Peter M. Attiwill

The disturbance of forest ecosystems: the ecological basis for conservative management

The extensive literature on natural disturbance in forests is reviewed in terms of the hypotheses: (1) that disturbance is a major force moulding the development, structure and function of forests; and (b) that management of forests for all their benefits can be controlled so that the effects can be contained within those which result from natural disturbance. The causal factors of natural disturbance are both endogenous and exogenous; there are major difficulties in the formal characterization of disturbance and of recovery after disturbance. As to the latter, the acceptance of classical generalizations of the nature of succession has led to particular difficulties in the assessment and interpretation of recovery. Tree fall, which creates gaps, is fundamental to the development of many forests, and has been most intensively studied in tropical forests of Central America and the Amazon and in temperate forests of North America. Tree fall is part of autogenic change; mechanisms of gap-filling and subsequent growth and species composition vary widely with forest type and geography. Disturbance by wind is particularly difficult to characterize. Wind varies along a continuum; the blow-down of an individual tree may be mostly due to autogenic processes of ageing and decay, whereas catastrophic hurricanes and cyclones may be defined as wholly exogenous. Nevertheless, the resilience in terms of species diversity of tropical forests following catastrophic disturbance by hurricane is remarkable. A number of studies support the view that the tropical forest in hurricane-prone areas is not a stable steady-state ecosystem but rather that heterogeneity is maintained by catastrophe. The ability to regenerate by suckers and the coincidence of regenerative space and gregarious flowering are important components of the response of rainforest following disturbance. For much of the world, ‘fire is the dominant fact of forest history’. As examples, fire and its effects are reviewed for the northern boreal forests, oak-pine forests and north-western sub-alpine forests of North America. The effect of fire on species composition varies with intensity and frequency. That, together with the popular view of fire as unnatural and therefore unacceptable, places great demands on management of forests for all of their benefits, including national parks and reserves. These difficulties also affect management of other ecosystems, such as Mediterranean-type shrublands and heathlands where species diversity, productivity and cycles of regeneration and degradation are governed by fire as a natural disturbance. Shifting agriculture is a traditional form of agriculture used by at least 240 million people in the humid tropics. Shifting agriculture, together with wind, lightning and fire, is an exogenous disturbance which has little effect on soil fertility and on structure and composition of the rainforest which re-establishes after abandonment. As the intensity of disturbance of rainforest increases, resilience of the forest decreases and the current problems of extensive clearing for improved pasture and of uncontrolled logging are resulting in degraded ecosystems. Regeneration follows the often extensive death of trees caused by outbreaks of insects in many coniferous forests of northern America. This disturbance by herbivory halts increasing stagnation (as measured by decreasing rates of ecosystem production and nutrient cycling) and reinitiates succession. Other disturbances to forests occur through damage from ice-storms, snow avalanches, erosional and earthquake landslides, and volcanic activity; the development of Nothofagus forests in Chile and New Zealand is determined by such catastrophic mass movements. An extensive literature supports the hypothesis that natural disturbance is fundamental to the development of structure and function of forest ecosystems. It follows that our management of natural forest should be based on an ecological understanding of the processes of natural disturbance. Whether or not we want to do this, and the extent to which we want to derive all of the benefits from the forest, including timber, depends on social attitudes. Whereas humanism may treat conservation as the wise husbanding of forests in the interests of social traditions and harmony, animism may give nature unalienable rights. The conclusion from this review is that the ecological framework of natural disturbance and the knowledge of its component processes and effects provides the basis on which we can manage our forests as a renewable resource which can be utilized so that the forests ‘retain their diversity and richness for mankinds continuing benefit’. Nowhere is this management more desperately needed than for the protection of the worlds tropical forests, its peoples and their cultures.

