Slide 13: Maximum tree basal area
So, given that tree cover limits grass growth potential, we might expect to see areas becoming increasingly treed until the grass dies out. This is not the case, however, and so clearly some other factor is guiding the maximum tree basal area.
Considering savanna data from all around the world, there is a clear upper limit to the amount of tree biomass or leaf area that a piece of land can support, given its water balance. This line represents the limit of tree-on-tree competition.