Forest fire model

The forest fire model is defined as a cellular automaton on a grid. A position can be empty or occupied by a tree which is ok, burning or burnt. We implement a slightly different ruleset to that of Drossel and Schwabl (1992), so that our implementation can be compared with other ABM frameworks

1. A burning position turns into a burnt position
2. A tree will burn if at least one neighbor is burning

The forest has an innate density, which is the proportion of trees initialized as green, however all trees that reside at x=1 on the grid are burning. The model is also available from the Models module as Models.forest_fire.

Defining the core structures

We start by defining the agent type

using Agents, Random, Plots

mutable struct Tree <: AbstractAgent
    id::Int
    pos::Dims{2}
    status::Symbol  #:green, :burning, :burnt
end

The agent type Tree has three fields: id and pos, which have to be there for any agent, and a status field that we introduce for this specific model. The status field will be :green when the tree is ok, :burning when on fire, and finally :burnt.

We then make a setup function that initializes the model.

function forest_fire(; density = 0.7, griddims = (100, 100))
    space = GridSpace(griddims; periodic = false, metric = :euclidean)
    forest = AgentBasedModel(Tree, space)
    # create and add trees to each position with a probability
    # determined by the `density`.
    for position in positions(forest)
        if rand() < density
            # Set the trees at position x=1 on fire
            state = position[1] == 1 ? :burning : :green
            add_agent!(position, forest, state)
        end
    end
    return forest
end

forest = forest_fire()

AgentBasedModel with 7045 agents of type Tree
 space: GridSpace with size (100, 100), metric=euclidean and periodic=false
 scheduler: fastest

Defining the step!

Because of the way the forest fire model is defined, we only need a stepping function for the agents

function tree_step!(tree, forest)
    # The current tree is burning
    if tree.status == :burning
        # Find all green neighbors and set them on fire
        for neighbor in nearby_agents(tree, forest)
            if neighbor.status == :green
                neighbor.status = :burning
            end
        end
        tree.status = :burnt
    end
end

Running the model

step!(forest, tree_step!, 1)
count(t->t.status == :burnt, allagents(forest))

125

step!(forest, tree_step!, 10)
count(t->t.status == :burnt, allagents(forest))

1328

Now we can do some data collection as well using an aggregate function percentage:

Random.seed!(2)
forest = forest_fire(griddims = (20, 20))
burnt_percentage(m) = count(t->t.status == :burnt, allagents(m)) / length(positions(m))
mdata = [burnt_percentage]

_, data = run!(forest, tree_step!, 10; mdata)
data

11 rows × 2 columns

Now let's plot the model. We use green for unburnt trees, red for burning and a dark red for burnt.

forest = forest_fire()
step!(forest, tree_step!, 1)

function treecolor(a)
    color = :green
    if a.status == :burning
        color = :red
    elseif a.status == :burnt
        color = :darkred
    end
    color
end

plotabm(forest; ac = treecolor, ms = 5)

or animate it

forest = forest_fire(density = 0.6)
anim = @animate for i in 0:10
    i > 0 && step!(forest, tree_step!, 5)
    p1 = plotabm(forest; ac = treecolor, ms = 5, msw = 0)
    title!(p1, "step $(i)")
end

gif(anim, "forest.gif", fps = 2)