Ecological disturbance and the conservative management of eucalypt forests in Australia

Abstract There is increasing recognition that natural disturbance is a dominant force in forest development. This paper sets out to review natural disturbances and their effects in forests in Australia, and to determine whether or not the effects of management of forests for all of their benefits can be contained within the known effects of natural disturbance. The history and evolutionary significance of fire in Australia is reviewed, and the differing fire ecologies of two representative species, Eucalyptus regnans in the south-east (which is killed by fire and regenerates prolifically from seed after fire) and Eucalyptus marginata in the south-west (which survives all but the most severe fires and regenerates from both shoots and seeds) are outlined. The development of E. regnans following stand-replacing fire can be defined as highly resilient (returning quickly to the pre-disturbance state) and that of E. marginata as resistant (difficult to move from the pre-disturbance state). The topic of forest management in relation to disturbance is discussed within the context of forests of mountain ash ( Eucalyptus regnans ), the worlds tallest flowering plant, in south-eastern Australia. These forests in their natural state are essentially even-aged or at the most include two to three age-classes, the result of regeneration following stand-replacing bushfires. They produce timber of high value, and current land planning and management prescriptions allow for about one-half of the forest area to be managed for timber production on a sustained-yield basis. The silvicultural system is clear-felling, followed by burning to give stand-replacing conditions. Mountain ash regenerating from this system is one of the fastest growing tree species in the world and at its best has a volume increment of 50 m 3 ha −1 year −1 and net primary production of 35 t ha-1 year-1 over the first 10 years. There is no definitive evidence of a loss of diversity of plant species in timber production forests. The greatest problem is to manage land for timber production and for the diversity of fauna which depend on nesting hollows in large dead or living trees. However, all of the species found in older forests can be found in regrowth forests which have been burned or logged over the past 50 years and which include large dead or living trees with nesting hollows. There has been speculation that harvesting mountain ash forests by clearfelling and regenerating them following slash-burning will lead to a loss of nutrients and a consequent loss of productivity. Nutrient cycling in mountain ash is shown to be resilient to disturbance. Rapid uptake of nutrients by the regenerating forest, immobilization of nutrients by microorganisms, and increased rates of nitrogen fixation are processes which lead to the conservation of nutrients following stand-replacing fires. There is no evidence of productivity decline following bushfire or timber harvesting. Another concern about forest harvesting is that there will be a reduction in carbon storage. The break-even point for E. regnans plantations yielding short-lived products (e.g. paper) is 37 years and for E. regnans forests grown for sawn timber, 60 years. Harvesting forests on rotations of 80–120 years will therefore result in an increase in carbon storage; however, it would take several rotations to restore carbon storage equivalent to that of old-age forest. Much of the literature on disturbance illustrates the difficulties of dealing with traditional and sometimes deeply entrenched concepts of how plant communities develop and the nature of the end-state of development. Development of mountain ash forest fits within Eglers model of initial floristic composition; there is no stable and self-perpetuating end-point, the climax is probably no more than the state where species change is immeasurably slow, and maximum diversity and productivity are maintained by random periodic disturbance. Does this set of conclusions provide the basis for management of the forest? The management of diversity in heath is discussed as an example; the issue is less contentious than in forests since no commercial product is harvested. Coastal heath on soils of poor fertility in Victoria also follows the model of initial floristic composition; diversity and productivity are maintained by fire, decreasing markedly within 20–50 years of disturbance. Management can use fire to maintain a mosaic of regenerated heathland to accommodate the range of responses of fauna and flora to disturbance, thereby maximizing diversity. The solution of maintaining diversity through an intermediate level of disturbance is well-established in the ecological literature; through disturbance, an equilibrium is never reached and higher diversity is maintained. In the managed forest, management dictates the disturbance regime (frequency, size and intensity) which must be fitted to the attributes, or life histories, of the organisms to be managed. This paper presents the view that timber harvesting in Australian forests is ecologically sustainable and that the effects of management can be contained within the framework of those caused by natural disturbance. Given that each plot in the forest differs from all other plots and that none is at steady state, a solution for the management of diversity is to use the whole of the forest estate (parks, stream reserves, catchment reserves, old-age forest, forests of different ages resulting from past fires and logging) so that diversity of the estate, rather than diversity of each plot, is maximized.

