Forests

Definition of old-growth Forest

2.1 Perceptions of Old-growth Forest
The RAC Forests and Timber Inquiry found that the public perception of old-growth forest often confuses two concepts.

First, 'old-growth' has been used to refer to the oldest forest that has remained relatively free of European disturbance within a region. It is difficult to unambiguously identify forest on the basis of this criterion because of the likelihood of opportunistic logging during the early years of European settlement. The Inquiry described these forests as 'unlogged forests'.

Second, the term 'old-growth' has been used to emphasise the ecological qualities of stands of trees approaching the limit of their life span. These stands are often not, or only slowly, increasing in biomass and are considered to support a high diversity of plant growth forms as well as a high diversity of plant and animal species. The RAC Inquiry described these forests as 'ecologically mature forests'

The RAC Inquiry concluded that "the concept of 'old-growth forest' encompasses a complex mixture of the measurable biological attributes of forest ecosystems and a range of intangible personal values associated with experience (direct or indirect) of the forest environment. It is this combination of the measurable and the immeasurable that creates the difficulty in defining and estimating the extent of old-growth forest ecosystems".

2.2 Characteristics of Old-growth Forests
From these different perceptions, a range of attributes might be used to characterise old-growth forest (Dyne 1991, RAC 1992, Woodgate et al 1994). These range from static and dynamic ecological properties, to disturbance influences, to cultural and intangible properties. They can be summarised as follows:

(a) Structural and compositional characteristics
(these are either measurable or inferred and contribute directly to a description of the old-growth state)

• relatively large trees and other plants for the area
• relatively old trees and other plants in terms of development stage
• presence of large crown gaps (in some forest types)
• presence of tree hollows and/or fallen trees
• characteristic biotic composition
• presence of indicator species
• presence of certain growth forms, such as epiphytes in some forest types

• characteristic levels of gross and net productivity
• stable nutrient cycles, high litter levels (in some vegetation classes)
• low or negative biomass increment
• low rates of change in species, forest structure and ecosystem functioning

• evidence of physical disturbance (like fire, logging, grazing, mining)
• evidence of biological disturbance (such as introduced weeds or pathogens)

• aesthetic considerations (like antiquity and grandeur - in some vegetation classes)
• wilderness quality
• public perceptions and opinion
• ease of long-term management or maintenance

2.3 Definition of old-growth forest

2.3.1 Working definition
The old-growth survey of East Gippsland (Woodgate et al 1994) analysed the range of characteristics, their relationships and degree of correlation with older-age forests. The study found that the most prominent old-growth characteristics were represented by forest stands that comprise the oldest possible growth stage (or combination of growth stages) and the least disturbed forest for a given forested vegetation class. That study also concluded that the definition of old-growth must have ecological meaning and be based on thresholds that can be physically delineated, both on the ground and on maps, so that old-growth values can be accommodated in on-ground management. Essentially, the Department of Natural Resources and Environment (NRE) defines old-growth forest as :

"Old-growth forest is forest which contains significant amounts of its oldest growth stage - usually senescing trees - in the upper stratum and has been subject to any disturbance, the effect of which is now negligible."

2.3.2 Explanation of terms to accompany working definition
It is difficult to fully portray the complexity of old-growth forest in a short concise definition so the following set of explanatory notes should be read in conjunction with the definition.

(a) Primary characteristics : The definition is based on the two primary characteristics of old-growth forest; that is, growth stage and level of disturbance.

(b) Forest : For the purpose of this study, 'forest' is defined as woody vegetation with a potential top height greater than five metres and with a crown cover projection greater than 10 percent.

(c) Crown cover : This is expressed as the percentage of the site occupied by the vertical projection of the periphery of the tree crowns in the upper stratum. For this purpose, crowns are considered to be opaque.

(d) Growth stages : It is assumed that, for a particular site and vegetation class, old-growth forests have attained their oldest, naturally achievable growth stage(s) under a contemporary regime of natural disturbance which, in the Central Highlands, is generally fire disclimax.

The oldest growth stage is usually the senescing growth stage and, to qualify as old-growth, must be present as a dominant, co-dominant or sub-dominant component of the stand.

More than one growth stage (senescing, mature or regrowth) may be present in the upper stratum.

