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Premature decline of eucalyptus and altered ecosystem processes in the absence of fire in some Australian forests.

Introduction

Aboriginal people used fire extensively and modified the pattern and structure of vegetation in Australia to improve access, to attract herbivores to 'green-pick', and to increase the abundance of food plants (Bowman, 1998). It is generally accepted that the frequency of fire has decreased, and the intensity and scale of fire has increased since European settlement of some Australian temperate forests (Bowman, 1998). Jackson (1968) developed the 'ecological drift' theory to describe how patterns of vegetation in western Tasmania are determined by fire interval, such that in a uniform landscape, low fire frequency is associated with the presence of rain forest, intermediate fire frequency is associated with eucalypt forest and high fire frequency is associated with button grass plain. A shift in mean fire frequency induces shifts (ecological drift) in vegetation composition and structure, and in soil physical and nutritional characteristics (Jackson, 1968). In Jackson's model (1968) the ecological drift away from eucalypts involved the death of trees by intense fire or old age, but not through premature decline and mortality. However, eucalypts can be replaced prematurely (in terms of their potential life-span) by rainforest species that can regenerate in the absence of fire in wet, temperate forests (Ellis, 1985; Werkman et al., 2008) and by Allocasuarina species in dry, temperate forests (Withers & Ashton, 1977; Lunt, 1998) in association with ecological drift in midstorey composition. These changes in the composition of midstorey vegetation have important implications for many forest values including biodiversity conservation, timber production, recreation, fire and water catchment management.

There is increasing evidence of premature decline of overstorey Eucalyptus in localised areas across a diverse range of forest types in temperate Australia (Archibald et al., 2005; Ellis, 1971, 1985; Kirkpatrick, 1986; Lunt, 1998; Withers & Ashton, 1977; Werkman et al., 2008). The forests in decline are not pathogen or pest affected, are not within agricultural landscapes, and have been subjected to changed fire regimes since European settlement. Below-average rainfall has prevailed across much of temperate Australia over the past three decades. However, the presence of declining forest adjacent to healthy forest (i.e., within identical soil and climatic environments), where the only difference is seemingly a management boundary (Davidson et al., 2007), suggests that reduced rainfall is not the sole or even the primary cause of premature and localised decline. Although anecdotal reports from forest and fire management authorities, coupled with our own observations, indicate that overstorey eucalypt decline is widespread in some forest types across temperate Australia (e.g. Jurskis, 2005), we currently lack quantitative information on the extent of decline and there is a clear need to investigate both the scale of the phenomenon and the ecological processes that underpin it. We emphasise that the research reported is site and species specific and that our model is thus applicable to only a sub-set of Australian ecosystems. For example some eucalypt species (Eucalyptus regnans in eastern temperate Australia and E. diversicolor in western temperate Australia) growing in relatively fertile soils and under high rainfall do not appear to exhibit premature decline with the development of a dense, shade adapted midstorey. We stress that in no way do we wish to infer that one vegetation type is qualitatively 'preferred' over another, but instead focus on the ecophysiological processes associated with premature decline of temperate overstorey eucalypts in Australia.

In this paper, we review the limited, but robust, published evidence for the change in forest structure in Australia associated with the suppression of fire in the recent past, and we highlight some ecological parallels and divergences between Australian eucalypt and some North American forests (Abrams 1992, 2003; Covington et al., 1997; Hart et al., 2005; Nowacki & Abrams, 2008). We propose a model of the ecophysiological processes that underpin premature decline of temperate overstorey eucalypts in Australia in the long absence of fire (Fig. 1), and describe the implications of these changes for forest, fire and biodiversity management.

Ecosystem Processes and Premature Decline of Eucalypts in the Absence of Fire

Our model of premature eucalypt decline predicts the impacts of an absence of fire on ecosystem processes to be: 1. the significant development of midstorey trees and shrubs and soil surface litter; 2. significantly increased leaf area (leading to increased inter-specific competition for soil water); 3. relatively cool and humid conditions at the soil surface (which affect mycorrhizal dynamics) and; 4. decreased availability of plant nutrients (Fig. 1).

