  In work with a variety of co-authors, Staver and Levin have argued that
savannah and forest coexist as alternative stable states with discontinuous
changes in density of trees at the boundary. Here we formulate a nonhomogeneous
spatial model of the competition between forest and savannah. We prove that
coexistence occurs for a time that is exponential in the size of the system,
and that after an initial transient, boundaries between the alternative
equilibria remain stable.
