Fire management implications of fuel loads and vegetation structure in rehabilitated sand mines near Newcastle, Australia

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Abstract

The objective of this study was to examine how vegetation structure and fuel loads change over time in sand mine rehabilitation that had never been subjected to fire. Ten sites were chosen incorporating eight rehabilitation pits of variable age (18 months to 17 years) and mining history (single mined sites or twice mined sites) and two forest sites with variable fire history (6 months and 18 years since fire). At each site, three 100 m transects were established on which six quadrats (0.5 m × 0.5 m) were randomly located in order to sample the fuel loads. All of the leaf litter and standing material (2.5 m high) present in a quadrat was collected, dried and weighed. To estimate the horizontal and vertical distribution of standing fuel at each site, a modified levy pole method was utilised. The average total mass of available fuel increased with the age of the rehabilitation (range 2.6 t ha−1 at 18 months to 17.0 t ha−1 after 17 years). This is indicative of the predominance of litter fuel in the total fuel load of the rehabilitated areas. The total fuel load and distribution of fuel in the 17-year-old rehabilitated sites was similar to that of the forest site that had not been burnt for 18 years. The incidence of dead standing material increased as the rehabilitated sites aged and appears to be related to vegetation senescing in the older sites. A lack of sub-canopy species in rehabilitated sites up to 17-years-old means that the majority of the vegetation is concentrated in the 0–2 m height range, in direct contrast to the forested site. The horizontal fuel distribution changes noticeably as the rehabilitation ages, but at no time does it resemble the distribution of the forested site, which indicates that the rehabilitated sites may require long-term management. The implications of these results to the fire management of the rehabilitated sand mines are discussed.

Introduction

Fire regime describes the frequency of fire occurrences, the fire intensity and the season in which the fire occurs for a particular region (Bacon and Dell, 1985). All three factors vary, though they are generally interlinked. The potential ignition sources and the prevailing weather conditions of an area determine the frequency of fire. The intensity of fire is generally determined by the quantity of fuel available and the rate at which it combusts. Only the fuel loads and vegetation structure of a particular site can be directly manipulated to influence the fire regime, which is the rationale behind much of the prescribed burning that occurs in Australia’s forests. More recently, it has been suggested that fuel loads may not directly influence fire spread but are instead a surrogate measure of fuel age, representing a range of structural factors such as height, continuity and greenness which do influence the behaviour of a fire (Cheney, 1996). This means that fuel loads can be used as an indirect measure of fire hazard for a particular area, although it is very site specific due to the variations in fuel loads across space and time (Adams and Simmons, 1996).

The vertical and horizontal distribution of fuel can be more important than the fuel loading in determining the behaviour of a fire (Conroy, 1993, Gould, 1993). In sclerophyll forests, the vertical distribution of fuel is significant, as crown fires will only develop if a continuous mass of fuel is available. Chandler et al. (1983) found that vertical gaps in the fuel continuum 1.5 times the height of the flames were sufficient to prevent crown fires from occurring, while a horizontal gap of 100 m or more was sufficient to ground a crown fire, forcing it to re-establish itself.

Until recently few comprehensive studies had investigated fuel loads, vegetation structure and subsequent fire management issues in rehabilitated areas on the east coast of Australia (Grant et al., 1998). However, a number of studies have investigated various aspects of fuel load management on rehabilitated sites in the jarrah forest of Western Australia (Collins, 1996, Grant et al., 1997), the tropical forests near Weipa in Queensland (Dahl and Mulligan, 1996) and the dune communities on North Stradbroke Island (M. Harwood pers. commun., 1998). For example, Grant et al. (1997) in a study examining fire ecology in bauxite mines in Western Australia found extremely high fuel loads in 11–13 year old rehabilitation with rapid reaccumulation following prescribed burning. They concluded that the fire management of rehabilitated areas will have to be different from that of the native jarrah forest due to differences in fuel characteristics, vegetation structure and fire behaviour of these two sites.

Mineral sand mining is an exogenous disturbance that totally removes both above and below ground vegetation. Rutile and Zircon Mines (RZM) Pty Ltd. has been engaged in mining the Tomago Sand Beds near Newcastle (New South Wales, Australia) since 1967 and has currently mined approximately 600 ha of open forest and swamp (5.7% of the Tomago Sand Beds). Fire exclusion has been the basic policy of the company for the duration of the mine life (J. Simpson, pers. commun., 1998). This policy may have, inadvertently, been creating a potential fire risk due to excessive build up of fuel loads within the rehabilitated areas. Extensive research has been performed at the Tomago mine site (Jackson and Fox, 1996, Prosser and Roseby, 1995), including a study on the effect of multiple disturbances on vegetation structure and growth (Fox et al., 1996). The overall objective of the current study was to examine how vegetation structure and fuel loads change over time in rehabilitated mineral sand mines at Tomago, and to determine the potential for fire, both uncontrolled and prescribed, in the rehabilitated mine sites and surrounding forests.

Section snippets

Site description

The Tomago Sand Beds Water Catchment Area (32° 52′S, 151° 45′E) is situated 16 km north of Newcastle on the New South Wales central coast, Australia (Fox et al., 1996, Prosser and Roseby, 1995). Economically significant deposits of the heavy minerals rutile, ilmenite and monazite are present in the sand beds, contributing 2–3% of the total volume of the sand mass. For a more detailed description of the study area see Fox et al. (1996).

Mining and rehabilitation

The mining at Tomago involves completely clearing existing

Fuel load and composition

The total average weight of available fuel increased with the age of the rehabilitation (R2 = 0.81, P < 0.001) ranging from a minimum of 2.6 t ha−1 at 18 months to a maximum of 17.0 t ha−1 after 17 years (Fig. 1). The major component contributing to this pattern was litter (R2 = 0.86, P < 0.001). The regression equation to predict the total mass of fuel from litter was:

Total Fuel (t ha−1 dry weight) =  0.679 + 1.069 × Litter (t ha−1 dry weight).

Generally, disturbing the top soil on more than one occasion did not

Fuel load and composition

Total fuel loads increasing with age is common within rehabilitated mine sites in Australia (Collins, 1996, Grant et al., 1997). Fuel loads will continue to increase until an equilibrium is established between fuel deposition and decomposition rates (McCaw et al., 1996). The fuel loads recorded in the current study are much lower than those recorded in rehabilitated bauxite mines in Western Australia for sites of similar age (Collins, 1996, Grant et al., 1997). For example, Grant et al. (1997)

Acknowledgements

A special thanks to Mr. John Simpson and Ms. Kate MacGregor for support and guidance throughout the duration of the project. Thanks also to Mr. Daniel Owen and Mr. Grant Sainsbery for assistance with fieldwork. We are also in debt to Rutile and Zircon Mines Pty Ltd. for both the opportunity to work at the site and also for the funding which made the project possible.

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