In situ studies of nitrogen mineralization and uptake in forest soils; some comments on methodology

Abstract Aspects of the methodology of and interpretation of results from, in situ studies of N-mineralization are discussed with reference to data collected from 17 eucalypt forests in south-eastern Australia during a 5-year period. Results suggest that: (i) it is possible to maintain moisture of soils contained within corers at levels not significantly different from those of the surrounding soil; (ii) inorganic-N is not produced linearly over time under field conditions, nor should we expect it to be; (iii) mineralization rates are affected by all in situ methods. In each of the forests examined, the average rate of net N-mineralization decreased as the period of containment increased; (iv) shorter periods of containment (e.g. 1–2 weeks) reduce artifacts due to containment and are therefore preferable to longer periods (e.g. 4–8 weeks); and (v) longer periods of containment cannot substitute for increased replication of sampling. Further, when annual or seasonal rates of N-mineralization and uptake are to be calculated, it is desirable to increase replication of the bulk soil sampling for the first and last sampling dates.

Nitrogen and phosphorus cycling in relation to stand age of Ecucalyptus regnans F. Muell

The N and P contents of the litter layer and the return of these nutrients in litterfall were measured in seven stands of Mountain Ash (Eucalyptus regnans) ranging in age from 5 years to about 250 years. Both annual litterfall and nutrient return were correlated with stand basal area and were high compared with other productive eucalypt forests. In contrast, the fall of dead eucalypt leaves was constant with stand age, demonstrating that sites are fully occupied at an early age. Similarly, amounts of N and P in total leaf fall (overstorey plus understorey) were constant with stand age, except for low amounts in the stand aged 40 years where Acacia spp., important fixers of atmospheric N, were not prevalent. The decomposition constant (k) of organic matter in the litter layer decreased with stand age, from 0.31 year-1 at age 5 years to 0.23 year-1 at age 250 years. These constants also applied to N and P, indicating a tight coupling between organic matter decomposition and release of these nutrients from litter. The litter layer released about 30 kg ha-1 of N at age 5 years, and about 70 kg ha-1 at age 80 years. These results are discussed in relation to growth of Mountain Ash following fire, and the subsequent retention and accumulation of N during stand development.

Effects of fire and harvesting on nitrogen transformations and ionic mobility in soils of Eucalyptus regnans forests of south-eastern Australia

SummaryEffects of fire and forest harvesting on inorganic-N in the soil, on net N-mineralization, and on the leaching of NOinf3sup--N and metallic cations were measured in forests of Eucalyptus regnans following a severe wildfire in 1983. E. regnans regenerates profusely by seed after fire, and this study compared unburnt forest with forests burnt at varying intensities (surface fire and crown fire), and with logged and burnt forest (slash fire). Total inorganic-N in soil (0–5 cm) increased with increasing fire intensity to a maximum of 158 μg g-1 in the slash fire plot (compared with 51 μg g-1 in the unburnt forest) over the first 205 days after fire. Total inorganic-N returned to a concentration equal to that in the unburnt forest after 485 days at the slash fire plot, and after only 205 days at the surface fire plot. Studies of net mineralization in situ and of NOinf3sup--N in soil solution support the hypothesis that inorganic-N was immobilized in all of the burnt forests; microbial immobilization after fire is identified as a key process in N-conservation, limiting the substrate available for nitrification and thereby limiting the loss of N from the system by leaching. The concentrations of NOinf3sup--N and metallic cations in soil solution increased with increasing fire intensity. For the first 318 days after the fire, [NOinf3sup--N] in soil solution at 10 cm averaged 0.6 μg ml-1 in the unburnt forest, 9.7 mg l-1 in the surface fire plot, 26 mg l-1 in the crown fire plot, and 70 mg l-1 in the slash fire plot. The concentration of metallic cations in soil solution was significantly correlated with [NOinf3sup--N], the observed order of mobility being Ca2+>Mg2+>K+>Na+. Processes which limit the production and persistence of NOinf3sup--N in soil solution following disturbance will significantly reduce nutrient losses or redistribution.

