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.

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.…”

“…At the individual level we found support for this model but, it was not supported at the plot level. Despite covering a range of basal area encompassing that considered by Cohn et al (2011) to characterise Callitris patches (approximately 0.13m 2 per 100m 2 ), there was no clear relationship between Callitris basal area and the overall proportion of trees that escaped from crown scorch even at low fire severity (Fig. 3a).…”

“…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.…”

“…At that severity the likely impact on the three most common species (E. albens, E. crebra and C. glaucophylla) was topkill for over 75% of the eucalypts and mortality for all the cypress pines, leading to a major alteration of structure and a change in dominance among the canopy species of this landscape. Death of adult and juvenile Callitris trees removes any severity ameliorating influence they may have across the landscape for several decades until post-fire seedlings reach a size or canopy density where they have some effect (Cohn et al 2011). A fire free period of at least 10 years will be necessary to allow Callitris trees to reach maturity .…”

“…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).…”