Vegetation sub-model
Design concepts
The succession sub-model is responsible for the simulation of time related changes in forest composition (Figure 1). The event is scheduled once every year at the end of the year. It affects all cells with dynamic vegetation species. It determines the characteristics of the potential transitions in terms of dominant tree species that affect each cell each year.
Post-fire changes in forest class
Post-fire changes in forest composition account for direct and non-direct (i.e. changes in dominant tree species) forest regeneration dynamics after the impact of fire in a given area. Transition changes were not located spatially at random but were forced to be aggregated. This was implemented by allowing transition to be based on the neighbors, which occurred with a predetermined probability. Otherwise, transitions were determined using transition probabilities.
When neighbor contagion was used, any neighbor available to define the transition was only taken into consideration if being the same species before fire and having the same time since last fire.
Succession from shrub to forest
Base transition probabilities of dominant tree species after a fire were based on Rodrigo et al. (2004). Given that post-fire transition probabilities are based on datafrom different fires located at different regions, we included different transitions probabilities for a given tree species in different region by including region as a spatial constant in the model. It is known that regeneration patterns are strongly linked to aspect value. In order to mimic this pattern, non-direct regeneration patterns where modified in order to increase the amount of shrub (i.e. non tree regeneration after fire) in southern slopes. On the other hand, given that transition probabilities after fire are known to depend on time since the previous last fire, we applied modified transition probabilities for recent re-burns in which those species with no direct regeneration after repeated fires (i.e. Pinus halepensis starts producing cones 15-20 years after a fire).
Succession (i.e. changes in species as time goes) is only modeled for shrub (i.e. forest are assumed to be in a relatively stable state, once forest is present it does not change). Shrub is allowed to have a yearly probability to change to forest after the a certain age. In case that transition from shrub to forest is possible, the transition probabilities are obtained from the distribution of mature forest types in the surroundings of the cell (150 m radius). Not all tree species have the same probability of shrub colonization. Therefore, we use aspect and tree species (i.e. mimicking differences among species in seed pressure) to weight the relative contribution of each species to distribution of tree species neighbor. Changes from shrub to forest could be evaluated by comparing temporal changes in shrub state in land use maps
Figures
Figure 1: Effects on the land cover types spatial distribution after fire.
References
Rodrigo, A., Retana, J., Picó, F.X., 2004. Direct regeneration is not the only response of Mediterranean forests to large fires. Ecology 85, 716–729.