How hot? How often? Getting the fire frequency and timing right for optimal management of woody cover and pasture composition in northern Australian grazed tropical savannas. Kidman Springs Fire Experiment 1993–2013

Abstract

A long-term (1993–2013) experiment in grazed semiarid tropical savannas in northern Australia tested the impact of varying the frequency (every 2, 4 and 6 years) and season (June – EDS versus October – LDS) of fire compared with unburnt controls on woody cover and pasture composition, in grassland and open woodland. Over an 18-year period, woody cover increased by 4% (absolute) in the woodland even with the most severe (i.e. frequent, late dry season) fire treatments. With less severe or no fire, woody cover increased by 12–17%. In the grassland, woody cover remained static when subjected to LDS fires every 2 or 4 years, but increased by 3–6% under other fire treatments, and by 8% when unburnt. Major shifts in understorey species composition occurred at both sites regardless of fire regime. The effect of fire on herbage mass and composition was compounded by higher grazing after fires. The herbage mass of perennial grasses declined and that of annual grasses and forbs increased following early or frequent fires. Brachyachne convergens, Gomphrena canescens and Flemingia pauciflora increased in response to fire while Aristida latifolia and Heteropogon contortus decreased. Four-yearly LDS fire provided the most effective management of woody cover and pasture composition. Although EDS fire is recommended for biodiversity management and reducing greenhouse gas emissions in wet tropical savannas, on grazed pastoral land, it can promote woodland thickening and pasture degradation. Optimal fire management, therefore, depends on vegetation type, land use and the prevailing seasonal timing and frequency of fire.

Incentivising fire management in Pindan (Acacia shrubland): A proposed fuel type for Australia's Savanna burning greenhouse gas emissions abatement methodology

“…); and fertile, pastorally productive open‐woodlands and grasslands where EDS fire management would very likely result in significant woody thickening and encroachment (Cowley et al . ; Russell‐Smith et al . ).…”

“…Registered Savanna burning projects can be conducted only on eligible 'Vegetation Fuel Types' (VFTs) of which there are a total of nine, including five in the LRZ (Australian Government 2015;Lynch et al 2015 (Woinarski & Fisher 1995;Russell-Smith et al 2010); and fertile, pastorally productive open-woodlands and grasslands where EDS fire management would very likely result in significant woody thickening and encroachment (Cowley et al 2014;Russell-Smith et al 2014).…”

Forb community responses to an extensive drought in two contrasting land-use types of a semi-arid Lowveld savanna

“…Cattle may even suppress forb cover through forb forage preferences towards the end of a dry season (Young et al 2005;Odadi et al 2007;Kimuyu et al 2017). Despite significant contributions to ecosystem goods and services, both annual and perennial forbs are often perceived negatively by land managers, because of their association with savanna land degradation (Siebert and Dreber 2019) and dominance over grasses under particular environmental conditions, such as on nutrient-rich soils (Reich et al 2003;Van Coller and Siebert 2015), when grazing pressure is moderate to high (Scholes 1987;Hejcmanová et al 2010;Cowley et al 2014) or after drought events (Fynn and O'Connor 2000;Buitenwerf et al 2011).…”

Beneficial land sector change in far northern Australia is required and possible – a refutation of McLean and Holmes (2019)

“…Although this is a somewhat interesting proposition since most rangeland managers and ecologists would argue that woody thickening is actually symptomatic of poor or inappropriate pastoral management (e.g. Burrows et al 2002;Cowley et al 2014;Crowley 2015), we have no in-principle opposition to this suggestion since, in effect, it could support the development of complementary ecosystem service market opportunities especially in pastorally marginal situations. There is, however, evidence for only slight woody thickening across our focal savanna region (Murphy et al 2014), with the notable exception of former grassland and open-woodland systems in Cape York (Crowley and Garnett 1998).…”