Fire and Its Interactions With Other Drivers Shape a Distinctive, Semi-Arid ‘Mallee’ Ecosystem

Abstract

Fire shapes ecosystems globally, including semi-arid ecosystems. In Australia, semi-arid ‘mallee’ ecosystems occur primarily across the southern part of the continent, forming an interface between the arid interior and temperate south. Mallee vegetation is characterized by short, multi-stemmed eucalypts that grow from a basal lignotuber. Fire shapes the structure and functioning of mallee ecosystems. Using the Murray Mallee region in south-eastern Australia as a case study, we examine the characteristics and role of fire, the consequences for biota, and the interaction of fire with other drivers. Wildfires in mallee ecosystems typically are large (1000s ha), burn with high severity, commonly cause top-kill of eucalypts, and create coarse-grained mosaics at a regional scale. Wildfires can occur in late spring and summer in both dry and wet years. Recovery of plant and animal communities is predictable and slow, with regeneration of eucalypts and many habitat components extending over decades. Time since the last fire strongly influences the distribution and abundance of many species and the structure of plant and animal communities. Animal species display a discrete set of generalized responses to time since fire. Systematic field studies and modeling are beginning to reveal how spatial variation in fire regimes (‘pyrodiversity’) at different scales shapes biodiversity. Pyrodiversity includes variation in the extent of post-fire habitats, the diversity of post-fire age-classes and their configuration. At regional scales, a desirable mix of fire histories for biodiversity conservation includes a combination of early, mid and late post-fire age-classes, weighted toward later seral stages that provide critical habitat for threatened species. Biodiversity is also influenced by interactions between fire and other drivers, including land clearing, rainfall, herbivory and predation. Extensive clearing for agriculture has altered the nature and impact of fire, and facilitated invasion by pest species that modify fuels, fire regimes and post-fire recovery. Given the natural and anthropogenic drivers of fire and the consequences of their interactions, we highlight opportunities for conserving mallee ecosystems. These include learning from and fostering Indigenous knowledge of fire, implementing actions that consider synergies between fire and other processes, and strategic monitoring of fire, biodiversity and other drivers to guide place-based, adaptive management under climate change.

Something went missing: cessation of Traditional Owner land management and rapid mammalian population collapses in the semi-arid region of the Murray–Darling Basin, southeastern Australia

“…The extent of all types of mature vegetation had dramatically declined and the Boodie (Bettongia lesueur) had become extinct over this 30-year period (Bird et al 2013). Perhaps coincidently, the first major bushfire in our region burnt through the Wimmera (and much of Victoria) on 6 February 1851 (Clarke et al 2021). Traditional Owner fires are embedded in many Australian ecosystems but fire regimes have dramatically changed since European colonization (Tindale 1959;Latz 1995b;Gott 2005Gott & 2008Cahir & McMaster 2018) (Figure 3).…”

In the Hot Seat: Behavioral Change and Old-Growth Trees Underpin an Australian Songbird’s Response to Extreme Heat

“…Our finding that the availability and value of thermal refuges increases with the size of mallee eucalypts has important management implications. Mallee stems are usually killed by wildfire and high intensity planned burns, hence trunk size and tree height increase with time since fire (Yates et al, 2017;Clarke et al, 2021). Indeed, it takes 60 years for mallees to attain full canopy height, and 40-60 years to develop hollows in living timber (Haslem et al, 2011;Clarke et al, 2021), likely reflecting their value as thermal refuges.…”

“…Mallee stems are usually killed by wildfire and high intensity planned burns, hence trunk size and tree height increase with time since fire (Yates et al, 2017;Clarke et al, 2021). Indeed, it takes 60 years for mallees to attain full canopy height, and 40-60 years to develop hollows in living timber (Haslem et al, 2011;Clarke et al, 2021), likely reflecting their value as thermal refuges. Climate change is expected to influence the incidence of wildfires in mallee habitat; in mallee regions of Victoria, for example, the number of days of extreme fire danger and the occurrence of dry lightning are predicted to increase (Clarke et al, 2021).…”

“…Indeed, it takes 60 years for mallees to attain full canopy height, and 40-60 years to develop hollows in living timber (Haslem et al, 2011;Clarke et al, 2021), likely reflecting their value as thermal refuges. Climate change is expected to influence the incidence of wildfires in mallee habitat; in mallee regions of Victoria, for example, the number of days of extreme fire danger and the occurrence of dry lightning are predicted to increase (Clarke et al, 2021). Whether this leads to increased fire incidence depends on their interaction with reductions in fuel biomass and continuity, associated with increasing aridity (Bradstock, 2010).…”

“…Managing fire to maintain and increase the area of mallee in older age classes is likely to maximize the availability of thermal refuges, thereby reducing mortality during heat waves. Later seral stages in mallee also provide significant habitat structural elements that support a diversity of reptiles, birds and mammals (Yates et al, 2017;Clarke et al, 2021). Another option worth considering relates to habitat structures known to act as critical refuges in other hot, arid ecosystems.…”