Australian Savanna Research Articles

Standing dead trees contribute significantly to carbon budgets in Australian savannas

Previous estimates of greenhouse gas emissions from Australian savanna fires have incorporated on-ground dead wood but ignored standing dead trees. However, research from eucalypt woodlands in southern Queensland has shown that the two pools of dead wood burn at similar rates. New field data from semiarid savannas across northern Australia confirmed that standing dead trees comprise about four times the mass of on-ground dead wood. Further, the proportion of total woody biomass comprising dead wood increases with decreasing fire frequency and a decreasing proportion of late dry season (August to December) fires. This gives scope for increasing the carbon stock in the dead wood pool with a reduced fire frequency. Following a previously published approach to quantify total dead wood loads in savannas, new and previously collected data on tree stand structures were used across the whole savanna zone to quantify dead wood loads in equilibrium with historic fire regimes. New parameters are presented for calculating dead wood dynamics including dead trees in Australia’s savannas.

Seasonal fine fuel and coarse woody debris dynamics in north Australian savannas

Several studies have separately explored accumulation of the dominant fuels (grass, fine litter (<6mm diameter) and coarse woody debris (CWD, 6–50mm diameter)) in north Australian savannas. We report an analysis of two longitudinal datasets describing how these three fuel components covary in abundance throughout the year in eucalypt-dominated savanna over a rainfall gradient of 700–1700mm mean annual rainfall (MAR). Our observations concur generally with previous observations that litter accumulation results in a late dry season (LDS) peak in biomass, whereas cured grassy fuels typically are seasonally invariant, and CWD inputs are associated with stochastic severe wet season storms and dry season fires. The distinct LDS litter peak contributes significantly to the potential for LDS fires to be of higher intensity, burn more fuel per unit area and produce greater emissions relative to early dry season (EDS) fires. However, Australia’s current (2018) formal savanna burning emissions avoidance methodology erroneously deems greater EDS fine fuel (grass and fine litter) biomass in four of nine designated vegetation fuel types. The study highlights the need to develop seasonally dynamic modelling approaches that better account for significant seasonal variation in fine fuel inputs and decomposition.

Vegetation change 10 years after cattle removal in a savanna landscape

Following the establishment of a conservation reserve, changes in ground stratum vegetation following removal of cattle were examined in a northern Australian savanna over a 10-year period. The floristic composition of 40 vegetation plots in lowland savannas were surveyed shortly after acquisition of the property, and then surveyed twice in the following 10 years after cattle removal. Some notable ecosystem-transforming introduced species (weeds) such as Themeda quadrivalvis remained relatively stable, whereas the pasture legume Stylosanthes scabra increased in cover. The species richness of both native and introduced plants increased. Various plant functional groups changed in relative cover, with a decline in relatively unpalatable grasses and a corresponding increase in palatable grasses, responses that are consistent with recovery from grazing pressure. Our results show that removal of cattle in highly disturbed savanna ecosystems can have both positive and negative results for native ground stratum vegetation in the first decade of recovery.

Feral cats are more abundant under severe disturbance regimes in an Australian tropical savanna

Abstract ContextThere is an increasing awareness that feral cats play a key role in driving the ongoing decline of small mammals across northern Australia; yet, the factors that control the distribution, abundance and behaviour of feral cats are poorly understood. These key knowledge gaps make it near-impossible for managers to mitigate the impacts of cats on small mammals. AimsWe investigated the environmental correlates of feral cat activity and abundance across the savanna woodlands of Melville Island, the larger of the two main Tiwi Islands, northern Australia. MethodsWe conducted camera-trap surveys at 88 sites, and related cat activity and abundance to a range of biophysical variables, either measured in the field or derived from remotely sensed data. Key resultsWe found that feral cat activity and abundance tended to be highest in areas characterised by severe disturbance regimes, namely high frequencies of severe fires and high feral herbivore activity. ConclusionsOur results have contributed to the growing body of research demonstrating that in northern Australian savanna landscapes, disturbance regimes characterised by frequent high-severity fires and grazing by feral herbivores may benefit feral cats. This is most likely to be a result of high-severity fire and grazing removing understorey biomass, which increases the time that the habitat remains in an open state in which cats can hunt more efficiently. This is due to both the frequent and extensive removal, and longer-term thinning of ground layer vegetation by severe fires, as well as the suppressed post-fire recovery of ground layer vegetation due to grazing by feral herbivores. ImplicationsManagement that reduces the frequency of severe fires and the density of feral herbivores could disadvantage feral cat populations on Melville Island. A firm understanding of how threatening processes interact, and how they vary across landscapes with different environmental conditions, is critical for ensuring management success.

