Differential demographic filtering by surface fires: How fuel type and fuel load affect sapling mortality of an obligate seeder savanna tree

Abstract

We studied the effects of fuel type, fuel load and their associated flammability attributes on growth and survival of Callitris intratropica saplings. Callitris intratropica is a fire‐sensitive conifer that is widespread across northern Australia, but its range is contracting because of frequent and intense fires. A small‐scale field experiment was used to compare the effect of three fuel types (grass, eucalypt litter and C. intratropica litter), and a mix of grass and eucalypt litter by varying fuel loads within their naturally occurring bounds, and measuring multiple flammability attributes. Fuel type had the strongest influence on flammability attributes and hence sapling survival. Grass burnt rapidly, producing high temperatures, while duration of flaming was longer for eucalypt litter. Grass–eucalypt litter mixtures had flammability attributes more like grass, while C. intratropica litter was difficult to burn. Fuel load had a secondary effect, with strong interactions between fuel type and load. Differences in sapling survival could be attributed to temperatures at 5 cm height; there was no additional effect of fuel type or canopy temperature. Sapling size variables were also important, and strongly correlated with bark thickness, so we could not identify the protective mechanism. Synthesis. Mortality of Callitris intratropica saplings was consistent with damage to the lower stem, because of the direct relationship with temperatures at 5 cm height. Our results demonstrate the existence of a grass–fire cycle in C. intratropica stands, whereby hot fires damage the stands and allow grass to invade, increasing the stand ignitability and combustibility and promoting further fires. Interrupting this cycle by reducing grass fuel loads and hence the frequency of hot fires should therefore be a management priority to safeguard C. intratropica populations. Our findings also highlight that, under a common climate, vegetation type can shape fire regimes, because fuel type strongly influences flammability attributes, which in turn act as a powerful filter of plant populations.

Grass Canopy Architecture Influences Temperature Exposure at Soil Surface

“…Canopy heating predicts post-fire woody species mortality by causing vascular cambium necrosis in bole and vegetative buds necrosis in the crown [25,26,58]. Although plant responses to crown or bole heating also depend on bark thickness and distribution of vegetative buds [59,60], with more heat release resulting in higher temperature exposure in plant tissues, fires usually result in higher mortality rate [61,62]. Canopy heating may also contribute to xylem dysfunction, and this is another pathway of fire-induced tree mortality [29,63,64].…”

Two climate‐sensitive tree‐ring chronologies from Arnhem Land, monsoonal Australia

“…, ), resilient to termite attack but sensitive to intense fire (Yates & Russell‐Smith ; Russell‐Smith ; Bowman et al . , ). The size‐class distribution of living and dead stems has been the basis for a number of landscape ecology studies (e.g.…”

Conceptualizing Ecological Flammability: An Experimental Test of Three Frameworks Using Various Types and Loads of Surface Fuels

“…Therefore attempts at ranking fuels on a single flammability scale hinge on the weight accorded to its various components [7], and are often also shaped by the ease or practical difficulties in measuring the associated metrics. Adding to these complications, values of flammability metrics are affected by the measurement scale (plant parts, whole plants or whole plant communities and landscapes), fuel loads and arrangement and mixtures of fuel types [6,8,[10][11][12][13][14][15][16]. The values also vary according to factors such as weather conditions, fuel moisture content and ignition method [17][18][19].…”