Forest Disturbance - Identification and Survey
5.1 Introduction
As discussed in Chapter 2, disturbance is a primary characteristic which diminishes, or detracts from, old-growth status. Significant disturbance renders a forest ineligible as old growth in accordance with the definition (see Section 2.3.1). Chapter 4 notes that the range of disturbances known to have altered the primary attributes (floristics, structure or growth stage) of the forests in the study area were investigated and quantified. Disturbances were described according to their cause - natural or unnatural (post-settlement and human induced). Research of historical and contemporary records was undertaken to delineate and map the extent and severity of these disturbances.
5.2 Agricultural Selection
Farming and mining were closely linked in the early history of the Central Highlands and dramatically altered its landscape. The 1869 Land Act, designed to encourage closer settlement and break the dominance of the early squatters, was the first piece of legislation to allow selection before survey. Many selectors were misled into believing that tall forests guaranteed rich and fertile soil and ringbarking, burning and clearing of their selections were features of the ensuing 25 years. Following clearing, these early selections were sown with pasture grasses while the more fertile alluvial flats were planted with cereal or vegetable crops. In the more remote hilly areas, agricultural blocks were cleared and farmed for a brief period before they were permanently abandoned and reverted to bushland.
Data relating to those areas of agricultural clearing within the study area that are now classified as State forest, was extracted from archival files. Information on the location, extent and approximate period of clearing for each selection was entered into the GIS database.
5.3 Wildfire
Eucalypt forests are extremely fire-prone and major bushfires are a feature of the Central Highlands. While these can be catastrophic to human life, property, and other assets, fire also has an important role in forest ecology, as the eucalypt is adapted to fire and dependent on it.
Aboriginal land use practices included the use of fire. Many authors and several historical accounts have described the manipulation of fire by Aborigines to assist in mobility, food-gathering and hunting - mainly in the plains and open woodlands and less so in the mountain forests.
The years 1851, 1898, 1902, 1908, 1919, 1926, 1932, 1939, 1962 and 1983 represent some of the major fires that have burnt across the study area. The most critical of these was in 1939. In January of that year, some 1.4 million hectares burned across the State, and whole settlements were incinerated. Most of the forests in the study area were killed or severely damaged.
The 1926 wildfires burned large tracts in the Acheron River Valley and south of the Yarra, extending to Toorongo, Toolangi and Noojee. Between them, the 1926 and 1939 fires destroyed almost 85% of the total of 175 000 ha of mountain forest in the study area. Natural regeneration and reforestation operations followed these fires. In 1983, 17 600 ha of mountain forest in the Upper Yarra Valley were burned. Also burnt were the forests of Mt Disappointment and parts of the Black Range.
Wildfire is a particularly important disturbance because of its impact on the growth rate characteristics and floristic composition of old-growth forest. Wildfire maintains the ecological dynamics of regeneration and decline for most eucalypt communities (Gill 1981). Clearly the period of time taken to recover from such a disturbance is dependant upon the fire intensity, frequency and the vegetation class involved.
No adequate large-scale maps of the area burnt by the 1939 fires were located. The outer boundaries of the 1939 fires and most of the wildfires for the last three decades were extracted from wildfire history records and re-drafted as a standard map layer for entry in the GIS database. Where aerial photograph interpretation confirmed significant disturbance to the growth stage or canopy within these areas, this was classified as 'significant natural' disturbance for the purposes of mapping old-growth forest.
5.4 Mining
Gold was discovered at Emerald in 1858, and short-lived gold rushes along the Little Yarra and Upper Yarra Rivers followed. The major auriferous belt in the Central Highlands, however, was the Jordan goldfield stretching from Jamieson in the north through Woods Point and Matlock to Walhalla in the south.
Jamieson developed from 1860 as a stopping place and market town for the new and rapidly expanding goldfield; Woods Point opened up in 1861 and Walhalla in 1863. By the 1880s about 2000 people lived in Walhalla, the centre of a goldfield that was second only to Bendigo in gold production. Following this boom period, mining continued along the Jordan goldfield in the 1920s and 1930s but the 1939 wildfire brought a virtual close to already dwindling mining communities.
Those areas of forest which were cleared for fuel and mining timbers, and which have subsequently revegetated, were identified from archival records and aerial photographs (where available) and were recorded as a 'significant disturbance'. Location and extent of mining sites, tracks and other disturbances associated with mining were also entered into the GIS database for subsequent analysis.
