Persistence of pyrophilous insects in fire-driven boreal forests: population dynamics in burned and unburned habitats

Several boreal insect species respond to smoke and heat generated by forest fires and use recent burns to reproduce in high numbers. Some of these species are rare or uncommon in undisturbed forests, and the contribution of recently burned habitats to their population dynamics has been deemed crucial by some to their long-term persistence. Consequently, the severe decline seen in some species in Fennoscandia has been frequently linked with fire suppression. In this paper, we explore some aspects of the spatial dynamics of pyrophilous insect populations in relation to the expected relative contribution of burned and unburned habitats to their global population dynamics. Forest fires are, throughout the boreal forest biome, generally highly aggregated in some years while rare in most other years. The low connectivity between fire events and the typical life cycle seen in these species make it improbable that recent burns act as significant population sources. This leads us to suggest that populations of pyrophilous species may be more limited by the adequacy of the unburned matrix than by the occurrence of fire events. Moreover, by combining an age-class distribution model and a dead wood availability model, we show that the quality of the unburned matrix increases in landscapes with longer fire cycles, in which pyrophilous insects should persist at higher population levels. We conclude that the degradation of the unburned habitat better explains the decline of pyrophilous insects than fire suppression alone.