FOREST-FIRE MODEL WITH IMMUNE TREES

We present a generalization of the forest-fire model of P. Bak et al. by including the immunity g which is the probability that a tree is not ignited although one of its neighbors is burning. When g reaches a critical value g(c)(p), which depends on the tree growth rate p, the fire cannot survive any more, i.e. a continuous phase transition takes place from a steady state with fire to a steady state without fire. We present results of computer simulations and explain them by analytic calculations. The fire spreading at the phase transition represents a new type of percolation which is called ''fluctuating site percolation''.