resource ratio model of the effects of changes in CO₂ on woody plant invasion

It is expected that elevated CO₂ levels may have an important positive effect on the dominance of woody plants over grasses in savannas and grasslands. I propose that these changes in the relative abundance of trees and shrubs over grasses may be explained by Tilman's resource ratio models. This change will occur because C₃ trees will have higher net photosynthetic rates than C₄ grasses which predominate in savannas. This will cause trees to have higher growth rates than grasses. An additional factor in trees and shrubs with carbon-based defences (such as tannins or other polyphenols) is that they may be better defended and, thus, lose less material to herbivory. Consequently, trees and shrubs should invade savannas and grasslands because their R* values will be lower. I compare this model to another less parsimonious model based on fire and carbon storage and allocation. Although these models are not necessarily mutually exclusive, the resource ratio model may be differentiated from the fire-carbon model on the basis of the presence of fire and/or the sensitivity to elevations in global CO₂ levels.