Vegetation Structure

There is strong evidence from a range of different forest types that a shift from historically frequent, low intensity fire to fire exclusion or infrequent, high intensity fire has promoted development of fire intolerant, shade tolerant midstorey trees and shrubs, soil surface litter accumulation and the premature decline of fire tolerant, shade intolerant overstorey eucalypt species. Ellis (1985, Table 1) and Werkman et al. (2008, Fig. 2) have quantified a general pattern of premature decline of E. delegatensis and E. coccifera forests in Tasmania in the absence of fire. Decline has progressively developed following the cessation of aboriginal burning 160 years earlier and is associated with the development of a rainforest midstorey, significant soil litter accumulation, and alteration in the rooting environment and microbial community (Werkman et al., 2008). Ellis et al. (1980) further demonstrated a reversal of premature decline in E. delegatensis through removal of the rainforest mid-storey. Similarly Withers and Ashton (1977) reported increased dominance of shade tolerant Allocasuarina species, the development of a thick soil litter layer and the decline of the overstorey E. viminalis and E. ovata in remnant eucalypt woodland that had not been burnt for at least 90 years in the central south coast of Victoria. Lunt (1998) re-sampled Withers and Ashton's (1977) plots after an additional 25 years in the absence of fire, and showed that the density of shade tolerant Allocasuarina species had increased further (Table 1) and that the fire tolerant eucalypts had continued to decline over the period. Seedlings planted in plots that had been experimentally burnt by Withers and Ashton (1977) had survived, indicating that recruitment was possible where the thick litter layer was eliminated. Kirkpatrick (1986) described a similar pattern of Allocasuarina/Eucalyptus dynamics in dry Tasmanian woodland in the absence of fire (Table 1) and related poor eucalypt health with high total tree basal area (Kirkpatrick et al., 2007).

In Western Australia, Archibald et al. (2005) studied Eucalyptus gomphocephala woodland in which fire frequency had declined since the Yalgorup National Park was declared in 1968. Fire had been suppressed since 1976, and by 2004 the midcanopy vegetation structure had shifted to a lower density of the post-fire seed regenerator Banksia attenuata and a higher density of the shade tolerant Agonis flexuosa (Fig. 3) in parallel with significant canopy decline across all mature age classes of the overstorey dominant, E. gomphocephala.

[FIGURE 1 OMITTED]

There are parallels between Australian eucalypt forests and North American pine and oak forests in the way vegetation structure is altered when fire becomes less frequent. Prior to 1900 there was a long history of frequent, low intensity fire in mixed-conifer forests dominated by Pinus ponderosa (2-15 year intervals) and Pinus jeffreyi (14-18 years) (McBride & Jacobs, 1980). After 1900, livestock grazing and logging occurred in mixed species P. ponderosa forests and fire management focussed on suppression. This led to a shift from a bunchgrass or shrub-dominated understorey to a dense, suppressed midstorey of mixed woody species. Mortality in the over-stocked midstorey contributed to a large and elevated fuel load that facilitated the development of crown fires. Crown fire and competition with the developed midstorey reduced survival of remaining dominant Pinus ponderosa and vegetation composition shifted to include less fire-resistant species (Covington et al., 1997). Change in vegetation structure also encouraged development of thick litter layers. While these serve to reduce diversity and abundance of understorey species (Covington et al., 1997) and can cause 'stagnation' in nutrient cycling processes due to microbial immobilisation in the high lignin 'pine needle' environment (Hart et al., 2005), they may also become 'inconspicuous' sources of high concentrations of mineral nitrogen (N) and phosphorus (P) in surface runoff waters (Miller et al., 2005).

