Fire regimes and interval-sensitive vegetation in semiarid Gregory National Park, northern Australia
Jeremy Russell-Smith A B E , Cameron P. Yates A B , Chris Brock C and Vanessa C. Westcott DA Bushfires NT, PO Box 37346, Winnellie, NT 0821, Australia.
B Tropical Savannas Cooperative Research Centre, Charles Darwin University, Darwin, NT 0909, Australia.
C Parks and Wildlife Service, Department of Natural Resources, Environment and the Arts, PO Box 2130, Alice Springs, NT 0871, Australia.
D Department of Resource Management and Geography, School of Land and Environments, The University of Melbourne, Carlton, Vic. 3010, Australia.
E Corresponding author. Email: jeremy.russell-smith@nt.gov.au
Australian Journal of Botany 58(4) 300-317 https://doi.org/10.1071/BT99210
Submitted: 1 November 2009 Accepted: 21 January 2010 Published: 22 June 2010
Abstract
Few data are available concerning contemporary fire regimes and the responses of fire interval-sensitive vegetation types in semiarid woodland savanna landscapes of northern Australia. For a 10 300 km2 semiarid portion of Gregory National Park, in the present paper we describe (1) components of the contemporary fire regime for 1998–2008, on the basis of assessments derived from Landsat and MODIS imagery, (2) for the same period, the population dynamics, and characteristic fine-fuel loads associated with Acacia shirleyi Maiden (lancewood), an obligate seeder tree species occurring in dense monodominant stands, and (3) the fire responses of woody species, and fine-fuel dynamics, sampled in 41 plots comprising shrubby open-woodland over spinifex hummock grassland. While rain-year (July–June) rainfall was consistently reliable over the study period, annual fire extent fluctuated markedly, with an average of 29% being fire affected, mostly in the latter part of the year under relatively harsh fire-climate conditions. Collectively, such conditions facilitated short fire-return intervals, with 30% of the study area experiencing a repeat fire within 1 year, and 80% experiencing a repeat fire within 3 years. Fine fuels associated with the interior of lancewood thickets were characteristically small (<1 t ha–1). Fine fuels dominated by spinifex (Triodia spp.) were found to accumulate at rates equivalent to those observed under higher-rainfall conditions. Stand boundaries of A. shirleyi faired poorly under prevailing fire regimes over the study period; in 16 plots, juvenile density declined 62%, and adult stem density and basal area declined by 53% and 40%, respectively. Although the maturation (primary juvenile) period of A. shirleyi is incompletely known, assembled growth rate and phenology data indicated that it is typically >10 years. Of 133 woody species sampled, all trees (n = 26), with the exception of A. shirleyi, were resprouters, and 58% of all shrub species (n = 105) were obligate seeders, with observed primary juvenile periods <5 years. Assembled data generally supported observations made from other northern Australian studies concerning the responses of fire-sensitive woody taxa in rugged, sandstone-derived landscapes, and illustrated the enormous challenges facing ecologically sustainable fire management in such settings. Contemporary fire regimes of Gregory National Park are not ecologically sustainable.
Acknowledgements
The authors thank the staff of Gregory National Park and the following people who assisted with botanical and related assessments over the years: Darren Larcombe, Rohan Fisher, David Cheal, Andrew Edwards, Sindre Langaas, Grant Allan, Di Napier, Diane Lucas, Carrie Michel, Raelee Kerrigan, Jenny Risler, Felicity Watt, Brett Murphy, Andrea Johnson, Wilfrid Russell-Smith. Funding for the studies described here was provided by the Tropical Savannas Cooperative Research Centre and the Natural Heritage Trust. The manuscript was improved by constructive suggestions from two referees.
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