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Winnellie, Australia

Lawes M.J.,Charles Darwin University | Adie H.,University of KwaZulu - Natal | Russell-Smith J.,Bushfires NT | Murphy B.,University of Tasmania | Midgley J.J.,University of Cape Town
Ecosphere | Year: 2011

To recruit to reproductive size in fire-prone savannas, juvenile trees must avoid stem mortality (topkill) by fire. Theory suggests they either grow tall, raising apical buds above the flames, or wide, buffering the stem from fire. However, growing tall or wide is of no advantage without stem protection from fire. In Litchfield National Park, northern Australia, we explored the importance of bark thickness to stem survival following fire in a eucalypt-dominated tropical savanna.We measured bark thickness, prefire height, stem diameter and resprouting responses of small stems under conditions of low to moderate fire intensity. Fire induced mortality was low (<10%), topkill was uncommon (<11% of 5 m to 37% of 1 m tall stems) and epicormic resprouting was common. Topkill was correlated only with absolute bark thickness and not with stem height or width. Thus, observed height and diameter growth responses of small stems are likely different pathways to achieving bark thick enough to protect buds and the vascular cambium. Juvenile height was traded off against the cost of thick bark, so that wide stems were short with thicker bark for a given height. The fire resilience threshold for bark thickness differed between tall (4-5 mm) and wide individuals (8-9 mm), yet tall stems had lower PTopkill for a given bark thickness. Trends in PTopkill reflected eucalypt versus non-eucalypt differences. Eucalypts had thinner bark than non-eucalypts but lower PTopkill.With deeply embedded epicormic buds eucalypts do not need thick bark to protect buds and can allocate resources to height growth. Our data suggest the only 'strategy' for avoiding topkill in fireprone systems is to optimise bark thickness to maximise stem bud and cambium protection. Thus, escape height is the height at which bark protects the stem and a wide stem per se is insufficient protection from fire without thick bark. Consequently, absolute bark thickness is crucial to explanations of species differences in topkill, resprouting response and tree community composition in fire-prone savannas. Bark thickness and the associated mechanism of bud protection offer a proximate explanation for the dominance of eucalypts in Australian tropical savannas. © 2011 Lawes et al.

Lawes M.J.,Charles Darwin University | Murphy B.P.,University of Tasmania | Midgley J.J.,University of Cape Town | Russell-Smith J.,Bushfires NT
Oecologia | Year: 2011

Eucalypts (Eucalyptus and Corymbia spp.) dominate (>60%) the tree biomass of Australia's tropical savannas but account for only a fraction (28%) of the tree diversity. Because of their considerable biomass and adaptation to environmental stressors, such as fire, the eucalypts may drive tree dynamics in these savannas, possibly to the exclusion of non-eucalypts. We evaluated whether the eucalypt and non-eucalypt components in tropical savannas are dependent so that changes in one component are matched by opposite trends in the other. Using tree inventory data from 127 savanna sites across the rainfall and fire frequency gradients, we found that eucalypt and non-eucalypt basal area and species richness had a negative relationship. This relationship was maintained across the rainfall gradient, with rainfall having a positive effect on the basal area and species richness of both components, but with a greater effect in non-eucalypts. Fire frequency negatively affected basal area, but not species richness, although basal area and species richness of eucalypts and non-eucalypts did not differ in their response to fire. Rainfall appears to set the upper bounds to woody biomass in these mesic savannas, while fire maintains woody biomass below carrying capacity and facilitates coexistence of the components. The magnitude of the component responses, particularly for non-eucalypts, is determined by rainfall, but their dependence is likely due to their differential response to both rainfall and fire, but not to competition for resources. Thus, while eucalypts dominate biomass overall, at high rainfall sites non-eucalypt basal area and diversity are highest, especially where fire frequency is low. © 2010 Springer-Verlag.

Russell-Smith J.,Bushfires NT | Russell-Smith J.,Charles Darwin University | Edwards A.C.,Bushfires NT | Price O.F.,University of Wollongong
Journal of Biogeography | Year: 2012

