Publications
Biodiversity Series, Paper No. 8
Biodiversity Unit
David Cheal
School of Botany, University of Melbourne
Since Specht's work in the 1950's heathlands have been subject to much ecological research. The regeneration of these fire-prone shrubby communities is now accepted to include a short period of heightened species richness for a few seasons after fire, and thence a gradual decline as the heathlands age. Regeneration is restricted to immediately post-fire. The dominant shrubs regenerate from both dormant buds at or near ground-level and from seed held high in woody fruit (such as Banksia cones). Other species also regenerate in abundance after fire but not between fires. This vigorous regeneration grows rapidly at first (thanks to the nutrients flush from ash and burnt plant material) but growth rates soon decline. Heathland soils are deficient in most plant nutrients - particularly nitrogen and phosphorus.
But is this relatively simple story always reliable? Are heathland soils poor? If they are nutrient limited, then how can nutrient-demanding plants (such as Coast Tea-tree) invade, establish and crowd out the heathland species? Is there truly no regeneration in the absence of fires? Do fires always return heathlands to an earlier stage of succession, or can fires (or their lack) change heathlands irreversibly?
Recent work suggests that earlier conclusions need reassessment. This improved knowledge should enable us to implement effective fire management - fire management to support and maintain the heathlands, and not just to rationalise our use of fires applied for other objectives.
Key words
fire succession, heathland, management implications, fire intensity, season of burn.
Heathlands are species-rich vegetation communities dominated by small-leaved sclerophyllous shrubs less than 3m tall. They are common in Mediterranean climates (cool, moist winters and hot, dry summers), notably around the Mediterranean itself (where known as garrigue and other local names), California (as chapparal), Chile (as macchia), South Africa (as fynbos) and in southern Australia (as heathland or kwongan) (Specht 1979). However, heathlands are not restricted to such climates, but also occur elsewhere (eg. Britain and other parts of northern Europe, the east coast of Australia and in many tropical mountain ranges).
The essential common features appear not to be climatic. Instead, heathland communities are characterised by nutrient-poor soils and a fire regime of relatively frequent, high intensity fires (Specht 1979; Gill 1981). These two features substantially determine all community processes, including maturation and regeneration, and also drive the whole of the dependant faunal community.
6.3.1 Characteristics
Typically, heathlands have :-
High species richness and diversity
Dominance by small-leaved sclerophyllous shrubs
Paucity of grasses and ephemeral herbs
Abundance of sedges and similar sclerophyllous monocots
Most species with particular adaptations to recovery from fires (eg. serotinous fruits, dormant protected buds at the base, long-term soil seed store with fire-induced germination)
Principal (sole ?) regeneration immediately post-fire (thus negligible regeneration between fires)
Low growth rates (low primary productivity)
Soils with low to very low concentrations of inorganic nutrients (leading to tight nutrient cycling; low foliage turnover, thus long-lived plant parts; low protein but high carbohydrate foliage; high root:shoot ratios)
Heathlands may contain an emergent tree layer. The presence or absence of scattered trees is a relatively unimportant habitat feature. A dense tree layer may so change the availability of nutrients that sclerophyll forest develops. As a result, sclerophyll forest and heathlands are usually different expressions of the one ecological continuum (Specht 1981a).
6.3.2 Soils
The extremely low levels of available nutrients attributed to heathland soils can be said to be the principal determinant or cause of heathland vegetation. Ecological research in Australian heathlands has concentrated on fire and its effects. Nutrient cycling and soil processes have, with a few exceptions, been largely ignored.
The fire behaviour of heathlands is determined by the climate and the nature of the component species. Hot dry summers immediately follow the growing season (spring to early summer) in southern Australia. Periods of high wind, with very low relative humidity are common in summer. The highly sclerophyllous vegetation, often laden with flammable essential oils, burns readily, even when green and growing. The low nitrogen and high carbohydrate litter produced by heathland plants decomposes slowly and accumulates as the heathland ages (Specht et al. 1958; Bradstock & Myerscough 1981; Millsom 1992). This combination of weather, foliage and litter characteristics leads to a relatively high frequency of fires (Kruger 1983; McMahon 1984).