Thresholds of fire response to moisture and fuel load differ between tropical savannas and grasslands across continents

AbstractAimAn emerging framework for tropical ecosystems states that fire activity is either “fuel build‐up limited” or “fuel moisture limited”, that is, as you move up along rainfall gradients, the major control on fire occurrence switches from being the amount of fuel, to the moisture content of the fuel. Here we used remotely sensed datasets to assess whether interannual variability of burned area is better explained by annual rainfall totals driving fuel build‐up, or by dry season rainfall driving fuel moisture.LocationPantropical savannas and grasslands.Time period2002–2016.MethodsWe explored the response of annual burned area to interannual variability in rainfall. We compared several linear models to understand how fuel moisture and fuel build‐up effect (accumulated rainfall during 6 and 24 months prior to the end of the burning season, respectively) determine the interannual variability of burned area and explore if tree cover, dry season duration and human activity modified these relationships.ResultsFuel and moisture controls on fire occurrence in tropical savannas varied across continents. Only 24% of South American savannas were fuel build‐up limited against 61% of Australian savannas and 47% of African savannas. On average, South America switched from fuel limited to moisture limited at 500 mm/year, Africa at 800 mm/year and Australia at 1,000 mm/year of mean annual rainfall.Main conclusionsIn 42% of tropical savannas (accounting for 41% of current area burned) increased drought and higher temperatures will not increase fire, but there are savannas, particularly in South America, that are likely to become more flammable with increasing temperatures. These findings highlight that we cannot transfer knowledge of fire responses to global change across ecosystems/regions—local solutions to local fire management issues are required, and different tropical savanna regions may show contrasting responses to the same drivers of global change.

Carbon isotope analysis shows introduced bovines have broader dietary range than the largest native herbivores in an Australian tropical savanna

AbstractAustralian savannas lack native megaherbivores (>500 kg body mass), but since the commencement of European colonisation in the 19th century bovine livestock, such as cattle (Bos sp.) and water buffalo (Bubalus bubalis), have established large feral populations that continue to geographically expand. The largest extant native herbivores are marsupials in the family Macropodidae (henceforth 'macropods': common wallaroo, Osphranter robustus [c. 40 kg]; antilopine wallaroo, O. antilopinus [c. 35 kg] and agile wallaby, Notamacropus agilis [c. 20 kg]). These species occur at low densities, with evidence that some species are in decline, the cause of which remains uncertain. We tested the hypothesis that bovines and macropods compete for nutritious forage in the North Kimberley, Western Australia by using carbon isotope analysis of feral cattle and native macropod dung (as a proxy for the relative contribution of C4 grass to their diet) and nutrient analysis of standing herbaceous biomass. Grass consumption varied between macropod species and was highest in larger wallaroo species and lowest in the smaller agile wallaby reflecting its broader diet. Grass consumption by wallaroos was maximal on fertile sites. The relative abundance of grass in the diet of cattle was lowest in the middle of the dry season with an interaction between fire and substrate fertility where grass consumption was highest on fertile sites, particularly those recently burnt. Grass consumption by cattle and wallaroos was negatively correlated with fibre content of live biomass, which was lowest on fertile and burnt sites. Introduced bovines shift their diets to non‐grasses as quality of herbaceous biomass declines with increasing fibre content, and by contrast, the largest macropod herbivores do not have this dietary flexibility. We conclude a plausible mechanism for the success of bovines and the decline of large macropods in Australian savannas is competition for nutritious grass that is abundant immediately after fire.

Regional seasonality of fire size and fire weather conditions across Australia's northern savanna

Australia’s northern savannas have among the highest fire frequencies in the world. The climate is monsoonal, with a long, dry season of up to 9 months, during which most fires occur. The Australian Government’s Emissions Reduction Fund allows land managers to generate carbon credits by abating the direct emissions of CO2 equivalent gases via prescribed burning that shifts the fire regime from predominantly large, high-intensity late dry season fires to a more benign, early dry season fire regime. However, the Australian savannas are vast and there is significant variation in weather conditions and seasonality, which is likely to result in spatial and temporal variations in the commencement and length of late dry season conditions. Here, we assess the temporal and spatial consistency of the commencement of late dry season conditions, defined as those months that maximise fire size and where the most extreme fire weather conditions exist. The results demonstrate that significant yearly, seasonal and spatial variations in fire size and fire weather conditions exist, both within and between bioregions. The effective start of late dry season conditions, as defined by those months that maximise fire size and where the most extreme fire weather variables exist, is variable across the savannas.