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

In the eastern U. S., vast expanses of deciduous forests were dominated by oak species throughout much of the Holocene epoch (Abrams, 2003). Low to moderate intensity understorey fires every 5-20 years were a critical ecological factor in the historical development and perpetuation of oak forests (Abrams, 1992, 1998, 2003). Factors such as thick bark, resprouting ability and the requirement of high-light, ash-bed conditions for seed germination and successful establishment (all of which parallel the ecological attributes of Eucalyptus species) led to the stability of oak populations on sites of extreme edaphic or climatic conditions, or areas that were periodically burned (Abrams, 1992). Nearly complete suppression of fire in forests in the eastern U.S. during the twentieth century resulted in dramatic reductions in recruitment of the dominant upland oaks on all but the most xeric and nutrient poor sites. Mixed-mesophytic and later successional hardwood genera, such as Acer, Betula, Fagus, Nyssa, Tsuga, and Prunus have rapidly replaced the oaks (Abrams, 1992, 1998, 2003). In the absence of fire, the increase in non-oak, less pyrogenic species is creating cooler and moister forest microenvironments and altering the rate of decomposition of the litter layer (Nowacki & Abrams, 2008).

Tree Water-Availability

Work investigating the effects of vegetation changes resulting from varied fire regimes on water availability for overstorey eucalypts is scant. Using sapflow technology, Hunt and Beadle (1998) showed that Eucalyptus nitens plantations with an Acacia midstorey transpired 30% more water than adjacent plantations without an Acacia midstorey. Kirkpatrick and Marks (1985) inferred that relatively high drought tolerance of Allocasuarina enabled it to displace eucalypts in a woodland of southern Tasmania. They further observed that fire, in eliminating a component of inter-specific competition, prevented drought damage to mature eucalypts (Kirkpatrick & Marks, 1985). Our recent findings (Close et al. unpubl. data, Pfautsch & Adams, unpubl. data) indicate similar increases in total water use, and thus a putative increase in competition, due to the developed midstorey in mature forests of E. regnans in Victoria, E. delegatensis in Tasmania and E. gomphocephala in WA.

A parallel process has been demonstrated in P. ponderosa forests of the North American. Significant mortality of overstorey P. ponderosa has been attributed to water stress induced by competition from midstorey vegetation that developed in the absence of fire in Yosemite National Park (Guarin & Taylor, 2005). In contrast, mortality of overstorey P. ponderosa was rare in northern Californian forest where the prevailing climatic conditions are similar to those in Yosemite National Park, but where prescribed fire was frequently applied (Guarin & Taylor, 2005). Sala et al. (2001) investigated transpiration in stands in which the fire intolerant/shade tolerant Abies lasiocarpa had encroached into the fire tolerant/shade intolerant Pinus albicaulis forest in the absence of fire. Transpiration of invaded stands was significantly increased due to increased leaf area index and leaf area to sapwood area ratios of 0.8 in Abies lasiocarpa relative to 0.3 in Pinus albicaulis. Further, Sala et al. (2005) reported that 8-9 years after being thinned, P. ponderosa had higher water potentials and stomatal conductances and consequently faster rates of photosynthesis at the end of the dry season than when not thinned.

Soil Microclimate and Microflora

Changes to vegetation structure have marked effects on soil microclimate. For example, significant development of a rainforest midstorey and litter accumulation in a declining E. delegatensis forest reduced average surface temperatures by 2[degrees]C and maximum soil temperatures by up to 8[degrees]C. Consequently, surface soils held considerably more water in summer relative to nearby, healthy E. delegatensis forest that was not in decline and did not carry significant midstorey development (Ellis, 1971). Note that this surface soil moisture accumulation occurred immediately under the relatively thick litter layer and was not necessarily indicative of plant water availability within the greater soil profile that plant roots access. This process is paralleled in eastern North American oak forests where the increased dominance of late-successional tree species has coincided with increased stand density and a microenvironment that is more shaded, humid and cooler. This has been termed the "mesophication" process, and it renders oak forests less likely to carry fire (Nowacki & Abrams, 2008).