The various vegetation stratums are not widely separated in space and thus fires usually burn the whole of the heathland stand. In forests and other vegetation communities it is not uncommon for fires to burn only the understorey, or the canopy or avoid the moister slopes and gullies. In heathlands fires are usually complete, with all layers of the vegetation at the one site being burnt (though there may be scattered unburnt islands).
At first, it may appear that fire behaviour is the prime community determinant. However, the vegetation is peculiarly flammable. In spite of the climatic pre-disposition to fires, the nature of the component species appears to increase the intensity of fires - the vegetation is markedly sclerophyllous, laden with essential oils and produces a highly flammable, long-lived litter layer - all exacerbating the likelihood, spread and intensity of fires. Why are heathland plants pre-disposed to fires?
6.3.3 Nutrient Relations
Heathland soils are characterised by extremely low levels of available nutrients (Westman & Rogers 1977; Groves 1981, 1983; Lamont 1983). Nitrogen and phosphorus concentrations are low to very low, when compared with other vegetation communities. Concentrations of other nutrients, notably essential cations, are also often very low - including deficiencies of trace elements, such as molybdenum and copper. Nearly all heathland species exhibit intricate mechanisms for capturing and retaining these nutrients (Bowen 1981; Coleman & Specht 1981a, 1981b; Malajczuk & Lamont 1981; Lamont 1983; Lamont 1984; Low & Lamont 1990). Limiting nutrients are usually concentrated in foliage, but even here they are at relatively low concentrations (Mooney 1983; Cunningham 1993). As a result, phosphorus, nitrogen and other critically limited nutrients are dedicated to the essential biochemistry of plant function and plant structure is concentrated on various carbohydrates (cellulose, lignin etc.) rather than protein (Shaver 1983; Kutbay et al. 1994). In addition, plant secondary compounds (which function to reduce browsing and other damage by heterotrophs)are dominated by carbohydrates, such as tannins, rather than alkaloids and similar compounds with an appreciable content of nitrogen and phosphorus (Mooney, 1983).
Plant material rich in carbohydrates is necessarily very flammable. High carbohydrate plant litter breaks down very slowly, leading to substantial accumulation of dry litter (fuel) (Cunningham 1993). This is an inevitable concomitant of growth on soils with very low nutrient concentrations, notably nitrogen and phosphorus.
6.3.4 Fires
As a result, fires are a frequent and decisive influence on heathland composition and structure. In the absence of fires, heathlands are relatively stable communities, showing little variation from one season to the next. Ephemeral and seasonal plants are few, the ground flora is perennial and varies little from one season to the next and there is little change in canopy and community structure and composition. However, immediately after fires, there are major changes in both soil and vegetation processes.
Fires consume both litter and above-ground plant material. The removal of plant canopies results in an immediate increase in the amount of light at ground level (Begon et al. 1990) and an associated increase in soil moisture. Plant canopies pump soil water to the atmosphere, but fires temporarily remove all plant foliage (Judd 1990); in addition, nutrient levels change substantially (Cunningham 1993; Judd 1990).
Much has been written of the ash bed effect after fires, ie. the increased availability of nutrients in the surface soil, resulting from the rapid conversion of unavailable, organic forms to readily available, inorganic forms and deposition of ash resulting from the fire (Specht et al. 1958; Wright & Heinselman 1973; Rundel & Parsons 1980; Rundel 1983; Trabaud 1983). For many limiting nutrients, particularly some cations (such as Ca, K, Mg) the ash represents a significant nutrient flush (Bell et al. 1984). For nitrogen and phosphorus, the scene is less simple. Up to 80per cent of the nitrogen available before the fire may be lost through volatilisation of organic forms of nitrogen to ammonia and various nitrogen oxides (Rundel 1983). In addition, there may be appreciable loss of phosphorus in particles. Nevertheless, there is an undoubted temporary and highly localised enrichment of surface soil immediately after fire (O'Connell et al. 1978). Nitrogen losses are often rapidly made up from the increased nitrogen fixation that results from the increased growth of legumes and other nitrogen fixers; these plants also benefit from the flush of otherwise limiting nutrients - particularly phosphorus (Bond 1957; Rundel 1983).
