A grass–fire cycle eliminates an obligate‐seeding tree in a tropical savanna

Abstract

A grass–fire cycle in Australian tropical savannas has been postulated as driving the regional decline of the obligate-seeding conifer Callitris intratropica and other fire-sensitive components of the regional flora and fauna, due to proliferation of flammable native grasses. We tested the hypothesis that a high-biomass invasive savanna grass drives a positive feedback process where intense fires destroy fire-sensitive trees, and the reduction in canopy cover facilitates further invasion by grass. We undertook an observational and experimental study using, as a model system, a plantation of C. intratropica that has been invaded by an African grass, gamba (Andropogon gayanus) in the Northern Territory, Australia. We found that high grass biomass was associated with reduced canopy cover and restriction of foliage to the upper canopy of surviving stems, and mortality of adult trees was very high (>50%) even in areas with low fuel loads (1 t·ha−1). Experimental fires, with fuel loads >10 t·ha−1, typical of the grass-invasion front, caused significant mortality due to complete crown scorch. Lower fuel loads cause reduced canopy cover through defoliation of the lower canopy. These results help explain how increases in grass biomass are coupled with the decline of C. intratropica throughout northern Australia by causing a switch from litter and sparse perennial grass fuels, and hence low-intensity surface fires, to heavy annual grass fuel loads that sustain fires that burn into the midstorey. This study demonstrates that changes in fuel type can alter fire regimes with substantial knock-on effects on the biota.

Effect of landscape fires on the demography of the endangered New Caledonian conifer Callitris sulcata

“…Selection pressure by crown fire regimes may have given rise to serotiny in the Callitris clade (Ladd et al, 2013) because they are extremely vulnerable to canopy defoliation by fire, given a very limited epicormic response. Many Australian species, however, have highly insulating bark that protects the cambium from high temperatures (Lawes et al, 2011;Bowman et al, 2014), consistent with adaptation to surface fire regime (Keeley, 2012). Although a previous study reported that C. sulcata has thin bark, which renders it vulnerable to fire (Ladd et al, 2013), we found that bark thickness of C. sulcata trees reached a plateau of 10 mm at a diameter of around 15 cm.…”

“…Although a previous study reported that C. sulcata has thin bark, which renders it vulnerable to fire (Ladd et al, 2013), we found that bark thickness of C. sulcata trees reached a plateau of 10 mm at a diameter of around 15 cm. Although this is substantially thinner than bark of C. intratropica trees (15 mm for trees with mean DBH of 21 cm), which grow in north Australian savannas subject to frequent surface fires (Bowman et al, 2014), it is thicker than that of Callitris macleayana (4.4 mm for trees with mean DBH of 20 cm), which grows in infrequently burnt Queensland rainforest (Lawes et al, 2011). It is therefore possible C. sulcata has evolved to tolerate infrequent surface fires.…”

“…For instance, we observed that trees that were completely scorched died, and no seedlings germinated beneath them (Table 1). We know from previous studies in the genus that the buds are killed by fire and that the damage is permanent, and if the trees are entirely defoliated they are killed (Bowman et al, 2014). In the other recently burnt plots, some trees that escaped complete canopy scorch survived, and many more seedlings established.…”

“…Despite tolerating seasonal dryness and soil infertility (Brodribb et al, 2014), environmental factors which are often associated with flammable foliage (Bowman et al, 2014), the limited development of fire recovery traits may reflect the low frequency of ignitions prior to prehistoric and historic human colonisation. There was very little landscape fire in New Caledonia in the 47,000 years prior to the arrival of humans 3000 years ago (Stevenson and Hope, 2005).…”

Determinants of growth of the flammable grass, Triodia scariosa: Consequences for fuel dynamics under climate change in the Mediterranean region of South Eastern Australia

“…This system provides an opportunity to examine the relative importance of shrub-grass interactions and other environmental controls on the balance of herbaceous and woody plants. An understanding of the strength of these determinants will provide insight into the way in which fuels and fire regimes are determined by community composition (Bowman et al 2014).…”