Interaction of Seasonal Sun-Angle and Savanna Phenology Observed and Modelled using MODIS

Remote sensing of phenology usually works at the regional and global scales, which imposes considerable variations in the solar zenith angle (SZA) across space and time. Variations in SZA alters the shape and profile of the surface reflectance and vegetation index (VI) time series, but this effect on remote-sensing-derived vegetation phenology has not been adequately evaluated. The objective of this study is to understand the behaviour of VIs response to SZA, and to further improve the interpretation of satellite observed vegetation dynamics, across space and time. In this study, the sensitivity of two widely used VIs—the normalised difference vegetation index (NDVI) and the enhanced vegetation index (EVI)—to SZA was investigated at four northern Australian savanna sites, over a latitudinal distance of 9.8° (~1100 km). Complete time series of surface reflectances, as acquired with different SZA configurations, were simulated using Bidirectional Reflectance Distribution Function (BRDF) parameters provided by MODerate Resolution Imaging Spectroradiometer (MODIS). The sun-angle dependency of the four phenological transition dates were assessed. Results showed that while NDVI was very sensitive to SZA, such sensitivity was nearly absent for EVI. A negative correlation was also observed between NDVI sensitivity to SZA and vegetation cover, with sensitivity declining to the same level as EVI when vegetation cover was high. Different sun-angle configurations resulted in considerable variations in the shape and magnitude of the phenological profiles. The sensitivity of VIs to SZA was generally greater during the dry season (with only active trees present) than in the wet season (with both active trees and grasses), thus, the sun-angle effect on VIs was phenophase-dependent. The sun-angle effect on NDVI time series resulted in considerable differences in the phenological metrics across different sun-angle configurations. Across four sites, the sun-angle effect caused 15.5 days, 21.6 days, and 20.5 days differences in the start, peak, and the end of the growing season derived from NDVI time series, with seasonally varying SZA at local solar noon, as compared to those metrics derived from NDVI time series with fixed SZA. In comparison, those differences in the start, peak, and end of the growing season for EVI were significantly smaller, with only 4.8 days, 4.9 days, and 3 days, respectively. Our results suggest the potential importance of considering the seasonal SZA effect on VI time series prior to the retrieval of phenological metrics.

Tree recruitment dynamics in fire‐prone eucalypt savanna

AbstractSavanna sites are idealized as exhibiting a demographic “bottleneck” physiognomy comprising a lower stratum of abundant resprouting persistent “juveniles” (albeit of indeterminate age), a mid‐stratum comprising relatively few released “saplings,” and a canopy‐layer cohort of “adults.” The magnitude and frequency of disturbance is considered to influence the critical transition from juvenile into adult phases. Under fire‐prone Australian savanna conditions, an extensive suite of both observational and manipulative studies have explored the responses of tree recruitment to fire disturbances. These studies oftentimes have produced seemingly highly disparate responses, particularly with respect to the differential responses of relatively fast‐growing eucalypts versus non‐eucalypts under different fire regime, and overstory competition, conditions. This study contrasts the responses of tree recruitment height classes to (1) the effects of total canopy removal from severe Cyclone Monica in 2006 over a subsequent 10‐yr period, (2) with observations from long‐term monitoring sites under relatively stable overstory conditions at Litchfield National Park over a six‐year period, (3) under ambient, frequent fire occurrence (mean > 0.5 fires/yr) at both locales including relatively severe late dry season fires. Recruitment at both study sites was represented mostly by resprouting, clonally reproducing juvenile trees <2 m tall, around half of which died over respective assessment periods. At post‐cyclone assessment plots, there was substantial release of eucalypts, including within the first five years, into the >5 m height class, with negligible corresponding release of non‐eucalypts. At Litchfield plots, there was negligible release of both eucalypts and non‐eucalypts. In discussion, we contrast these results with findings from relevant regional studies. We contend that collective disparate observations feasibly can be reconciled as reflecting significant interactions between fire regime characteristics and variable site overstory competition effects, such that the rate of recruitment of fast‐growing savanna eucalypt individuals into the midstory is relatively independent of the fire regime, but is significantly regulated by resource competition interactions especially with the overstory, whereas recruitment of non‐eucalypts is relatively independent of overstory competitive effects, but is suppressed under fire regimes dominated by frequent, especially severe fires.