The morphology of the senescing growth stage in many eucalypts is characterised by declining crowns and dead or dying branches, although these and other characteristics may vary according to the vegetation class or forest type.

(e) Crown dominance : The terms dominant, co-dominant and sub-dominant refer to the area occupied by the tree crowns of a given growth stage in the upper stratum of the stand (they do not refer to the vertical structure through the profile of the crown). These dominance classes occupy more than 50%, 30% to 50% and 11% to 30% respectively of the total crown cover of the stand. When present in these proportions, the senescing growth stage is considered to significantly influence the ecological processes of the stand; for example, growth of younger trees, development of branch and bole hollows and nutrient recycling.

For a stand to qualify as old-growth, the regrowth growth stage, if present, must be sparse (less than 10% of the total crown cover of the stand). More regrowth than this generally indicates a greater than negligible disturbance of recent origin.

(f) Vegetation classes and forest types : The morphological characteristics that identify each growth stage vary according to the vegetation class (based on floristic composition and biophysical attributes) and forest type (dominant species, growth stage and density of overstorey trees), both of which are influenced by environmental site quality. For this reason the old-growth state manifests itself in different ways for different vegetation classes and forest types.

(g) Disturbance : All forests are assumed to have been subjected to some form of disturbance. Undisturbed forest is considered to be that for which there is no record or evidence of disturbance although wildfires almost certainly occurred in the past.

Negligibly disturbed forest is that in which disturbance is known to have occurred, but the disturbance is unlikely to have altered the structure (growth stage and crown cover) or the usual species composition which characterises a given vegetation class; or, if the alteration did occur in the past, it is no longer measurable.

Disturbance may be natural (like wildfire) or unnatural (anthropogenic or human-induced disturbance such as agricultural clearing, timber harvesting, grazing and mining). In the context of this study and in the absence of records that enable a systematic search, disturbances induced by indigenous people before European settlement are also treated as being natural.

(h) Intangibles : Old-growth forests have considerable intangible characteristics which are not directly addressed by the definition. They may include elements such as grandeur, antiquity, naturalness, spiritually or aesthetics. The type of disturbance influences the intangible characteristics of forest and determines their values within the old-growth domain.

(i) Old-growth forest dynamics : Significant anthropogenic disturbances may cause long-term changes to forest structure and floristic composition. Although such forests may not fulfil the old-growth definition after one cycle of regrowth and senescence, they may do so after several cycles provided they are not significantly disturbed in the meantime. The definition thus recognises the dynamic nature of old-growth and allows for forests to be recruited into, or excluded from, the old-growth domain over a period of time.

2.3.3 National forest policy statement
The working definition of old-growth forest was developed for the East Gippsland study and used in the Central Highlands to facilitate the mapping of old-growth forest over large areas in a quantitative and practical way using conventional field survey techniques. The NRE working definition outlined above is broadly consistent with, but essentially a refinement of, the more theoretical definition contained in the NFPS which defines old-growth forest as:

'Forest that is ecologically mature and has been subjected to negligible unnatural disturbance such as logging, roading and clearing. The definition focuses on forest in which the upper stratum or overstorey is in the late mature to overmature growth phases.'

'Ecologically mature forests are stands of trees approaching the limit of their life span. These stands are often either not or only slowly increasing in biomass and they usually support a high diversity of plant growth forms as well as a high diversity of plant and animal species.'

The relationship between old-growth forest, ecological maturity and Jacobs' growth stages is portrayed in conceptual terms in Figure 1.

Figure 1 Conceptual relationships between old-growth forest, ecologically mature forest and forest growth stages in wet and damp forest types

The NRE working definition of disturbance is more explicit than that in the NFPS. The latter confines 'negligible disturbance' to unnatural causes only and is silent on the question of natural forms of disturbance, although by inference it permits forest to be considered as 'old-growth' even if it has been subject to recent natural disturbance such as wildfires or cyclones. The NRE working definition provides for forests that have been subject to any disturbance, natural or unnatural, to be considered as old-growth provided that the effects of that disturbance are currently negligible. Thus a recent wildfire that has completely killed the overstorey trees would remain as old-growth under the NFPS but would not be considered as old-growth under NRE's working definition until it had recovered from the effects of that disturbance.