Fire directly influences soil microbial communities by decreasing total organism biomass and selecting for heat-tolerant soil microbes. However, given the strong links between plant species or functional groups and soil microbial communities, Hart et al. (2005) argued where frequent fire has shaped the evolutionary history of North American forests, the effects of fire on soil microbial communities are mediated primarily by fire-induced changes in the vegetative community. Ellis and Pennington (1992) investigated the growth of E. delegatensis seedlings in soils from: (a) stands with healthy eucalypt that had been burnt 4 years previously, (b) secondary rainforest with dead and dying eucalypts, and (c) rainforest. Eucalypt seedlings grown in soil from healthy stands had vigorous root systems, with ubiquitous ectomycorrhizae. Seedlings grown in rainforest soils had localised ectomycorrhizae with occluded root tips. By contrast, seedlings grown in soil from stands with unhealthy eucalypts had sparse primary roots, with few short roots and few or no ectomycorrhizae (Ellis & Pennington, 1992). Inoculation of soil from unhealthy stands with soil from healthy stands resulted in increased seedling growth and favourable associations with ectomycorrhizae.

Evidence that fire-driven changes to vegetation modify soil microbial communities includes the significantly increased rates of soil respiration and nutrient-mineralizing enzymatic activities under grass (the pre-European dominant canopy type) than under Pinus ponderosa canopies that dominate southwestern North American ponderosa pine forests in the absence of frequent fire (Boyle et al., 2005). Hart et al. (2005) calculated that net N released directly via fire under the historical frequent surface fire regime provided far less plant-available N than did net N mineralisation from soil organic matter during the fire-free interval. Hart et al. (2005) further hypothesised that the relative importance of fire-induced changes to vegetation structure compared to post-fire nutrient mineralisation increases as the mean fire return interval increases.

Tree Nutrient-Availability

N Cycling

Soil N accumulates (via N fixation and atmospheric deposition) in the absence of fire. In wet temperate Australian forests, N-fixing Acacia can replace the soil N lost (via volatilisation) during fires within 10-20 years (Adams & Attiwill, 1991). Turner and Lambert (2005) suggested that regenerating forests undergo three stages of stand development and soil N dynamics:

1. Initially, N losses, due to tree uptake, are greater than inputs, leading to a net decline in soil N.

2. N uptake declines and inputs become greater than outputs. Soil N and carbon (C) accumulate. Mineral N increases while C:N decreases--burning regulates this process.

3. N accumulates in the soil and large increases in mineral N lead to N saturation. At this stage, stands lose nitrate through leaching that can be measured in soil and runoff water.

Jurskis and Turner's (2002) model of stand dynamics predicts that eucalypt decline in the absence of fire occurs because "... increased soil moisture and N status stresses the roots of established eucalypt trees". However, no data support this model in undisturbed native forests, although decline of E. ovata and E. camphora has been postulated as due to increases in available N from nearby land uses in an intensively managed agricultural landscape (Granger et al., 1994). In contrast our recent results (Close et al. unpubl. data) suggest that soil N accumulation in the absence of fire did not translate to increased N uptake by overstorey eucalypts in trials in Tasmania (E. delegatensis) and Western Australia (E. gomphocephala). The question of whether total soil N accumulation in the absence of fire translates to increased plant-available soil nitrogen and increased eucalypt uptake of N is a key issue that remains to be answered.

P Cycling

Generally P becomes increasingly limiting over time relative to N in the humus layer, and this is subsequently followed by reduced concentrations of P in litterfall (Wardle et al., 2004). O'Connell and Mendham (2004) concluded that regular fire was critical for cycling P and maintaining plant P uptake in the E. marginata forest of south-west Western Australia. Fire converts soil organic P, and P immobilized in vegetation and litter, into plant-available orthophosphate, leading to significantly increased plant-available P (Adams & Byrne, 1989). More recently, in a review of nutrient cycling, Adams (2007) noted that "widening (of) N:P without fire is the cause of large or even wholesale changes in diversity and productivity". Wittkuhn (2002) reported that 28-43% of P locked up in plant biomass pre-fire was plant-available in ash following low intensity fire in a E. marginata forest. These studies, and other studies that report increased availability of P in soils after fire (e.g. Tomkins et al., 1991), confirm that the vital P-mineralizing role of fire is an essential part of the ecology of eucalypt forests. We recently found that in a forest of E. delegatensis in premature decline with a rainforest midstorey, and where fire had been absent for >120 years, foliar P was almost three-fold less than in adjacent healthy E. delegatensis (Close et al. unpubl. data). The result implicates altered ectomycorrhizal relations.