Discharge-driven seasonal pattern of ionic solutes, suspended sediment and water clarity for a tropical savanna river in northern Australia

River water quality is driven primarily by discharge and a catchment’s natural and anthropogenic features. In this study, the underpinning role of discharge in driving water quality is examined for an Australian tropical savanna river. The seventh-order Daly River has a near-natural discharge regime and catchment that is highly weathered, and has low topographic relief, with most of its natural vegetation retained. Four discharge periods are defined according to surface and groundwater water source, and the magnitude and frequency of run-off events. On a seasonal scale, the river’s water quality reflected the mix of water sources. These comprised extremes of high surface water-driven discharge, high suspended sediment concentrations and low water clarity during the wet season, and low groundwater-fed discharge during the dry season with high water clarity. The suspended sediment concentration of run-off events during the dry-to-wet transition and the wet season were directly and inversely related to event maximum discharge respectively, suggesting the exhaustion of sediment supply to the river during the wet season. Variability associated with the discharge–water quality relationships was attributed to water source, hysteresis and event magnitude and timing. Suspended sediment yields and concentrations were relatively low compared with more anthropogenically affected Australian savanna catchments.

Are stacked species distribution models accurate at predicting multiple levels of diversity along a rainfall gradient?

AbstractWe use observed patterns of species richness and composition of ant communities along a 1000 mm rainfall gradient in northern Australian savanna to assess the accuracy of species richness and turnover predictions derived from stacked species distribution models (S‐SDMs) and constrained by macroecological models (MEMs). We systematically sampled ants at 15 sites at 50 km intervals along the rainfall gradient in 2012 and 2013. Using the observed data, we created MEMs of species richness, composition and turnover. We built distribution models for 135 of the observed species using data from museum collections and online databases. We compared two approaches of stacking SDMs and three modelling algorithms to identify the most accurate way of predicting richness and composition. We then applied the same beta diversity metrics to compare the observed versus predicted patterns. Stacked SDMs consistently over‐predicted local species richness, and there was a mismatch between the observed pattern of richness estimated from the MEM, and the pattern predicted by S‐SDMs. The most accurate richness and turnover predictions occurred when the stacked models were rank‐ordered by their habitat suitability and constrained by the observed MEM richness predictions. In contrast with species richness, the predictions obtained by the MEM of community similarity, composition and turnover matched those predicted by the S‐SDMs. S‐SDMs regulated by MEMs may therefore be a useful tool in predicting compositional patterns despite being unreliable estimators of species richness. Our results highlight that the choice of species distribution model, the stacking method used, and underlying macroecological patterns all influence the accuracy of community assembly predictions derived from S‐SDMS.

When rare species are not important: linking plot-based vegetation classifications and landscape-scale mapping in Australian savanna vegetation

Plant communities in extensive landscapes are often mapped remotely using detectable patterns based on vegetation structure and canopy species with a high relative cover. A plot-based classification which includes species with low relative canopy cover and ignores vegetation structure, may result in plant communities not easily reconcilable with the landscape patterns represented in mapping. In our study, we investigate the effects on classification outcomes if we (1) remove rare species based on canopy cover, and (2) incorporate vegetation structure by weighting species’ cover by different measures of vegetation height. Using a dataset of 101 plots of savanna vegetation in north-eastern Australia we investigated first, the effect of removing rare species using four cover thresholds (1, 5, 8 and 10% contribution to total cover) and second, weighting species by four height measures including actual height as well as continuous and categorical transformations. Using agglomerative hierarchical clustering we produced a classification for each dataset and compared them for differences in: patterns of plot similarity, clustering, species richness and evenness, and characteristic species. We estimated the ability of each classification to predict species cover using generalised linear models. We found removing rare species at any cover threshold produced characteristic species appearing to correspond to landscape scale changes and better predicted species cover in grasslands and shrublands. However, in woodlands it made no difference. Using actual height of vegetation layer maintained vegetation structure, emphasised canopy and then sub-canopy species in clustering, and predicted species cover best of the height-measures tested. Thus, removing rare species and weighting species by height are useful techniques for identifying plant communities from plot-based classifications which are conceptually consistent with those in landscape scale mapping. This increases the confidence of end-users in both the classifications and the maps, thus enhancing their use in land management decisions.