Even where the heathland regrowth consists primarily of resprouter species (those that regenerate on mature root systems from dormant buds below ground level), the fire opens up space within the community, ie. unexploited resources (of light, nutrients and water (Wright & Heinselman 1973). These spaces, the first for many seasons, provide the best opportunity for regeneration from seed. Thus most regeneration in heathlands is restricted to the period immediately after fires. Characteristically, heathland regrowth consists of a mixture of resprout regeneration of many of the species present before the fire, seedling regeneration of other species that cannot resprout after burning and a suite of ephemerals that grow for one to fifteen years or more and then exist solely as a soil seed bank until the next fire (Bell et al. 1984).
This window of opportunity 'post-fire' is so important that many species have specific adaptations to regeneration at this time and only at this time (Gill 1981; Posamentier et al. 1981; Groves 1983; Bell et al. 1984). The seeds of serotinous species are retained in their large, elevated, woody structures - these protect the seed between and during fires. Banksia, Callitris and Casuarina cones, the fruit of Hakea and Isopogon and the huge, woody capsules of some eucalypts (eg. Eucalyptus macrocarpa and Eucalyptus tetragona) are familiar examples (Gill 1981; Bell et al. 1984). These protective, woody fruits dehisce and release their seeds after being burnt and the ensuing rain of seeds maximises germination as well as satiating seed predators (Bradstock & Myerscough 1981). At the same time, soil stored seed germinates en masse, presumably in response to poorly-understood germination cues from the burnt landscape (Gill 1981; Bell et al. 1984).
Heathlands are most species rich in the first few seasons after fires (McMahon 1977; Russell & Parsons 1978; Cheal et al. 1979; Posamentier et al. 1981; Bell et al. 1984; McFarland 1988) (Figure 6.1). All species in the mature community are present within the first season or two, though later dominants may be relatively unimportant at this early stage (Purdie & Slatyer 1976; Cheal et al. 1979). In addition, many fire ephemerals and disturbance ephemerals are also present and growing vigorously. Indeed, these short-lived herbs and weakly woody shrubs may dominate for the first few seasons (Bell et al. 1984).
Figure 6.1: Heathland at Wilsons Promontory, Victoria - Apparent Species Richness (ie. excluding species present as seeds or underground storage structures such as corns or tubers) (error bars are 95% confidence intervals) - from Cunningham 1993. Dates of most recent fire below the four bars to the left. Second bar from the right - unburnt since 1951 and lacking the invasive shrubs Kunzea ambigue & Leptospermum laevigatum. Right hand bar - heavily invaded by K. ambigua & L. laevigatum.
As the heathland ages, species richness apparently decreases - rapidly by about five years post-fire and then gradually as the long-lived, large woody dominants re-establish more or less complete canopies (McMahon 1977; Russell & Parsons 1978; Specht & Morgan 1981; Keeley 1986; Specht & Specht 1989). The oldest heathlands usually appear to be the most species-poor, but the likelihood of another fire is so great that stands rarely manage to reach the stage of decrease in the woody dominants (Hazard & Parsons 1977; Kruger 1983). Subsequent fires both rejuvenate the heathlands and recycle the nutrients (Campbell & van der Meulen 1980; Rundel 1983).
6.3.5 Fire Regime
Fire is a determining ecological condition within heathlands, rather plants rarely respond to fire itself, but to a particular fire regime. Important components of all fire regimes include intensity, frequency and season. These interrelated variables all contribute to the role of fire within heathlands.
Fire intensity is clearly related to fuel loads (weight of fuel available for combustion - and thus dependent on fuel moisture levels), air temperature, relative humidity and wind speeds.
Fuel loads accumulate with age, though the rate of accumulation varies and there is evidence that fuel loads do not increase with time in old (senescent) heathlands; they may even decrease as the heathland ages further (Specht 1981b; Bell et al.1984). The combustibility of these fuels is also dependent on seasonal features, such as fuel moisture levels and air temperatures (Gill 1975). As a result, summer fires are characteristically high intensity, whereas the prescribed burns of spring and autumn often burn at a much lower intensity; these prescribed fires are intentionally lit to establish or maintain firebreaks or reduce fuel levels. Nevertheless, it is important to distinguish the low intensity fires of forests (where whole vegetation stratums at the one site may remain unburnt) from the low intensity' fires of heathlands, where the fires may be decidedly patchy (with many scattered unburnt islands) but where all the vegetation at the one site is usually burnt or unburnt.
