaf and plant size (PFLs a-b-c-d-m-n and i-k-c-d-m-n) (Fig. 2) and was evidenced most importantly in the Mississippian and Permian by the formation of vast coal deposits (3).
Given that proxy CO2 data indicate that CO2 levels stabilized at low values in the Late Paleozoic (Fig. 2 A), negative feedbacks must have operated to counterbalance these positive examples. The two most important involved reduced rates of rock weathering as CO2 levels decreased and the climate cooled (pathway i-j-g), and gradual CO2 limitation of terrestrial photosynthetic primary production eventually slowing both weathering (pathway h-e-f-g) and organic carbon burial (pathway h-m-n). The cooler climate also should have led to less rainfall and less CO2 uptake by weathering (pathway i-p-q-r-g).