An experimental test of whether pyrodiversity promotes mammal diversity in a northern Australian savanna

Abstract The increasing awareness that a fire regime that promotes biodiversity in one system can threaten biodiversity in another has resulted in a shift away from fire management based on vague notions of maximising pyrodiversity, towards determining the optimal fire regime based on the demonstrated requirements of target species. We utilised a long‐running, replicated fire experiment on Melville Island, the largest island off the northern Australian coast, to test the importance of pyrodiversity for native mammals in a northern Australian savanna landscape. We first developed statistical models to determine how native mammal abundance has responded to nine years of experimentally‐manipulated fire frequency. Next, given each species' modelled response to fire frequency, we identified the level of pyrodiversity and optimal mix of fire frequencies that would be expected to maximise mammal diversity and abundance, and minimise extinction risk. This was done for both the entire mammal assemblage and for the mammal species currently declining on Melville Island. Fire frequency was a significant predictor of abundance of the northern brown bandicoot Isoodon macrourus, black‐footed tree‐rat Mesembriomys gouldii, brush‐tailed rabbit‐rat Conilurus penicillatus, grassland melomys Melomys burtoni, pale field‐rat Rattus tunneyi, and mice/dunnarts but not for the common brushtail possum Trichosurus vulpecula. The geometric mean abundance (GMA) of the entire mammal assemblage was positively associated with pyrodiversity, but peaked at an intermediate value. Hence, maximising pyrodiversity would reduce native mammal assemblage GMA below its potential maximum. The fire history for an area that maximised the entire native mammal assemblage GMA consisted of 57% long‐unburnt, 43% triennially burnt and <1% annually burnt. Pyrodiversity did not reduce the extinction risk, nor increase the GMA of declining mammals above that predicted in areas entirely annually or triennially burnt. Synthesis and applications. We demonstrate a useful approach with which to develop fire management strategies based on the demonstrated requirements of target species. By comparing the optimal fire regime identified for the conservation of threatened species and that identified for the entire mammal assemblage, we demonstrate the flexibility of this approach to tailor fire management to address specific management priorities in other fire‐prone environments.

Facultative and Obligate Trees in a Mesic Savanna: Fire Effects on Savanna Structure Imply Contrasting Strategies of Eco-Taxonomic Groups.

Fire is a major determinant of savanna tree communities and, as such, manipulation of fire frequency is an important management tool. Resolving the effects of fire management on tree size class distributions can help managers predict and plan for short-term ecological and economic outcomes, reveal different strategies by which woody plants cope with frequent fire, and help us predict vegetation changes under future fire scenarios. Savanna structure and size class distribution are strongly influenced by the ability of suppressed tree resprouts to escape stem death by frequent fire. A widespread assumption is that resprouts have an imperative to escape fire to reach sexual maturity in the canopy and thereby ensure long-term species viability. We use a census of Australian mesic savanna tree communities subjected to annual, triennial, and fire exclusion (unburnt) fire treatments to ask how fire frequency affects size class distributions within and between eco-taxonomic groups of species. Total tree densities did not significantly differ, but were highest in the triennial (7,610 ± se 1,162 trees ha−1) and unburnt fire treatments (7,051 ± se 578 trees ha−1) and lowest in the annual fire treatment (6,168 ± se 523 trees ha−1). This was caused by increased sapling densities in the triennial and unburnt fire treatments, predominantly of Acacia and pantropical genera. Eucalypts (Eucalyptus and Corymbia spp.) dominated the canopy across all fire treatments indicating relatively greater success in recruiting to larger sizes than other species groups. However, in the sub-canopy size classes eucalypts co-dominated with, and in some size classes were outnumbered by, pantropicals and Acacia, regardless of fire treatment. We hypothesize that such results are caused by fundamental differences in woody plant strategies, in particular sexual reproduction, that have not been widely recognized in Australian savannas.