In summer-moist climate regimes (such as along the east coast) low-intensity heathland fires may stimulate resprouting but provide inadequate stimulus for many obligate seed regenerators (Posamentier et al. 1981; Benson 1985). Elsewhere, all fires promote some seed regeneration from co-ordinated seed release from protective woody fruit or the soil seed bank and generally enhance establishment by the removal of litter and competition (McMahon 1977; Cheal et al. 1979; Bell et al. 1984).
There has been some research on fire frequency. Frequent burning reduces both structural and floristic diversity. If burnt within their primary juvenile period (time to first fruit set) species dependent on seedling regeneration are eliminated (Siddiqi et al. 1976; Wark et al.1987; Cunningham 1993). This applies particularly to species with woody (or serotinous) fruit that protect the seed from predation and from fire; such fruits usually open immediately after being burnt and release the seed en masse to germinate in the highly suitable conditions post-fire. But even though many species flower soon after fire, there may be little seed-set until some seasons afterwards and thus heightened vulnerability to future fires persists. Repeated burning may reduce the capacity of resprouting species to regenerate and may eventually kill resprouting species (Bradstock & Myerscough 1988). Even the archetypically tolerant mallee eucalypts may die after repeat fires in close succession (Noble 1989).
By contrast, the effects of the long-term absence of fire are less clear. Fires are such a common feature in heathlands that very few stands remain unburnt for long periods. Nevertheless, it appears that the long-term absence of fire may have just as dramatic an effect on species composition and vegetation processes as fires that are too frequent. This will be discussed later.
Little is known of the effects of season of burn in heathlands. It has been assumed that prior to the arrival of humans (European settlement ?) fires were most frequent in summer (Gill 1975; Gill & Groves 1981), although there is some evidence that at least in some areas 'natural' ignition is most common in late spring to early summer (Cheal et al. 1979). Prescribed burns are usually lit in the cooler months of spring and autumn (when fire behaviour is more predictable and manageable). Cool-season burns often result in poor seed regeneration; from the plants being burnt whilst in flower or with immature fruit; from a reduced seed release due to the lower intensity fires; from greater mortality of released seed due to the higher soil temperatures over the following summer; and from increased seed predation on the seed released from the protective fruit and exposed on the soil surface (Gill & Groves 1981; Bond 1984; McMahon 1984; Cowling & Lamont 1987).
Fire intensity, fire frequency and season of burn are all components of the fire regime at a site. The temporal and spatial variability of this fire regime may be an important element in maintaining the local species richness so characteristic of heathlands.
6.3.6 Woody Plant Invasions
Heathlands are a major habitat component of many reserves and other protected situations. The character of those reserves, the habitats they offer to flora and fauna and the landscapes preserved may depend on the maintenance of their vegetation communities and processes. In many situations species formerly absent from heathlands, though native to adjoining plant communities, have established and are completely changing the nature of the heathlands (Hobbs 1991). Woody weed invasion has become a major consideration of heathland management (Kruger 1981).
Most work on woody weed invasion of heathlands has been done at Wilsons Promontory. As early as 1969 the Coast tea-tree (Leptospermum laevigatum) was recognised as a major invader of heathlands (Burrel 1969). Kunzea ambigua is also rapidly establishing in heathlands of Wilsons Promontory (Frood 1979; Judd 1990). Establishment of these tall shrubs results in a major reduction in apparent species richness, elimination of the former dominants and many associated species as well as major change in regeneration patterns, fire behaviour and nutrient processes (Specht 1981b; Cheal 1984; Judd 1990). Both shrubs have invaded from adjoining communities and were not components of heathlands before the last few decades (Burrell 1981).
Elsewhere, other woody weeds have become invasive, such as Acacia longifolia var. sophorae and Pinus radiata at Lower Glenelg in south-western Victoria and Pinus pinaster on French Island and in Langwarrin Flora Reserve (Cheal 1984). Woody weed invasion is also a major management problem in South African fynbos and similar communities, though in South Africa the major weeds are Australian species, such as Acacia cyclops, Acacia saligna and Hakea sericea, and the Mediterranean Pinus pinaster (Kruger 1981).
