The role of succession in the evolution of flammability

Fire-prone ecosystems contain plants that are both fire-adapted and flammable. It has been hypothesized that these plants were under selection to become more flammable, but it is unclear whether this could be adaptive for an individual plant. We propose arrested succession as a robust mechanism that supports the evolution of flammability in surface fire ecosystems without the need to invoke group selection or additional fitness benefits. We used the natural history of lodgepole pine (Pinus ponderosa) forests, longleaf pine (Pinus palustris) forests, and tall grass prairies to create a general mathematical model of surface fire ecosystems and solved for the evolutionarily stable strategy (ESS) level of flammability. In our model, fires always kill understory plants and only sometimes kill overstory plants. Thus, more flammable plants suffer increased mortality due to fires, but also more frequently arrest succession by clearing their understory of late successional competitors. Increased flammability was selected for when the probability of an overstory plant dying from an individual fire was below a maximum threshold and the rate of succession relative to fires was above a minimum threshold. Future studies can test our model predictions and help resolve whether or not plants have been selected to be more flammable.