Exotic grass invasion alters microsite conditions limiting woody recruitment potential in an Australian savanna

Andropogon gayanus Kunth. is a large African tussock grass invading Australia’s tropical savannas. Invasion results in more intense fires which increases the mortality rate of adult woody plants. Invasion may also affect community structure by altering the recruitment potential of woody plants. We investigated the effects of A. gayanus invasion on ground-level microclimate, and the carbon assimilation potential and recruitment potential of two Eucalyptus species. We compared microclimatic variables from the early wet-season and into the mid-dry season to coincide with the period of growth of A. gayanus. We assessed Eucalyptus recruitment by monitoring seedling establishment, growth and survival of experimentally sown seed, and estimating seedling density resulting from natural recruitment. A. gayanus invasion was associated with increased grass canopy height, biomass and cover. Following invasion, the understorey microclimate had significantly reduced levels of photon flux density, increased air temperatures and vapour pressure deficit. The conditions were less favourable for woody seedling with aboveground biomass of seedlings reduced by 26% in invaded plots. We estimated that invasion reduced daily carbon assimilation of woody seedlings by ~30% and reduced survivorship of Eucalyptus seedlings. Therefore, A. gayanus invasion reduces recruitment potential, contributing to the transformation of savanna to a grassland ecosystem.

Cyclones, fire, and termites: The drivers of tree hollow abundance in northern Australia’s mesic tropical savanna

Tree hollows are a vital wildlife feature, whose abundance and availability has declined in many regions due to broad-scale vegetation clearance, timber-harvesting, and disturbance such as fire. In the temperate forests and woodlands of eastern and southern Australia, the loss of large, old trees and associated tree hollows has severely impacted populations of hollow-dependent fauna. In the tropical savannas of northern Australia, many hollow-dependent mammals are in decline, and habitat simplification and resultant hollow limitation have been suggested as potential contributors to these declines. Hence, we sought to quantify the abundance of hollows, and identify the key drivers of this abundance in northern Australian savannas. We modelled the environmental and ecological correlates of hollow abundance across an extensive area of eucalypt savanna in Australia’s Northern Territory. We found that hollow abundance was significantly related to tree characteristics (size, species) and broad environmental gradients (annual rainfall, soil depth). Key disturbances – cyclones, fire, and termites – substantially disrupted these relationships and led to high variation in hollow abundance, even at a local scale. Hollow abundance across the study area was high by both Australian and global standards (hollows >5 cm entrance diameter: 88 ha–1, hollows >10 cm: 23 ha–1) and greatest in high rainfall areas (associated with the abundance of large eucalypts). Many arboreal mammal species in northern Australia have now contracted to higher rainfall parts of their former range (where hollows are at highest density); however such higher rainfall areas are also more likely to be affected by stochastic cyclonic events that can severely reduce the abundance of hollows. Hollow abundance was also affected by recent fire history and, in many areas, the current regime of frequent, high intensity fires will lead to marked reduction in this resource.

Efficiency of Individual Tree Detection Approaches Based on Light-Weight and Low-Cost UAS Imagery in Australian Savannas

The reliability of airborne light detection and ranging (LiDAR) for delineating individual trees and estimating aboveground biomass (AGB) has been proven in a diverse range of ecosystems, but can be difficult and costly to commission. Point clouds derived from structure from motion (SfM) matching techniques obtained from unmanned aerial systems (UAS) could be a feasible low-cost alternative to airborne LiDAR scanning for canopy parameter retrieval. This study assesses the extent to which SfM three-dimensional (3D) point clouds—obtained from a light-weight mini-UAS quadcopter with an inexpensive consumer action GoPro camera—can efficiently and effectively detect individual trees, measure tree heights, and provide AGB estimates in Australian tropical savannas. Two well-established canopy maxima and watershed segmentation tree detection algorithms were tested on canopy height models (CHM) derived from SfM imagery. The influence of CHM spatial resolution on tree detection accuracy was analysed, and the results were validated against existing high-resolution airborne LiDAR data. We found that the canopy maxima and watershed segmentation routines produced similar tree detection rates (~70%) for dominant and co-dominant trees, but yielded low detection rates (<35%) for suppressed and small trees due to poor representativeness in point clouds and overstory occlusion. Although airborne LiDAR provides higher tree detection rates and more accurate estimates of tree heights, we found SfM image matching to be an adequate low-cost alternative for the detection of dominant and co-dominant tree stands.

