Tree cover-fire interactions promote the persistence of a fire-sensitive conifer in a highly flammable savanna

1. Callitris intratropica is a long-lived, obligate-seeding, fire-sensitive overstorey conifer that typically occurs in small groves (<0.1 ha) of much higher tree densities than the surrounding, eucalypt-dominated tropical savanna in northern Australia. We used C. intratropica groves of varying canopy cover to examine the role of feedbacks between fire and tree cover in the persistence of a fire-sensitive tree species and the maintenance of habitat heterogeneity in a highly flammable savanna. 2. We examined the population structure and floristic composition of C. intratropica groves and conducted controlled burns with Aboriginal landowners to test the prediction that groves of C. intratropica with closed canopies inhibit savanna fires by physically altering the fuel arrays below trees. We measured pre- and post-burn fuel availability, the probability of burning and fire intensity along transects (5575 m) spanning entire C. intratropica groves and extending into the surrounding savanna matrix. 3. We found that closed-canopy groves of C. intratropica had higher densities of seedlings and saplings than open-canopy groves and supported a distinct plant community. Closed-canopy groves also had a lower probability of burning and less severe fires due to a lower availability of fine fuels than the surrounding savanna. 4. Synthesis. Our results suggest that the observed regeneration within closed-canopy C. intratropica groves within frequently burnt savanna reflects a vegetationfire feedback. A significant, negative relationship between canopy cover and the probability of burning provides strong evidence that closed-canopy C. intratropica groves are capable of excluding low-intensity savanna fires, thereby enabling the persistence of patches of fire-sensitive forest or woodland amid open, highly flammable savanna vegetation. We present our findings as evidence that alternative stable state dynamics may play a role in determining savanna diversity and structure.