Soil pH and Exchangeable Cations

Soil pH is usually increased following fire as a result of the oxidation of organic acids and the release of alkaline cations (Ca, Mg, K and Mn) previously bound to organic matter. Significant increases in soil pH (in excess of one pH unit for high fuel load treatments) were found following fire in a E. obliqua/E. rubida forest in Victoria (Tomkins et al., 1991). Increased soil pH after fire serves also to maintain availability of plant nutrients. Wittkuhn (2002) reported that 18-50% of cations and 27-85% of micronutrients were returned from plant detritus to ash after low intensity fire in E. marginata forest of south-west Australia. Significantly increased concentrations of Ca, Mg, Cu, Zn, S and B in leaf bases has been related to mineralisation and increased availability of those nutrients in fine ash following fire (e.g. Tomkins et al., 1991). Consistent with this we noted significantly reduced concentrations of foliar Cu in prematurely declining E. gomphocephala over significantly developed midstorey in the long absence of fire, relative to healthy E. gomphocephala that had little midstorey due to frequent (every 10 years) burns (Close et al. unpubl. data). Further, we found that E. gomphocephala seedlings planted immediately following a prescribed bum had significantly higher foliar Cu than seedlings planted at the same time in adjacent, unburnt E. gomphocephala woodland (Close et al. unpubl. data).

Conclusions

The main conclusions drawn from this review underpin our model of premature decline of temperate Australian overstorey eucalypts (Fig. 1). These are: (1) low fire frequencies since European settlement have promoted the development of dense, shade-tolerant midstorey vegetation (ecological drift) and the decline of overstorey eucalypts in particular areas across a wide range of forest types in temperate Australia. Where this occurs, the developed midstorey vegetation (2) competes with overstorey eucalypts for soil water, and (3) alters soil microclimate conditions that deleteriously affect overstorey eucalypt-ectomycorrhizal interactions. Thus, (4) fire plays a crucial role in controlling tree nutrient-availability by increasing soil pH and the availability of P and cations.

We highlight four clear parallels between the decline of northern American forests and Australian eucalypt forests in response to the long absence of fire: (1) the increased development of midstorey vegetation, the decline of fire tolerant tree species and the dominance of shade tolerant species; (2) increased total stand water use and increased mortality due to drought of fire tolerant/shade intolerant overstorey trees; (3) altered soil microclimate and microbial dynamics and; (4) increased occurrence and risk of wildfire in xeric P. ponderosa systems but decreased occurrence and risk of wildfire in mesic oak systems (mesophication; Nowacki & Abrams, 2008)--the latter is clearly similar to the mesophication of wet eucalypt forests, such as temperate E. delegatensis forests (Werkman et al., 2008). The main divergence between ecological processes appears to be the effect of a decrease in fire frequency on plant-availability of P and cations that occurs in Australian but not in North American forest systems. We speculate that this is due to inherently younger and generally less weathered soils in North American relative to Australian forest systems.

Our integrated model of premature tree decline in the absence of fire (Fig. 1) provides a valuable synthesis of current research on forest dynamics and ecosystem functions, and provides a framework to guide research on this issue. Further research is required to document the spatial extent of temperate eucalypt decline in Australian forests, and to relate the potential for decline to environmental parameters; for example, can forests susceptible to eucalypt decline in the absence of fire be readily identified, in a similar manner to Nowacki and Abrams' (2008) model of forests susceptible to mesophication in the eastern United States? Such research would greatly assist forest planning and management. From a mechanistic perspective, key areas of research include the ecophysiology of inter-species competition for water, and the effects of structural vegetation change on tree-ectomycorrhizal relations, soil and tree nutrition. Additionally, we recommend that further research is urgently required to develop ecological fire regimes that address a wide range of forest attributes and values, including ecosystem processes, fuel dynamics and biodiversity conservation.