A comparison and validation of satellite-derived fire severity mapping techniques in fire prone north Australian savannas: Extreme fires and tree stem mortality

Severe fires in tropical savanna systems are recognised as incurring significant impacts on a variety of ecological attributes, including woody vegetation structure and greenhouse gas emissions. However, knowledge of the frequency and extent of severe fires is restricted given challenges associated with the development of reliable remotely sensed mapping procedures. This study takes advantage of three wildfires, 900–5300km2 in extent, containing very severely affected areas, occurring in semi-evergreen, eucalypt-dominated, tropical Australian savanna, which resulted in significant areas of complete canopy scorch, very significant tree stem mortality (24–55%), and associated loss of living above ground biomass (47–69%) at respective sites. Although increased map scale is generally considered to improve the reliability of fire severity mapping, our analysis found >90% agreement between Landsat and MODIS-derived burnt area mapping, and >80% for binary (severe vs. non-severe) fire severity mapping. Mapping of internal fire (unburnt) patchiness was enhanced with finer resolution Landsat imagery, but the much longer orbital return cycle precluded its use at two of the three sites given extended cloudy conditions. Application of an automated MODIS-derived fire severity mapping algorithm (overall reliability in 2015=75%) calibrated for generalised north Australian savanna conditions, suggests that 15% and 12% of Australia's 1.3Mkm2 tropical savannas region were burnt by severe fires in 2015 and 2016, respectively. The study illustrates the potential for MODIS-derived fire severity mapping, the impacts of very severe fires on stand structure, and ongoing challenges associated with deriving reliable fire severity mapping products in Australian savanna systems.

Fire-Driven Decline of Endemic Allosyncarpia Monsoon Rainforests in Northern Australia

Although contemporary fire regimes in fire-prone Australian savannas are recognised as having major impacts on an array of biodiversity and environmental values, a number of studies have observed significant monsoon rainforest expansion in recent decades. Here we assess the status of a locally extensive endemic monsoon rainforest type, dominated by Allosyncarpia ternata (Myrtaceae), restricted to sandstone terrain including in the World Heritage property, Kakadu National Park. We undertook assessments of: (1) geographic correlates of Allosyncarpia forest distribution; (2) change in canopy cover at 40 representative forest patches at topographically exposed sites with reference to a 60-year aerial photo and fine-scale image archive, and fire mapping data; and (3) structural characteristics associated with sites exhibiting stable, contracting, and increasing canopy cover. Mean canopy cover at sampled forest patches declined by 9.5% over the study period. Most canopy loss occurred at the most fire-susceptible patches. Assessment of structural characteristics at sampled sites illustrated that canopy expansion represented vegetative recovery rather than expansion de novo. The study (1) confirms the vulnerability of exposed margins of this forest type to fire incursions; (2) illustrates the magnitude of, and describes solutions for addressing, the regional conservation management challenge; and (3) serves as a reminder that, in savanna environments, severe fire regimes can substantially outweigh the woody growth-enhancing effects of other regional (e.g., increased rainfall) and global-scale (e.g., atmospheric CO2 fertilisation) drivers.

Intentional Fire-Spreading by “Firehawk” Raptors in Northern Australia

We document Indigenous Ecological Knowledge and non-Indigenous observations of intentional fire-spreading by the fire-foraging raptors Black Kite (Milvus migrans), Whistling Kite (Haliastur sphenurus), and Brown Falcon (Falco berigora) in tropical Australian savannas. Observers report both solo and cooperative attempts, often successful, to spread wildfires intentionally via single-occasion or repeated transport of burning sticks in talons or beaks. This behavior, often represented in sacred ceremonies, is widely known to local people in the Northern Territory, where we carried out ethno-ornithological research from 2011 to 2017; it was also reported to us from Western Australia and Queensland. Though Aboriginal rangers and others who deal with bushfires take into account the risks posed by raptors that cause controlled burns to jump across firebreaks, official skepticism about the reality of avian fire-spreading hampers effective planning for landscape management and restoration. Via ethno-ornithological workshops and controlled field experiments with land managers, our collaborative research aims to situate fire-spreading as an important factor in fire management and fire ecology. In a broader sense, better understanding of avian fire-spreading, both in Australia and, potentially, elsewhere, can contribute to theories about the evolution of tropical savannas and the origins of human fire use.