Interplay between wildfires and forest age structure in steady state.

Stationary forests can be characterized by a spatiotemporal distribution of species of trees in respect of its location and age. Several external factors like wildfires, recurrent plagues and forest managements for harvesting of timber, should perturb the dynamics of the evolution of the populations of tree species, superimposed to the inherent topographical and hydrological factors as well as the climate seasonal variations.
We investigate the interplay between the distribution of long-term and large-scale forest fires and the forest tree age distribution, using a cellular automata model. For this, we associate the age of a tree with its robustness and hence to its degree of flammability, so that much younger or older trees, shall be more susceptible to burning than those at the ripe age. Particularly, we investigate the effects of wildfires in the most simplest case of single-species forests focusing on the dynamic regime where the probability of interaction between fires is null. In such scenario, the density of trees can evolves in large time scales to one of two possibles steady state attractors, \emph{dense forest} or \emph{savana forest}, regardless of the initial configuration of trees. The time dependent profile of the density of trees and its steady-state age-frequency histograms and fire-size distributions were estimated through various simulations and the records analyzed according to the model parameters.

The \emph{dense forest} state is characterized by a high density of trees with an uniform age histogram for almost all classes, except for the one of the very young trees and those of the senescence period, the later exhibiting an exponential decay. Moreover a typical exponential decay for the fire-size distribution function appears indicating that the presence of a massive number of mature trees prevents the spread of large fires. On the other hand, the \emph{savana forest} state is characterized by a low density of very young trees exhibiting an power-law like behavior for the fire-size distribution function.
In short, the inclusion of correlation between the age of trees and its flammability even in a mono-species forest environment, leads to a possible dynamic phase transition between dense forest to a savanna forest state. An outline of the phase diagram according the model parameters is discussed.