A proposed CO2-controlled mechanism of woody plant invasion in grasslands and savannas

We propose that elevated CO2 may have a significant positive effect on wood y plant success and thus favour tree invasion and thickening in grass-domin ated ecosystems. We note that savanna tree biomass is strongly constrained by disturbance, particularly fire, and that elevated CO2 could act to reduc e this constraint. Our argument combines knowledge of tree recovery from in jury after grassland fires, with theory about carbon acquisition and carboh ydrate storage patterns in C3 woody plants in response to elevated CO2. We propose simply that elevated CO2 will tend to favour regrowth of juvenile t rees trapped (sometimes for decades) in the 'topkill' zone, thus allowing t hem to escape more readily from periodic fires as CO2 continues to rise. Li ttle empirical evidence exists to test this hypothesis, even though the pro cess may have important implications for tree/grass codominated ecosystems currently in a dynamic equilibrium.