Perennial pasture grass invasion changes fire behaviour and recruitment potential of a native forb in a temperate Australian grassland

Zac C Walker,John W Morgan

doi:10.1007/s10530-022-02743-4

Abstract

Invasive grasses can modify fire regimes of native ecosystems leading to changed ecosystem structure, composition, and functioning. Temperate grasslands in Australia are currently being invaded by a suite of exotic perennial pasture grasses, but their effects on ecosystems remain largely unknown. We aimed to determine the effect of invasion by the exotic perennial grass Phalaris aquatica on fire behaviour, as well as the regeneration potential of an endangered forb in temperate native grasslands in south-eastern Australia. Frequently burnt native grasslands invaded by exotic grasses were found to have two times more fuel than grasslands dominated by native grasses; in less-frequently burned native grasslands, exotic grasses contributed to fuel loads that were five times higher than native grasslands. Exotic-dominated grasslands burned differently than native grasslands; fire intensities were three times higher in exotic-dominated grasslands and had a wide variability in fire residence times. Soil heating was positively related to fire residence time but had no clear relationship with fire intensity. Seed germinability of Leucochrysum albicans var. tricolor (Hoary Sunray, Asteraceae) was reduced by exotic grass-fueled fire and increasing fire residence times. The observed changes in fire behaviour represent an invasion-driven shift in the ecosystem’s fire regime. By increasing fuel mass, fire residence time increased, and this influenced seed survival and subsequent germinability. Increased fire intensity following invasion highlights that invasive grasses can increase the fire-risk of grasslands. Maintaining native grasslands free of invasive pasture grasses therefore has environmental and fire-risk benefits.

Highlights

Exotic grasses have invaded substantial areas of vegetation on almost every continent and are regarded as a major threat to global biodiversity (D’Antonio and Vitousek 1992; Heywood 1989; Vitousek et al 1997)
The increase in soil temperature was positively related to fire residence time (R2 = 0.76, Fig. 2a) but had no clear relationship with fire intensity ( R2 = 0.04, Fig. 2b)
Conservation of temperate native grasslands depends on understanding key ecological processes and how these processes may be altered by the invasion of exotic perennial grasses (D’Antonio and Vitousek 1992; Mack and D’Antonio 1998; Brooks et al 2004)

Summary

Introduction

Exotic grasses have invaded substantial areas of vegetation on almost every continent and are regarded as a major threat to global biodiversity (D’Antonio and Vitousek 1992; Heywood 1989; Vitousek et al 1997). One of the most detrimental effects of exotic grass invasion on ecosystems are their effects on the fire regime (D’Antonio and Vitousek 1992; Brooks et al 2004). In tropical and arid Australia, where exotic pasture grasses such as Andropogon gayanus (Gamba Grass) and Cenchrus ciliaris (Buffel Grass) have invaded savanna, they have greatly altered the amount and distribution of fuels in native ecosystems (Miller et al 2010; Rossiter et al 2003), with subsequent effects on the frequency and intensity of fire (Butler and Fairfax, 2003; Miller et al 2010; Setterfield et al 2010), as well as negative impacts on woody plant persistence (Butler and Fairfax 2003; Miller et al 2010; Setterfield et al 2010). Interactions between invasive grasses, fire, and ecosystem processes are well-studied globally in woody ecosystems, but much less is known about the impact of invasive grasses on fire and ecosystem processes in grasslands

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