Forest Fires

Self-Organized Criticality (SOC)

A system exhibits SOC if it limits itself to some optimal state or size. This can be seen in the density of forests. Instead of largest possible density, forests seem to limit themselves to the largest possible density that still prevents forest fires from spreading through the entire forests.

In models/Earth Sciences/Fire turtles represent fire and patches represent the forest. A green patch has a tree growing on it. A black patch is bare. If a fire turtle is on a green patch, it burns it, then ignites all four neighboring green patches. This is accomplished by asking the patch to sprout a fire turtle.

to go
if not any? turtles ;; either fires or embers
[ stop ]
ask fires
[ ask neighbors4 with [pcolor = green]
[ ignite ]
set breed embers ]
fade-embers // turtle fades to black then dies
tick
end

;; creates the fire turtles
to ignite ;; patch procedure
sprout-fires 1
[ set color red ]
set pcolor black
set burned-trees burned-trees + 1
end

Experiment with different forest densities. Run it 10 times at 50% density. Record the percent burned in a spread sheet. Repeat at 70% density and 59% density. Use the spreadsheet to calculate the averages.

Notice also the fractal shape of the fire.