We have shown that ecological drift (sensu Jackson, 1968) of midstorey composition and structure, and consequent mesophication and changes to the soil physical and chemical environment, has occurred in response to European fire management since settlement in Australia. In some forest types this alters competitive relationships between the midstorey and overstorey, leading to crown decline and premature mortality of the dominant overstorey trees.

Acknowledgements We thank Ian Abbott, Paul Barber, Frank Batini, Bob Ellis, Vic Jurskis, Rick Sneeuwjagt, Perry Swanborough, Kevin Tolhurst, John Turner, Dave Ward and Roy Wittkuhn for discussions during the preparation of this manuscript. David Bowman provided insightful comments on an earlier version of the manuscript. The Bushfire CRC provided Fellowship to DC Close and the operational funds for the unpublished research results by Close et al. referred to in this manuscript.

Published online: 3 April 2009

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D. C. Close (1,2,3,12) * N. J. Davidson (1,4) * D. W. Johnson (5) * M. D. Abrams (6) * S. C. Hart (7) * I. D. Lunt (8) * R. D. Archibald (3,4) * B. Horton (1,2,9) * M. A. Adams (10,11)

(1) School of Plant Science, University of Tasmania, Private Bag 55, Hobart 7001, Australia

(2) Bushfire Cooperative Research Centre, Level 5, 340 Albert Street, East Melbourne, Victoria 3002, Australia

(3) School of Biology and Biotechnology, Murdoch University, South Street, Murdoch, Perth 6150, Australia

(4) Cooperative Research Centre for Forestry, Private Bag 12, Hobart 7001, Australia

(5) Department of Natural Resources and Environmental Science, University of Nevada, Reno, NV 89557, USA

(6) School of Forest Resources, The Pennsylvania State University, 307 Forest Resources Building, University Park, PA 16802, USA

(7) School of Natural Sciences and Sierra Nevada Research Institute, University of California, Merced, CA 95344, USA

(8) Institute for Land, Water and Society, Charles Sturt University, P.O. Box 789, Albury, NSW 2640, Australia

(9) School of Agricultural Science, University of Tasmania, Private Bag 54, Hobart 7001, Australia

(10) School of Biological, Earth and Environmental Sciences, The University of New South Wales, Sydney, NSW 2052, Australia

(11) The University of Sydney, Sydney, NSW 2006, Australia

(12) Author for Correspondence; e-mail: dugald.close@utas.edu.au
Table 1 Examples of Increased Density of Fire Intolerant Species
and Decreased Density of Eucalypt Species in the Absence of Fire
in Temperate Australia

 Fire intolerant
Absence of 5 (years)
 Species % change

160 Nothofagus +77
 cunninghaii
Urban remnant Allocasuarina +257
 bushland verticillata
 1-2 low
 intensity burns
 between 1975-1985
115 (changes from Allocasuarina +266
 1975 to 2000) verticillata

 Allocasuarina +633
 littoralis

 Fire tolerant
Absence of 5 (years) Reference
 Species % change

160 Eucalyptus -73.5 Ellis
 delegatensis (1985)
Urban remnant Eucalyptus -10 Kirkpatrick
 bushland viminalis (1986)
 1-2 low
 intensity burns
 between 1975-1985
115 (changes from Eucalyptus -59 Lunt
 1975 to 2000) ovata (1998)

All are based on individual stem density plot data except
that of Ellis (1985), which is based on inference using
dead stems to indicate past E. delegatensis forest
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Author:Close, D.C.; Davidson, N.J.; Johnson, D.W.; Abrams, M.D.; Hart, S.C.; Lunt, I.D.; Archibald, R.D.; H
Publication:The Botanical Review
Article Type:Report
Geographic Code:8AUST
Date:Jun 1, 2009
Words:5122
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