The response of growth and foliar nutrients to fertilizers in young Eucalyptus globulus (Labill.) plantations in Gippsland, southeastern Australia

Abstract Fertilizer trials, which included rates of application of up to 400 kg ha−1 N, 200 kg ha−1 and 200 kg ha−1 K, were established in experimental plantations of E. globulus at three sites in Gippsland, southeastern Australia. These sites cover a range of annual rainfall and soil fertility, from 620 mm on a deep infertile sand, to 1000 mm on a rich gradational clay loam. Early growth at all sites was significantly increased by the addition of fertilizers and by age 4 years was consistently best at the highest combined rates of N and P. Foliar N and P concentrations were significantly increased at all sites by combined additions of N and P at Age 1, but were insensitive to treatment at Ages 2 and 4. Relationships between foliar nutrient concentrations and growth were site dependent. Correlations between foliar nutrients and growth in the same year were strongest at Age 1. Foliar nutrients at Age 1 were also strongly associated with growth at later ages at all sites. The best growth was associated with foliar concentrations at Age 1 of about 2.5% N and 0.23% P. Additions of P, alone or with N, consistently decreased foliar N P ratios at Age 1 to between 11 and 12 whereas N and P additions resulted in foliar N P ratios of 15 to 16 at Age 4. These ratios indicate a greater requirement for P than N in the first year suggesting that the N P ratio of fertilizers should be 1:1 within the first year, increasing to 2:1 in later applications. General recommendations for fertilizer addition in the routine establishment of E. globulus are presented. Mean tree volumes at Age 4 ranged from 0.014 to 0.019 m3 in control treatments and from 0.031 to 0.055 m3 at the highest rate of fertilizer addition. Growth responses to fertilizers were greatest, in both absolute and relative terms, at the most fertile site and appeared to be constrained by unfavourable soil texture and limited water availability at the other sites. Because the responses of E. globulus are site specific, detailed assessments of fertilizer requirements on soil types other than those described here require the establishment of further trials. A schedule for the establishment and monitoring of such trials is proposed.

Clearfelling and burning effects on nitrogen mineralization and leaching in soils of old-age Eucalyptus regnans forests

An argument against clearfelling and burning operations in forests is that nutrient reserves may be diminshed, leading to productivity decline over successive rotations. Nitrogen is of primary concern as it is readily volatilised and may be leached and thus the retention of nitrogen is a key recovery process following perturbation. In this study we measured N mineralization in situ and nitrogen concentrations in soil water from an old-age Eucalyptus regnans forest (about 250 years old) and from clearfelled forest in which treatment areas of unburnt ground, burnt ground and burnt ground maintained free from regrowth were established. Total inorganic N in the soil (0–5 cm) increased to a maximum of 168 μg g−1 of dry soil in clearfelled forest following burning, compared with 33 μg g−1 of dry soil in undisturbed forest. Increased total inorganic N in the soil returned to a concentration equal to that in undisturbed forest most rapidly in the clearfelled unburnt forest (6–9 months) and persisted for the longest amount of time in the most severely disturbed site (clearfelled burnt + herbicide-treated forest; 15–18 months). Net annual N mineralization in undisturbed forest soils (0–5 cm) averaged 74.9 μg g−1 of dry soil over the two years of the study. In contrast, annual average N mineralization was negative in two of the three clearfelled sites with 148.3 μg g−1 of dry soil of N immobilized in clearfelled and burnt forest. The concentration of NO3− in soil water increased with increasing forest disturbance. Over the first 260 days following clearfelling NO3−-N concentrations in soil water at 10 cm depth averaged 2.6 mg l−1 in undisturbed forest, 8.5 mg l−1 in clearfelled unburnt forest, 24.2 mg l−1 in clearfelled burnt forest, and 60.3 mg l−1 in clearfelled burnt + herbicide treated forest. Studies of net N mineralization in situ, and of NO3− in soil water, support the hypothesis that inorganic N was immobilized in all disturbed forests. Immobilization of N by soil micro-organisms is alone not sufficient to limit nitrification and NO3− leaching in disturbed E. regnans forests. Rapid uptake of N by regrowing vegetation is essential in reducing the availability of substrate for nitrification (NH4+) as well as in reducing NO3− concentrations in soil water. Clearfelling of the E. regnans catchment in this study did not significantly increase streamwater NO3− concentrations and demonstrates the resilience of E. regnans forests to leaching losses of N following destructive disturbance.

