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

Abstract

Summary1. 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 (55-75 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 vegetation-fire 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.

Responses of tree species to a severe fire indicate major structural change to Eucalyptus–Callitris forests

“…In contrast, C. glaucophylla showed a strong capacity to escape from low severity fire across the range of size classes, consistent with other studies of this and other coniferous species (e.g. Callitris - Cohn et al 2011;Trauernicht et al 2012;Larix occidentalis -Belote et al 2015;Pseudotsuga menziesii -Cocking et al 2012). These studies indicate that these conifers alter the fuel characteristics in their immediate vicinity by suppressing understorey vegetation and by having litter that is less flammable than angiosperm litter.…”

“…This hypothesis (Cohn et al 2011;Trauernicht et al 2012) considers that Callitris reduce understorey litter flammability and vegetation with the effect size related to the density and or local biomass of Callitris, both of which are expected to be correlated with basal area and stem density.…”

“…F.M.Bailey) that are obligate seeders (killed by fire) are often co-dominant. When of sufficient size and density, Callitris reputedly reduce local fire severity via characteristics of their foliage and litter and by reducing the cover of the herbaceous understorey (Cohn et al 2011(Cohn et al , 2013Trauernicht et al 2012). These Callitris species retain a canopy seed bank (although the temporal dynamics of this are poorly known -Ladd et al 2013), can recruit in the absence of fire (Ross et al 2008;Whipp et al 2012) and are also capable of recruitment into burnt patches from unburnt areas through the dispersal of winged seeds (Cohn et al 2011).…”

Climate, not Aboriginal landscape burning, controlled the historical demography and distribution of fire-sensitive conifer populations across Australia

“…By contrast, under frequent, low-intensity burning characteristic of Aboriginal management, healthy populations of C. intratropica are maintained [80]. The mechanism for the latter is a vegetationfire feedback where C. intratropica groves with closed-canopies suppress grassy fuels, thereby enabling the persistence of patches of fire-sensitive Callitris woodland in highly flammable savannahs with heavy grass fuel loads, provided that the fires are of low intensity [81]. We therefore propose that the traditional Aboriginal landscape burning in the northern monsoon tropics, and possibly southern Australia, helped to maintain Callitris populations that may otherwise have contracted into fire-protected refugia during the LGM.…”

Future directions for forest restoration in Hawai‘i

“…The dense canopies of some tree species can shade out grasses enough to break the grass-fire cycle (Trauernicht et al 2012). In fire-prone sites on the island of Moloka'i, plantations of non-native trees have been integrated into fuel breaks as a barrier to grass growth and thus fire spread (Lance DeSilva, pers.…”