How do slow-growing, fire-sensitive conifers survive in flammable eucalypt woodlands?

Abstract

Question: To what extent do low flammability fuel traits enhance the survival and persistence of fire-sensitive (slowing-growing, non-serotinous, non-resprouting) dominant trees in highly flammable landscapes, under varying fire-weather conditions? Location: Mixed forests co-dominated by flammable Eucalyptus species and fire-sensitive Callitris glaucophylla in Pilliga State Forest, southeast Australia. Methods: The influence of vegetation composition (relative abundance of Callitris and flammable Eucalyptus) on fire intensity and survival of fire-sensitive Callitris was assessed across gradients of Callitris abundance in mixed Eucalyptus–Callitris forests that burned under low-moderate and extreme fire-weather conditions. Results: In areas that burned under low-moderate fire-weather conditions, as Callitris abundance increased, fire intensity declined and Callitris survival increased (46%). By comparison, in extreme fire-weather conditions, lower fire intensity at higher levels of Callitris abundance, was not sufficient to increase Callitris survival (4%). Callitris survival was also positively related to trunk diameter. Ground fuel type, but not biomass, varied with vegetation composition. Conclusions: These results demonstrate that flammable feedbacks, mediated by low flammability fuel traits of dominant trees, can provide an important mechanism for enhancing the survival and persistence of slow-growing, non-serotinous, non-resprouting, fire-killed trees in highly flammable landscapes. By modifying vegetation and fuel structure, patches of fire-sensitive Callitris reduce fire intensity, and thereby reduce Callitris mortality, enhancing population persistence. However, this feedback loop is insufficient to ensure Callitris survival under extreme fire-weather conditions, when fire intensity is greater. After burning, stands remain vulnerable to future fires, until trees grow large enough to modify fuel levels and reduce stand flammability.