BIODIVERSITY RESEARCH: Turning up the heat: the impacts of Andropogon gayanus (gamba grass) invasion on fire behaviour in northern Australian savannas

Abstract

Aim This study aimed to quantify changes in fire severity resulting from the invasion of Australia’s tropical savannas by the African grass Andropogon gayanus Kunth. (gamba grass). Location Mesic savannas of the Northern Territory, Australia. Method Byram’s fire‐line intensity (If), fuel load and architecture, and two post‐fire indicators of fire intensity – scorch height (SH) and char height (CH) of woody vegetation – were determined for fires in native grass savanna and A. gayanus invaded savanna. Leaf scorch is the height at which the fire’s radiant heat browns leaf tissue, and leaf char is the height that radiant heat blackens or consumes leaf tissue and provides an indirect measure of flame height. These data, and 5 years of similar data collected from the Kapalga Fire Project in Kakadu National Park, were used to develop empirical relationships between If and the post‐fire indices of fire intensity. Results A relationship between A. gayanus If and SH could not be developed because complete canopy scorch occurred in most A. gayanus fires, even at low If. In contrast, A. gayanus If was strongly correlated with CH. This empirical relationship was substantially different from that for native grass fires. For a given If, there was a significantly greater CH in invaded sites. This increase in radiant heat is attributable to the increased biomass (mean 3.6 t ha−1 in native grasses compared to 11.6 t ha−1 in A. gayanus) and height (approximately 0.5 m in native grasses compared to 4 m in A. gayanus) of the standing fine fuel. Main conclusion Andropogon gayanus invasion resulted in substantial changes in fire behaviour. This has important regional implications owing to the current (10,000–15,000 km2) and predicted (380,000 km2) area of invasion and the negative consequences for the native savanna biota that has evolved with frequent but relatively low‐intensity fire.

Learning from the past to predict the future: an historical analysis of grass invasions in northern Australia

“…This data set was used previously to test for predictors of high-impact species (van Klinken et al 2013). Grasses are typically one of the bestrepresented naturalised families (Groves 1998;Pysek et al 2012) and their resulting negative impacts to the environment and industry can be particularly severe (Mack 1989;D'Antonio and Vitousek 1992;Ferdinands et al 2005;Groves et al 2005;Setterfield et al 2010;Friedel et al 2011;van Klinken et al 2013).…”

Why non-native grasses pose a critical emerging threat to biodiversity conservation, habitat connectivity and agricultural production in multifunctional rural landscapes

“…Fire regime change is arguably the best example, with the dramatic ecosystem-transforming effects of the grass/fire cycle associated with A. gayanus threatening the long-term viability of much of the tropical savanna belt of northern Australia (Rossiter et al 2003;Brooks et al 2010;Setterfield et al 2010). There has been recent scientific debate over whether C. ciliaris causes the same phenomenon in semi-arid and arid ecosystems (Miller et al 2010;Fensham et al 2013), but the capacity for this species to modify fire regimes is also enormous (Grice 2006).…”

Monitoring the Distribution and Dynamics of an Invasive Grass in Tropical Savanna Using Airborne LiDAR

“…A. gayanus alters nitrogen cycling [5,12] and reduces plant species richness and abundance [10]. Invasion results in fuel loads at least three times higher than native grasses, resulting in significantly more intense fires [8,11,13]. The rate of spread from initial source paddocks of A. gayanus has been rapid [14] with invasion now covering ~15,000 km 2 .…”

“…The species considered the greatest threat to the region's savanna woodlands and forests is Andropogon gayanus Kunth. (gamba grass) [10,11]. A. gayanus is a perennial C4 grass that forms large tussocks in excess of 4 meters high and displaces the much shorter native vegetation [10].…”