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


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We propose that elevated CO2 may have a significant positive effect on woody plant success and thus favour tree invasion and thickening in grass-dominated ecosystems. We note that savanna tree biomass is strongly constrained by disturbance, particularly fire, and that elevated CO2 could act to reduce this constraint. Our argument combines knowledge of tree recovery from injury after grassland fires, with theory about carbon acquisition and carbohydrate storage patterns in C3 woody plants in response to elevated CO2. We propose simply that elevated CO2 will tend to favour regrowth of juvenile trees trapped (sometimes for decades) in the ‘topkill’ zone, thus allowing them to escape more readily from periodic fires as CO2 continues to rise. Little empirical evidence exists to test this hypothesis, even though the process may have important implications for tree/grass codominated ecosystems currently in a dynamic equilibrium.