178 Age Structure of the karri forest: l. Defining and mapping structural development stages Age structure of the karri forest: 1. Defining and mapping structural development stages F.J. Bradshaw and M.E. Rayner Department of Conservation and Land Management, Manjimup, Western Australia 6258 Revised manuscript received 18 June 1997 Summary nan! tree in tall open (wet sclerophyll) forest in the extreme Even-aged stands of karri (Eucalyptus dive~:,icolor) develop south west of Western Australia. As one of the tallest through distinct stages of development - Establishment, Ju­ hardwoods in the world, its spectacular size and relatively venile, Immature, Mature and Senescent. This paper describes limited distribution give it considerable value for recreation a study in which Immature, Mature and Senescent stages were and conservation as well as for its timber resource and asso­ interpreted from aeiial photography. The age range of these ciated industry. Approximately half of the publicly owned stages was estimated by sampling diameters of dominant trees forest is available for timber harvesting and the other half is for which regressions of diameter and age had been estab­ managed for nature conservation. Almost all of the public lished. The stage and age of uneven-aged stands was based forest is managed by the Department of Conservation and on the biologically dominant cohort- the oldest cohort with a Land Management (CALM 1992). crown cover exceeding 25%. The results confirm that broad One of CALM's management objectives is the maintenance structural development stages can be mapped from API, and of an appropriate representation of each of the recognisable that the stages, defined from stand dynamics and silvicultural stand development stages oftheforest (CALM 1992). This is characteristics, correlate well with the age of the dominant required in the context of both sustained timber production cohort. Data for stages derived from interpretation and from and sustained habitat representation. The objective applies to historical records were combined to produce a map of devel­ the whole public forest, regardless of tenure and land use. opment stages for the karri dominant forest in the south west To meet this objective requires a knowledge of the present ofWestern Australia. This was part of a broader study to model distribution of stand development stages, their age and the the development of forest age structure into the future. way that they might change over time. This study was under­ taken in two parts. Part I reported here describes the factors Introduction influencing the distribution of the karri forest, its response to fire disturbance, the characteristics of the structural develop­ Karri (Eucalyptus diversicolor F. Muell.) occurs as a domi- ment stages, and the techniques used to map those stages. We LOCALITY ,,_.8 ····- ) ..,..N KEY • Karrl occurrence Cl Figure3 $CALE 0 10 20 30 -'10 50Km ·-aoo _ lsohyet (mm) Figure 1. Karri distribution Australian Forestry Vol 60, No. 3 pp. 178-187 179 confirm the relationship between age and development stage a sunogate for bio-diversity - the 'coarse filter' approach and map the location of the existing stand development stages (Ranis 1984, Hunter 1990). To be useful for this purpose, in the main karri belt. Part 2 of the study (Bradshaw and development stages need to have easily recognised and quan­ Rayner 1997b) estimates the presen't age distribution of the tifiable physical attributes that facilitate mapping. They also forest, makes a projection of age and development stage into need to reflect the dynamics of the stand throughout its life, the future and considers the implications for management. and should be relevant to habitat requirements. Even-aged stands of karri develop through several distinct Background stages from establishment to senescence and therefore fit the first two ofthe requirements mentioned above. Although these stages describe key structural changes in the forest we make Description of the karri forest no assumptions conceming specific habitat which may be Karri occurs in three broadly discrete areas - the 'main karri associated with these stages. Our purpose is to provide a spa­ belt' and two outliers on the west and the south coast (Figure tial and a temporal framework on which any of these particu­ 1). The climate is distinctly Meditenanean and the forest oc­ lar values may be extrapolated from the stand to the forest as curs generally in areas that receive 1000 to 1400 mm of an­ a whole. nual rainfall with a summer (driest quarter) rainfall exceed­ Five development stages are recognised in the overstorey of ing 70 mm per month. even-aged kani forest (CALM 1992). These are the Estab­ Within the climatic zone suited to karri, distribution is deter­ lislunent, Juvenile, Immature, Mature and Senescent stages. mined by soil type (McArthur and Clifton 1975, Bradshaw These are described in detail in