Nutrient balance in forests of northern Tasmania. 2. Alteration of nutrient availability and soil-water chemistry as a result of logging, slash-burning and fertilizer application

Abstract In situ measurements of nitrogen mineralization and nutrient mobility in severely perturbed forest soils (the forest was clear-felled, slash-burnt and then fertilizer amended) were compared with those in the undisturbed forest. Nitrification was absent in all soils. Nitrate concentrations in soil-water were low and probably not responsible for cation movement. Cation concentrations in soil-water were immediately increased by fire and by fertilizer additions and decreased continuously after these perturbations. Addition of strong-acid anions (SO 4 2− ) in fertilizer increased cation movement. Soil-water sampled at 10 cm depth was often coloured and contained high concentrations of oxidizable organic carbon; lower concentrations were found in samples collected from 30 cm depth. Uptake and mineralization of N were increased by logging and slash-burning. N-mineralization was promoted further by the addition of fertilizer-N and was reduced by the addition of P, or N and P together. Nutrient losses are exacerbated by fertilizer additions and restricted by the lack of nitrification, by nutrient uptake by soil microorganisms and vegetation, and probably by the formation of complexes between organic anions and cations in the soil profile.

The effects of fertilizers on early growth and foliar nutrient concentrations of three plantation eucalypts on high quality sites in Gippsland, southeastern Australia

Abstract Fertilizer trials of a standard design were established in experimental plantations of Eucalyptus regnans, Eucalyptus nitens and Eucalyptus globulus on sites of high potential productivity in Gippsland, southeastern Australia. The design involved factorial combinations of three rates of N (up to 400 kg ha −1 elemental) and four rates of P (up to 200 kg ha −1 ) with four additional treatments including K and trace elements. Treatments were complete after up to four applications at 2, 9, 14 and 26 months post-planting. P additions significantly increased foliar P concentrations and growth of E. regnans to 45 months but poor survival and competition from woody weeds limited growth responses to treatments. Foliar concentrations of N and P in E. nitens and E. globulus at age 1 year were significantly increased by N and P additions and were positively correlated with both early and later growth. Added P significantly decreased the foliar N to P ratios of all species at age 1 year, indicating that P additions, in particular, were required for good early growth. Combined additions of N and P significantly increased growth of E. nitens in the first year but growth responses to further additions of fertilizer were minimal. In contrast, growth of E. globulus increased in response to all fertilizer additions, leading to the greatest mean volumes at the highest combined rates of N and P. Relative gains in productivity were therefore greater in E. globulus (m.a.i. range at 71 months: 10–24 m 3 ha −1 year −1 ) than E. nitens (16–33 m 3 ha −1 year −1 ). There were no additional growth responses of any species to added K or trace elements. However, concentrations of K in E. regnans foliage at age 1 year were increased by K additions and were correlated with later growth. Low concentrations of Mg in E. nitens foliage suggest a probable response to Mg additions. General fertilizer prescriptions for the establishment phase of eucalypt plantations on sites of high quality are presented. Refinement of these prescriptions will rely on a better understanding of the nutritional requirements of individual species.

Nutrient balance in forests of northern Tasmania. 1. Atmospheric inputs and within-stand cycles

Abstract Eucalypt forest soils in northeastern Tasmania are ammonifying and contain low concentrations of extractable P, inorganic N and exchangeable cations, irrespective of underlying geology. Litterfall was proportional to productivity and could be described by a climatic index. Nutrient return in litterfall was unaffected by soil type and was the dominant transfer of N, P and Ca. Significant amounts of N (up to 8 kg ha−1 year−1) and other nutrients were added to the forests in rain. Although rainwater quality varied with distance from the coast, total nutrient input was dominated by rainwater quantity. Nutrient cycles and inputs were comparable with those found in other Australian forests of similar productivity. Soil in a cool-temperate rainforest was more organic than eucalypt forest and soils and contained higher concentrations of extractable P, inorganic N and other nutrients. Litterfall in the rainforest was rich in P, relative to the eucalypt forests, but poor in Mg. Nutrient absorption by the rainforest root mat was more effective than that in eucalypt forests.