It has been suggested that fires are the key that enable these shrubs to establish in heathlands (Burrell 1981; Offor 1990). Fires produce a temporary increase in nutrients, notably phosphorus, and a synchronised opening of mature capsules, thus satiating seed predators (O'Dowd & Gill 1983). Other localised disturbance may also provide regeneration opportunities (see the discussion below)
Curiously, most of the invasive woody weeds are obligate seed regenerators and do not resprout after being burnt. This has led to recommendations that two (planned) fires in quick succession (ie. within the period of reproductive immaturity, viz. approx. five years) would eliminate these invasive shrubs (Burrell 1981; Judd 1990). However, this will also eliminate other obligate seed regenerators that have similar or longer periods of reproductive immaturity, such as Hakea ulicina, Hakea teretifolia and the rare Olearia allenderae. Furthermore, many species regenerate from both seed and resprouts post-fire and the ecological impacts of eliminating the seed-regenerated component are unknown. There is clearly need for great care in recommending the planned application of fire to control invasion by woody weeds.
6.3.7 Recent Research
Soil Nutrient Status
A paradigm of heathland ecology is that heathland soils are nutrient deficient. A number of soil analyses has demonstrated low concentrations of nutrients, particularly nitrogen and phosphorus. However, comparisons of heathland soils around the world have somewhat confused this accepted situation. Soils under Calluna and Erica-dominated heathland in northern Europe may have relatively high concentrations of nitrogen and phosphorus. The calcareous soils common around the Mediterranean often have high nutrient concentrations, and the dominant scrub vegetation (maquis etc.) may develop into forest or savanna woodland in the absence of continuing disturbance.
The soils under Californian chapparal and Chilean macchia are poor by comparison with these European sites; but they still have much higher levels of nitrogen, phosphorus and other nutrients than the leached podsols typical of heathland in Australia and fynbos in South Africa (Mooney 1983). These last two are considered to have some of the lowest concentrations of nutrients measured for any terrestrial vegetation in the world, with Australian soils having lower concentrations than those of South Africa (Groves 1983).
Whilst it is now accepted that Australian heathland soils are very low in nutrients when compared with heathland soils elsewhere, there have been very few studies comparing Australian heathland soils with soils from nearby forests, scrub or other vegetation communities. It has often been assumed that the heathland/forest boundary is substantially determined by a soil difference indicating increased nutrients in forest soils. However, there are very few comparisons of the nutrient status of adjoining heathland and forest soils.
Recent work on the nutrient status of soils from forest and heathland at Wilsons Promontory (Adams et al. 1994) has found that the surface soil in long-unburnt heathland has relatively high concentrations of carbon, nitrogen and phosphorus. Indexes of nitrogen and phosphorus availability are similar to those in highly productive eucalypt forests. This raises the question, that, if nutrient limitation is one of the principal determinants of heathland occurrence and development, of why heathlands are so common at Wilsons Promontory.
The concentrations of nutrient available to a plant are important in determining the growth rates of the plants. But there is more to available nutrients than their concentration in the soil. Relatively high phosphorus concentrations have occasionally been measured in soils that support heathland or other vegetation indicative of low phosphorus availability (Bowen 1981). In these situations phosphorus may be abundant in the soil, but in a form unavailable to the plant (eg. in stable minerals, such as monazite).
In high-rainfall environments, heathland is often found on soils that become seasonally waterlogged. The upper soil horizons are frequently podsolised (somewhat leached). More importantly, there is a strongly impeding layer at moderate depth, such that during the wettest period of the year, the entire soil profile is waterlogged (and thus relatively anaerobic). There is thus a dramatic contrast between the waterlogged, anaerobic soil of winter and the free-draining, dry & sandy root-zone of summer. This may explain the development of heathland on soils that have relatively high concentrations of nitrogen and phosphorus at Wilsons Promontory. Nitrogen and phosphorus are relatively unavailable during winter (active nutrient uptake is strongly limited by the anaerobic soil conditions) and growth is restricted to a short period either side of mid-summer (when seasonal drought may limit growth). The restricted volume of soil available for root exploitation may explain the low total amount of limiting nutrients available to the flora - thus leading to the development of a vegetation community more typical of soils with very low concentrations of nutrients. It should be noted that the litter layer of these soils has a relatively high carbon:nitrogen ratio and thus it is unlikely that acid conditions have led to the formation of peat (or peaty sands at least). Plant decomposition products, such as tannins, may lock up many plant nutrients in acid peats. Heathlands on Wilsons Promontory are largely restricted to the lower (gentler) slopes and flats. Winter rainfall readily flows off the steeper upper slopes, thus preventing the development of an anaerobic soil profile there.
