Evidence for the equal resilience of Triodia spp. (Poaceae), from different functional groups, to frequent fire dating back to the late Pleistocene

Rainfall-Linked Megafires as Innate Fire Regime Elements in Arid Australian Spinifex (Triodia spp.) Grasslands

Large, high-severity wildfires, or “megafires,” occur periodically in arid Australian spinifex (Triodia spp.) grasslands after high rainfall periods that trigger fuel accumulation. Proponents of the patch-burn mosaic (PBM) hypothesis suggest that these fires are unprecedented in the modern era and were formerly constrained by Aboriginal patch burning that kept landscape fuel levels low. This assumption deserves scrutiny, as evidence from fire-prone systems globally indicates that weather factors are the primary determinant behind megafire incidence, and that fuel management does not mitigate such fires during periods of climatic extreme. We reviewed explorer’s diaries, anthropologist’s reports, and remotely sensed data from the Australian Western Desert for evidence of large rainfall-linked fires during the pre-contact period when traditional Aboriginal patch burning was still being practiced. We used only observations that contained empiric estimates of fire sizes. Concurrently, we employed remote rainfall data and the Oceanic Niño Index to relate fire size to likely seasonal conditions at the time the observations were made. Numerous records were found of small fires during periods of average and below-average rainfall conditions, but no evidence of large-scale fires during these times. By contrast, there was strong evidence of large-scale wildfires during a high-rainfall period in the early 1870s, some of which are estimated to have burnt areas up to 700,000 ha. Our literature review also identified several Western Desert Aboriginal mythologies that refer to large-scale conflagrations. As oral traditions sometimes corroborate historic events, these myths may add further evidence that large fires are an inherent feature of spinifex grassland fire regimes. Overall, the results suggest that, contrary to predictions of the PBM hypothesis, traditional Aboriginal burning did not modulate spinifex fire size during periods of extreme-high arid zone rainfall. The mechanism behind this is that plant assemblages in seral spinifex vegetation comprise highly flammable non-spinifex tussock grasses that rapidly accumulate high fuel loads under favorable precipitation conditions. Our finding that fuel management does not prevent megafires under extreme conditions in arid Australia has parallels with the primacy of climatic factors as drivers of megafires in the forests of temperate Australia.

Mechanisms behind persistence of a fire-sensitive alternative stable state system in the Gibson Desert, Western Australia

Alternative vegetation types that switch from one to another under contrasting fire regimes are termed fire-mediated alternative stable states (FMASS). Typically, pyrophylic communities (i.e., vegetation assemblages favored by burning) dominate under high frequencies or intensities of fire. Conversely, fire-sensitive (pyrophobic) vegetation types persist under long fire-free conditions. As the persistence traits of plants of FMASS systems are generally poorly researched, threshold levels of pyric disturbance that trigger 'state-switching' are often unknown. Dense thickets of the obligate-seeder shrub waputi (Aluta maisonneuvei ssp. maisonneuvei [Myrtaceae]) form fire-retarding woody islands within highly flammable spinifex (Triodia spp.) grasslands in arid Australia. To examine the tolerance of Aluta thickets to burning, we investigated: (1) the influence of post-fire rainfall and fire severity on recruitment (a field study); (2) soil seedbank densities (a field study); and (3) fire-related dormancy cues in seeds (a germination trial). We found a positive relationship between recruitment and post-fire rainfall volume, and much higher mean recruitment at sites with high- (5.9 seedlings/m2) than low-severity-burnt (2.2 seedlings/m2) and unburnt shrubs (0.03 seedlings/m2). Post-fire regeneration was mediated by dense soil-borne seedbanks, and the germination trial indicated that smoke promoted germination. Although Aluta shrubs are invariably fire-killed, high-severity fires are unlikely to lead to state shifts from shrubland to grassland because of the ability of mature stands to regenerate from dense, fire-cued seedbanks. Nevertheless, given that Aluta seedlings are exceptionally slow-growing, post-fire droughts combined with fire-return intervals less than the Aluta primary juvenile period of c. 5 years could drive conversion from Aluta- to Triodia-dominated vegetation.

A review of the prevalence, utility, and caveats of using chloroplast simple sequence repeats for studies of plant biology

Microsatellites occur in all plant genomes and provide useful markers for studies of genetic diversity and structure. Chloroplast microsatellites (cpSSRs) are frequently targeted because they are more easily isolated than nuclear microsatellites. Here, we quantified the frequency and uses of cpSSRs based on a literature review of over 400 studies published 1995-2013. These markers are an important and economical tool for plant biologists and continue to be used alongside modern genomics approaches to study genetic diversity and structure, evolutionary history, and hybridization in native and agricultural species. Studies using species-specific primers reported a greater number of polymorphic loci than those employing universal primers. A major disadvantage to cpSSRs is fragment size homoplasy; therefore, we documented its occurrence at several cpSSR loci within and between species of Acmispon (Fabaceae). Based on our empirical data set, we recommend targeted sequencing of a subset of samples combined with fragment genotyping as a cost-efficient, data-rich approach to the use of cpSSRs and as a test of homoplasy. The availability of genomic resources for plants aids in the development of primers for new study systems, thereby enhancing the utility of cpSSRs across plant biology.

Fire history from life-history: determining the fire regime that a plant community is adapted using life-histories

Wildfire is a fundamental disturbance process in many ecological communities, and is critical in maintaining the structure of some plant communities. In the past century, changes in global land use practices have led to changes in fire regimes that have radically altered the composition of many plant communities. As the severe biodiversity impacts of inappropriate fire management regimes are recognized, attempts are being made to manage fires within a more 'natural' regime. In this aim, the focus has typically been on determining the fire regime to which the community has adapted. Here we take a subtly different approach and focus on the probability of a patch being burnt. We hypothesize that competing sympatric taxa from different plant functional groups are able to coexist due to the stochasticity of the fire regime, which creates opportunities in both time and space that are exploited differentially by each group. We exploit this situation to find the fire probability at which three sympatric grasses, from different functional groups, are able to co-exist. We do this by parameterizing a spatio-temporal simulation model with the life-history strategies of the three species and then search for the fire frequency and scale at which they are able to coexist when in competition. The simulation gives a clear result that these species only coexist across a very narrow range of fire probabilities centred at 0.2. Conversely, fire scale was found only to be important at very large scales. Our work demonstrates the efficacy of using competing sympatric species with different regeneration niches to determine the probability of fire in any given patch. Estimating this probability allows us to construct an expected historical distribution of fire return intervals for the community; a critical resource for managing fire-driven biodiversity in the face of a growing carbon economy and ongoing climate change.