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

David Bowman, Harry MacDermott, Scott Nichols, Brett Murphy

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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.
Original languageEnglish
Pages (from-to)4185-4194
Number of pages10
JournalEcology and Evolution
Volume4
Issue number21
DOIs
Publication statusPublished - Nov 2014
Externally publishedYes

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seeding
savanna
savannas
sowing
grass
grasses
canopy
biomass
Callitris
Andropogon gayanus
scorch
grass fire
mortality
Northern Territory
fire regime
defoliation
foliage
tree crown
conifers
coniferous tree

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Bowman, David ; MacDermott, Harry ; Nichols, Scott ; Murphy, Brett. / A grass-fire cycle eliminates an obligate-seeding tree in a tropical savanna. In: Ecology and Evolution. 2014 ; Vol. 4, No. 21. pp. 4185-4194.
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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.",
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A grass-fire cycle eliminates an obligate-seeding tree in a tropical savanna. / Bowman, David; MacDermott, Harry; Nichols, Scott; Murphy, Brett.

In: Ecology and Evolution, Vol. 4, No. 21, 11.2014, p. 4185-4194.

Research output: Contribution to journalArticleResearchpeer-review

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