Thus heathlands develop on soils where access to nutrients is limited. This may be because: there are very low concentrations of nutrients in the soil; or because those nutrients may be present but in a form that the plants cannot exploit; or because soil conditions limit the plants' abilities to access those nutrients.
Communities 'Cycling'
Heathlands are frequently subject to fire. Indeed, the regeneration patterns of component species are predicated and largely dependent on fire - the community seems pre-disposed to fire. The likelihood of fire is high enough that heathlands rarely mature into other communities and certainly do not reach a climax state sensu Clements (1936) - change is on-going.
Given the low growth rates of heathlands, it is not surprising that research has concentrated on the post-fire vegetation and regeneration patterns. Nevertheless, fires are not a certainty and, as fire control becomes increasingly effective in parks and other reserves, long-term changes in heathlands are becoming apparent as they age.
In the Big Desert region of Victoria, heathlands dominated by Banksia ornata, Leptospermum spp. and Casuarina spp. may change to low woodland dominated by Callitris verrucosa as the heathland ages (Cheal et al. 1979; Cheal & Parkes 1989) (Figure 6.2). This is not an example of a woody weed invasion, as C. verrucosa is a component of the heathlands from the earliest post-fire regeneration. It does not become dominant until many decades after fire, by which time the former dominants and nearly all the associated species have disappeared from the community. In the continued absence of fire a suite of small annuals enters the community and becomes seasonally common in the field layer. Note that by this stage the serotinous former dominants have been eliminated from the community. For these species there is no soil seed-store and regeneration is solely dependent on seed stored in elevated woody fruit. In addition, the resprouting heathland shrubs have also been locally eliminated as they also lack a soil seed-store. A fire at this (over-mature) stage of heathland will not reinstate the former mixed dominance. The vegetation has changed into an entirely different community with different regeneration patterns after the next fire and different floristic composition, structure and growth patterns.
Figure 6.2: Vegetation "succession" in heathlands of the Big Desert of Victoria.
In the long term absence of fire comparable communities elsewhere mature into woodlands or forests. For example from chapparal to woodlands of Quercus spp. in California (Keeley 1986) and from garrigue to maquis to woodlands dominated by Quercus spp. and/or Pinus spp. in the Mediterranean (Gill & Groves 1981). Even fynbos may change as Widdringtonia spp. establish to form an open woodland at 20 to 30 years post-fire (Kruger 1983). It is appropriate at this point to emphasise that Widdringtonia spp. (family Cupressaceae) are remarkably similar to the local Callitris spp. (family Cupressaceae) in appearance and ecology
Australian heathlands are subject to a higher fire frequency, with lower variance, than comparable nutrient-poor vegetation communities elsewhere (Kruger 1983). The Australian perception of cyclical succession based on frequent fires and subsequent vigorous regeneration may have determined the local approach to heathland management. Heathland senescence or over-maturity may be merely the local equivalent of seral change and maturity in the local sclerophyllous shrublands.
Regeneration
It has become generally accepted that regeneration in heathlands is restricted to the period immediately after fires. Classical (Clementsian) vegetation succession theory postulated that the growing conditions produced by early stages in vegetation succession enabled subsequent plant species to establish and eventually change the growing conditions (Clements 1936). These changes led to the disappearance of the earlier successional species and their replacement with species better adapted to the changed conditions, with a consequent change in the community dominance and processes. This process then continued until a stable climax was formed; the particular climax that eventuated at each site being dependent on the local climate (Clements 1936).
By contrast, it is now accepted that apparent succession in heathlands is a result of the gradual emergence and dominance of the species which were present before the fire, owing to their various rates of regeneration, development and senescence (Purdie & Slatyer 1976; Gill & Groves 1981; Bell etal. 1984). Thus it appears that the early dominance of herbs, particularly fire ephemerals, gives way to short-lived shrubs and finally to long-lived shrubs even though all these species have been present from the outset. Species dominant only in older vegetation have been present since fire, but their low growth rate prevented them assuming dominance until many years later. A corollary of this delayed succession is that species characteristic of the early successional stages disappear from the vegetation as it ages, and do not reappear until after the next fire.
