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

1. Callitrisintratropica is a long-lived, obligate-seeding, fire-sensitive overstoreyconifer that typically occurs in small groves (<0.1 ha) of much highertree densities than the surrounding, eucalypt-dominated tropical savanna innorthern Australia. We used C. intratropica groves of varyingcanopy cover to examine the role of feedbacks between fire and tree cover inthe persistence of a fire-sensitive tree species and the maintenance of habitatheterogeneity in a highly flammable savanna.

 

2. Weexamined the population structure and floristic composition of C. intratropica grovesand conducted controlled burns with Aboriginal landowners to test theprediction that groves of C. intratropica with closed canopiesinhibit savanna fires by physically altering the fuel arrays below trees. Wemeasured pre- and post-burn fuel availability, the probability of burning andfire intensity along transects (55–75 m) spanning entire C. intratropica grovesand extending into the surrounding savanna matrix.

 

3. We foundthat closed-canopy groves of C. intratropica had higherdensities of seedlings and saplings than open-canopy groves and supported adistinct plant community. Closed-canopy groves also had a lower probability ofburning and less severe fires due to a lower availability of fine fuels thanthe surrounding savanna.

 

4. Synthesis.Our results suggest that the observed regeneration within closed-canopy C. intratropica groveswithin frequently burnt savanna reflects a vegetation–fire feedback. Asignificant, negative relationship between canopy cover and the probability ofburning provides strong evidence that closed-canopy C. intratropica grovesare capable of excluding low-intensity savanna fires, thereby enabling thepersistence of patches of fire-sensitive forest or woodland amid open, highlyflammable savanna vegetation. We present our findings as evidence thatalternative stable state dynamics may play a role in determining savannadiversity and structure.