Appendix I. and Lush 1981 ). Karri occurs within a mosaic of other forest types and, within the main karri belt, karri dominant forest The role of fire in stand initiation and structural occurs in more than 2000 discrete patches, 30% of the area occurring in patches of less than I 00 ha. Karri commonly variation occurs in mixture with marri (Corymbia calophylla) and less The establishment stage ofkani begins with disturbance, usu­ commonly with jarrah (E. marginata). In the southernmost ally fire, that exposes mineral soil and provides the opportu­ part of the distribution it also occurs in mixture with red tin­ nity for gennination. Regeneration will occur if seed is present gle (E. jacksonii) and yellow tingle (E. guilfoylei) (Wardell­ at the time of disturbance and will persist if there is vacant Johnson and Coates 1996). space in the canopy of sufficient size to allow it to become Although it is a wet sclerophyll forest there is an absence of established (Breidahl and Hewett 1995). Large gaps will give the cool, temperate rainforest elements common to many wet rise to even-aged stands which will develop through the dis­ sclerophyll forests of the east coast of Australia (Ashton 1981, tinct stages described in Appendix I. Progression through Christensen 1992). the whole series of stages will only occur where there is no significant subsequent disturbance to the overstorey during The Mediterranean climate of the south west ensures that this period. Such a situation would be rare since most stands weather conditions conducive to fire occur every summer. In will have been affected by fire, the most common form of the driest part of the year, when the Soil Dryness Index (Mount disturbance, to some degree during that time. 1972) exceeds 50, there are approximately 110 rain-free days. This contrasts with about 165 rain-free days for the dry Fire may result in a number of different outcomes. If it is sclerophyll jarrah forest and about 80 rain-free days for the severe it may kill a large proportion of the overstorey, trun­ wet sc!erophyll E. regnans forest in Tasmania. McCaw 1 (un­ cating the cycle and providing the opportunity for regenera­ published data) suggests that ignition is possible on about 70 tion. The regenerating patch size will vary widely depending of the 110 rain-free days in the kani forest. on the severity of the disturbance and the development stage (and hence fire sensitivity) of the stand. Following intense The present age distribution reflects natural disturbance events wildfires a substantial proportion of the overstorey may be such as fire and cultural disturbances such as harvesting and killed and patches of regeneration will be larger and more abandoned agricultural clearing. Harvesting began in 1913 frequent. While stand-replacing fires do occur, the large con­ in the main belt using at different times both clearfelling and tiguous areas of many hundreds or even thousands of hec­ group selection systems (Bradshaw and Lush 1981). tares described for many conifer forests of north America or This study applies to the publicly owned karri dominant for­ the Mountain Ash in Australia (Attiwill 1994) are not ob­ est within the main kani belt ( 138,000 ha) of which 63,000 served in karri forest. Severely affected areas of several hun­ ha is virgin forest. (Virgin forest is unlogged but has had a dred hectares are known, although totally single-aged patches history of fire disturbance of varying frequency and inien­ of greater than twenty hectares are uncommon. Observation sity.) suggests that a single severe fire event will seldom cause com­ plete stand replacement, though it may result from multiple Stand development stages severe fire events. The maintenance of an appropriate representation of devel­ Milder fires produce smaller gaps in the miginal canopy, as opment or seral stages is a common requirement in forest will other disturbances such as storms or simply old age, even management plans and is seen as one means of providing down to the loss of a single tree. Very small gaps are unlikely and maintaining a broad level of diversity throughout the for­ to regenerate successfully unless the surrounding overstorey est. In the absence of detailed knowledge about the relation­ trees have reached maturity and are no longer able to extend ships between these stages and other organisms, it can act as their crowns to occupy the vacated site. Where small patches are occupied with a new cohort of regeneration, a multi-aged stand is produced and the structural phases described in Ap­ pendix I become somewhat blurred and overlapping 1 L. McCaw, Department of Conservation and Land Man­ (Mackowski 1984). agement, Manjimup, WA. n 180 Age Structure of the karri forest: 1. Defining and mapping structural development stages I I I Further complexity will develop where karri grows in mix­ Two hectares was selected as the minimum patch size for ture with other tree species.