However, some recent work has alluded to the possibility of regeneration in old, undisturbed heathlands (Bradstock & O'Connell 1988; Bradstock 1990). It has been suggested that seedling establishment is unlikely and infrequent, but may occur often enough to maintain an obligately seed regenerating species in heathland, even without the regeneration opportunities presented by fire (Bradstock & O'Connell 1988).
An opportunity to test the ability of species to regenerate in the absence of fire was presented by reassessment of heathland quadrats in Wyperfeld National Park. These quadrats were first assessed in 1978, as part of a preliminary study on the effects of fire in the Mallee National Parks of Victoria (Cheal et al. 1979). The identical quadrats were reassessed in 1992, as part of on-going research at the School of Botany, University of Melbourne. On both occasions I assessed the 20 quadrats (thus differences can not be attributed to operator effects). Thirteen of the quadrats had not been burnt between 1978 and 1992 - the most recent fire in this part of Wyperfeld was in 1959.
These results are preliminary and await confirmatory statistical analysis, nevertheless for 12 of the 13 unburnt quadrats there is an increased species number in 1992 compared with 1978! In spite of the absence of fire for the intervening 14years, species not present in the quadrats in 1978 established and grew. This is in marked contradiction of the expected gradual decline in species number over that period. This increase in species number is not due to an increase in the annuals or the dominant serotinous shrubs. The increase is due to establishment and growth of a number of subordinate woody shrubs (both seed regenerators and root resprouters), eg. species of Astroloma, Baeckea, Calytrix, Cryptandra, Leucopogon and the like. How can these typically heathland species have established and grown in the absence of fire?
There was an unprecedentedly severe frost in this part of Wyperfeld in 1981. Three succeeding nights of less than -11o C were recorded, with individual records reaching as low as -13oC (O'Brien 1989). One dramatic effect of this frost was to kill most (>90per cent) of the dominant Banksia ornata of the heathlands. Regeneration of B. ornata is abundant after fires as the serotinous cones release much seed. However, after the frost, the follicles of the Banksias opened gradually over many years and very few B. ornata seedlings managed to re-establish (O'Brien 1989). The resultant gaps in the community enabled a number of other (subordinate) shrubs to establish, and thus species number increased as the vegetation aged.
It appears that most heathlands regenerate only after fires as these present the major opportunities for regeneration; these are the only times when gaps appear in these communities. But, with the exception of serotinous shrubs and the obligate fire ephemerals, whose fruit dehiscence or germination are cued by fire, many heathland species are able to regenerate into any gaps - whether resulting from fire, frost or any other disturbance. In drier heathlands, such as those of the Big and Little Deserts of Victoria, fires are much less frequent than in heathlands of higher rainfalls. This lower frequency is thought to be due to the slower accumulation of fuel and the more open nature of older stands (large gaps breaking the fuel continuity). Sporadic disturbances and the resultant gaps are likely to be particularly important in maintaining species richness in these heathlands as they age; they may be especially important in maintaining many lower shrubs in the community (particularly the fleshy-fruited epacrids and the low, shrubby myrtles).
Regeneration is restricted to the gaps produced by disturbance. That disturbance is commonly fire, but may also derive from other natural events.
6.3.8 Management Implications
1. Fire is a normal component of the heathland environment. The regeneration of most of the component species is dependant on gaps in the community and these gaps (or regeneration opportunities) most often result from fires. Many species depend on fires for successful regeneration and some are only apparent in the first few seasons after fires.
2. Not all fires are identical. The frequency of burning is a major determinant of persistence and regeneration possibilities for many of the dominant species. Some research on fire frequency has been done and it is now possible to incorporate these results into fire management plans. However, there has been very little research on the effects of season of burning. Preliminary work strongly indicates that season of burn has a major and determining impact on post-fire regeneration and vegetation composition. Cool season (prescribed) fires do not mimic hot season (wild-) fires. Fire management plans that pretend otherwise will produce long-lasting (or even irreversible) changes in the vegetation.
3. Heathlands may change irreversibly in the long-term absence of fire. Fires are not necessarily an intrusive disruption in heathlands. They provide the only growing opportunities for some species, and the only regeneration opportunities for many others. Heathlands continue to change as they age - they do not reach a climax (unchanging) state.
