At the end of last year M. Sankaran et al had a paper Determinants of woody cover in African savannas (Nature 2005 438(8) 846-849) that maps the possibility of savannas that can exist in alternative states based on rainfall.  This is the first map I have seen that maps the possibility of alternative states at a large  scale.

Figure: The distributions of MAP-determined (‘stable’) and disturbance determined (‘unstable’) savannas in Africa. Grey areas represent the existing distribution of savannas in Africa. Vertically hatched areas show the unstable savannas (>784mm MAP); cross-hatched areas show the transition between stable and unstable savannas (516–784mm MAP); grey areas that are not hatched show the stable savannas (<516mm MAP).

In the abstract the author writes:

Savannas are globally important ecosystems of great significance to human economies. In these biomes, which are characterized by the co-dominance of trees and grasses, woody cover is a chief determinant of ecosystem properties. The availability of resources (water, nutrients) and disturbance regimes (fire, herbivory) are thought to be important in regulating woody cover, but perceptions differ on which of these are the primary drivers of savanna structure. Here we show, using data from 854 sites across Africa, that maximum woody cover in savannas receiving a mean annual precipitation (MAP) of less than 650mm is constrained by, and increases linearly with, MAP. These arid and semi-arid savannas may be considered ‘stable’ systems in which water constrains woody cover and permits grasses to coexist, while fire, herbivory and soil properties interact to reduce woody cover below the MAP-controlled upper bound. Above a MAP of 650mm, savannas are ‘unstable’ systems in which MAP is sufficient for woody canopy closure, and disturbances (fire, herbivory) are required for the coexistence of trees and grass. These results provide insights into the nature of African savannas and suggest that future changes in precipitation6 may considerably affect their distribution and dynamics.