Aim Fire is a key agent in savanna systems, yet the capacity to predict fine-grained population phenomena under variable fire regime conditions at landscape scales is a daunting challenge. Given mounting evidence for significant impacts of fire on vulnerable biodiversity elements in north Australian savannas over recent decades, we assess: (1) the trajectory of fire-sensitive vegetation elements within a particularly biodiverse savanna mosaic based on long-term monitoring and spatial modelling; (2) the broader implications for northern Australia; and (3) the applicability of the methodological approach to other fire-prone settings. Location Arnhem Plateau, northern Australia. Methods We apply data from long-term vegetation monitoring plots included within Kakadu National Park to derive statistical models describing the responses of structure and floristic attributes to 15years of ambient (non-experimental) fire regime treatments. For a broader 28,000km 2 region, we apply significant models to spatial assessment of the effects of modern fire regimes (1995-2009) on diagnostic closed forest, savanna and shrubland heath attributes. Results Significant models included the effects of severe fires on large stems of the closed forest dominant Allosyncarpia ternata, stem densities of the widespread savanna coniferous obligate seeder Callitris intratropica, and fire frequency and related fire interval parameters on numbers of obligate seeder taxa characteristic of shrubland heaths. No significant relationships were observed between fire regime and eucalypt and non-eucalypt adult tree components of savanna. Spatial application of significant models illustrates that more than half of the regional closed forest perimeters, savanna and shrubland habitats experienced deleterious fire regimes over the study period, except in very dissected terrain. Mainconclusions While north Australia's relatively unmodified mesic savannas may appear structurally intact and healthy, this study provides compelling evidence that fire-sensitive vegetation elements embedded within the savanna mosaic are in decline under present-day fire regimes. These observations have broader implications for analogous savanna mosaics across northern Australia, and support complementary findings of the contributory role of fire regimes in the demise of small mammal fauna. The methodological approach has application in other fire-prone settings, but is reliant on significant long-term infrastructure resourcing. © 2012 Blackwell Publishing Ltd.

Murphy B.P.,South Dakota State University | Murphy B.P.,University of Melbourne | Bradstock R.A.,University of Wollongong | Boer M.M.,University of Western Sydney | And 7 more authors.
Journal of Biogeography | Year: 2013

Aim: Comparative analyses of fire regimes at large geographical scales can potentially identify ecological and climatic controls of fire. Here we describe Australia's broad fire regimes, and explore interrelationships and trade-offs between fire regime components. We postulate that fire regime patterns will be governed by trade-offs between moisture, productivity, fire frequency and fire intensity. Location: Australia. Methods: We reclassified a vegetation map of Australia, defining classes based on typical fuel and fire types. Classes were intersected with a climate classification to derive a map of 'fire regime niches'. Using expert elicitation and a literature search, we validated each niche and characterized typical and extreme fire intensities and return intervals. Satellite-derived active fire detections were used to determine seasonal patterns of fire activity. Results: Fire regime characteristics are closely related to the latitudinal gradient in summer monsoon activity. Frequent low-intensity fires occur in the monsoonal north, and infrequent, high-intensity fires in the temperate south, demonstrating a trade-off between frequency and intensity: that is, very high-intensity fires are only associated with very low-frequency fire regimes in the high biomass eucalypt forests of southern Australia. While these forests occasionally experience extremely intense fires (>50,000kWm-1), such regimes are exceptional, with most of the continent dominated by grass fuels, typically burning with lower intensity (<5000kWm-1). Main conclusions: Australia's fire regimes exhibit a coherent pattern of frequent, grass-fuelled fires in many differing vegetation types. While eucalypts are a quintessential Australian entity, their contribution as a dominant driver of high-intensity fire regimes, via their litter and bark fuels, is restricted to the forests of the continent's southern and eastern extremities. Our analysis suggests that the foremost driver of fire regimes at the continental scale is not productivity, as postulated conceptually, but the latitudinal gradient in summer monsoon rainfall activity. © 2012 Blackwell Publishing Ltd.

Murphy B.P.,Bushfires NT | Murphy B.P.,University of Tasmania | Russell-Smith J.,Bushfires NT | Russell-Smith J.,Charles Darwin University | And 2 more authors.
Global Change Biology | Year: 2010

Tropical savannas are typically highly productive yet fire-prone ecosystems, and it has been suggested that reducing fire frequency in savannas could substantially increase the size of the global carbon sink. However, the long-term demographic consequences of modifying fire regimes in savannas are difficult to predict, with the effects of fire on many parameters, such as tree growth rates, poorly understood. Over 10 years, we examined the effects of fire frequency on the growth rates (annual increment of diameter at breast height) of 3075 tagged trees, at 137 locations throughout the mesic savannas of Kakadu, Nitmiluk and Litchfield National Parks, in northern Australia. Frequent fires substantially reduced tree growth rates, with the magnitude of the effect markedly increasing with fire severity. The highest observed frequencies of mild, moderate and severe fires (1.0, 0.8 and 0.4 fires yr-1, respectively) reduced tree growth by 24%, 40% and 66% respectively, relative to unburnt areas. These reductions in tree growth imply reductions in the net primary productivity of trees by between 0.19 tCha-1 yr-1, in the case of mild fires, and 0.51 tCha-1 yr-1, in the case of severe fires. Such reductions are relatively large, given that net biome productivity (carbon sequestration potential) of these savannas is estimated to be just 1-2 tCha-1 yr-1. Our results suggest that current models of savanna tree demography, that do not account for a relationship between severe fire frequency and tree growth rate, are likely to underestimate the long-term negative effects of frequent severe fires on tree populations. Additionally, the negative impact of frequent severe fires on carbon sequestration rates may have been underestimated; reducing fire frequencies in savannas may increase carbon sequestration to a greater extent than previously thought. © 2009 Blackwell Publishing Ltd.

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