4. Old heathlands or woodlands that develop when heathlands age are not unnatural. The vegetation present at a site is the result of the local environment, both past and present. An alteration to the fire regime will inevitably lead to a change in the local vegetation communities. Past (manipulated) fire regimes are one of the principal causes of the current vegetation patterns. The future vegetation patterns are dependant on the management practices currently being implemented. As heathlands have not developed in the complete absence of anthropogenic influences it is neither practical nor sensible to remove all such influences in order to maintain natural heathland communities.
5. Current ecological knowledge is inadequate to implement comprehensive land management plans for the long term future. Although we have some knowledge of the regeneration requirements for some heathland species, we are largely ignorant of: the effects of season of burn; of the regeneration possibilities in the absence of fire; of the determining soil nutrient processes; of the environmental variables that permit many similar species to persist in the same community; of the effects of stochastic major perturbations such as droughts or frosts; and of the regeneration behaviour of most of the rare or threatened species of both flora and fauna.
The comments made by Dr Mark Adams and Prof. Peter Attiwill on an early draft of this paper substantially improved it. I am also grateful for the intellectual environment offered by the staff and postgraduate students of the Ecology Laboratory at the School of Botany, Melbourne University.
The financial support of the Save the Bush Program (of the Australian Nature Conservation Agency) enabled completion of much of the above-mentioned field work.
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Quad.Id. Tot spp Tot Ann Ann Serot Serot Serot Serot Leg Leg Rrnon- Rrnon- Srnon- SRnon- Per. Per. Vegetation Community
(1978) spp. (1978) (1992) 1978 1992 1978 1992 ser.78 ser.92 ser.78 ser.92 ser.78 ser.92 Herbs'78 Herbs'92
(1992)
A03037 20 31 9 7 1 2 0 1 0 2 4 4 1 2 5 14 Leptospermum coriaceum
Heath
A03038 30 43 23 17 2 2 1 1 0 2 2 7 0 5 4 11 Callitris verrucosa
Shrubland
A03039 24 35 13 10 0 2 0 1 1 1 4 7 0 0 5 14 Triodia Grassland
A03040 30 33 14 8 0 3 0 2 1 1 5 5 0 2 7 12 Kunzea pomifera Heathland
A03041 17 29 8 6 1 2 0 1 0 2 3 5 0 4 4 10 Leptospermum coriaceum
Heath
A03042 29 35 16 11 0 2 0 2 1 2 4 6 1 1 6 12 Kunzea pomifera Heathland
A03043 33 37 20 17 0 0 0 0 1 1 4 7 1 1 7 10 Callitirs gracilis
Woodland
A03056 19 25 1 0 2 3 1 2 2 2 4 8 1 2 10 10 Mixed Heathland
A03057 23 25 14 13 3 2 2 2 0 1 2 4 1 1 4 4 Callitris verrucosa
Shrubland
A03058 21 27 3 4 2 1 1 0 2 2 5 6 0 1 8 13 Mixed Heathland
A03059 20 24 5 3 1 1 0 0 1 1 4 6 0 0 9 12 Leptospermum coriaceum
Heath
A03062 25 30 1 2 3 3 2 2 1 1 7 9 2 2 12 14 Mixed Heathland
A03063 24 26 5 6 2 1 1 0 1 2 4 8 0 1 11 8 Leptospermum coriaceum
Heath
A03064 26 23 1 3 2 1 1 0 2 2 7 6 0 1 15 10 Leptospermum coriaceum
Heath
A03065 34 35 3 3 5 6 3 4 1 2 11 9 4 4 12 14 Mixed Heathland
A03066 30 31 3 2 6 6 4 4 0 2 10 9 2 2 11 12 Mixed Heathland
A03067 35 38,36 2 2 6 6 4 4 2 3 8 11 6 4 11 13 Mixed Heathland
A03106 28 33 10 11 2 2 1 1 2 2 7 6 3 2 4 10 Leptospermum/Callitris
Heath
A03109 36 45 27 31 1 1 1 1 1 2 1 2 0 0 5 9 Callitris verrucosa
Shrubland
A03109 27 32 16 15 1 1 0 0 1 1 3 5 0 0 5 8 Leptospermum coriaceum
Heath
incl. both excl. excl. excl. Non-serot excl. Non-serot excl. incl. exclu. Stipa
Leptospermu Callitris Leptospermum Callitris legumes legumes Triodia, mollis, Pora
m gracilis gracilis Stipa micr
falcata