5.5 Grazing
Licences and leases for private agriculture, mainly for the grazing of cattle under licence conditions that specify stocking rate, may be issued over public forested land. Historical and current data on the location and extent of grazing licences were researched as part of the old-growth forest study and entered as one of the layers in the GIS database.
The impact of grazing obviously varies for each vegetation class. It is unlikely to have an appreciable effect on classes with unpalatable vegetation, for instance, even if the records indicate that the licensed area included these vegetation classes. From the records it appears that significant areas of unpalatable vegetation were only grazed in years of severe drought. This form of disturbance was considered to be significant (that is, compromising old-growth forest values) in those areas which contained 'palatable' vegetation classes and where the most recent grazing records were younger than 50 years for sub-alpine vegetation classes and younger than 30 years for non-alpine vegetation classes.
5.6 Timber Harvesting
The more accessible forests in the study area near Melbourne provided building materials and firewood for the growing city in the 19th Century. Gold discoveries in the 1850s vastly increased the demand and large areas of forests around Walhalla, Healesville, Toolangi and Warburton were clearfelled and the timber used for fuel, building materials and mining timbers. Split palings and shingles were produced from the earliest days of settlement, but sawn-timber uses were restricted by seasoning problems. By 1910 seasoning techniques had been developed and sawmills, associated settlements and tramways for transporting sawlogs were established, firstly in the Yarra River valley near Warburton, and soon after in the Upper La Trobe and Thomson River valleys.
Sawmilling activity peaked in the 1920s (the number of sawmills in the forest reached 241 in 1921) and immediately after World War II with fire salvage operations. The war also resulted in increased demand for timber, which accelerated in the post-war boom in housing construction.
The history of fire in the mountain forests of the Central Highlands had a critical influence on the development of the hardwood sawmilling industry in Victoria. The 1939 fires claimed the lives of many timber workers and destroyed 41 mills. Salvage harvesting following these fires meant that the tramway network was used and extended in some areas until the 1950s. Salvage of sawlogs from the 1939 fire-killed trees largely provided Victoria's requirements for sawn hardwood timber until the early 1950s, when the older trees had deteriorated and the harvesting operations would have caused unacceptable damage to the developing regrowth.
Prior to the development and application of regeneration techniques suitable for eucalypts, logging was selective and trees and logs considered to be low quality for sawn timber were usually left on the harvesting site. Investigations subsequently proved that low quality eucalypt logs were suitable for paper-making. In 1936, Australian Paper Manufacturers Ltd entered into an agreement with the government to obtain pulpwood from both foothill and mountain forests within a region that includes much of the study area. This agreement was ratified by Act of Parliament and has been revised several times, most recently in 1996. As a result, harvesting of pulpwood in conjunction with sawlog harvesting operations has been a feature of forests in the study area for more than 50 years.
Accurate information on the areal extent of timber harvesting was obtained for this study from historical records and maps, and for more recent harvesting from aerial photographs. Significant areas of timber harvesting following the 1939 fires were mapped from aerial photographs taken between 1944 and 1954. Both clearfelled and selectively harvested areas were mapped and incorporated in the GIS spatial data base together with all former forest-based sawmill sites and the tramway network. When these mapped sources of disturbances were confirmed by the crown cover or growth stage mapping the disturbance level was classified as 'significant unnatural'.
5.7 Fuel Reduction Burning
Fuel reduction burning is the controlled use of fire to reduce the amount of flammable material on the forest floor in order to reduce the risk of damage from wildfire. Fuel reduction burns normally differ significantly from wildfire. They are deliberately ignited on days designed to produce a low intensity burn thereby causing minimal damage to the crowns of trees. They may be more frequent than wildfires, occur earlier or later in the season, and produce a different mosaic of burn.
Fuel reduction burning of forest has been routinely conducted in the drier forests of the Central Highlands since the 1960s. Boundary information is recorded on maps and supported by field reports and aerial observation in respect of the proportion of the proposed area which had actually been burnt. In areas where crown-cover and growth-stage mapping from aerial photos revealed no damage, fuel reduction burning was classified as 'negligible unnatural' disturbance.
5.8 Other Disturbances
Native forest and other vegetation cleared and otherwise disturbed for the establishment of softwood plantations were identified and mapped from plantation map records. Other disturbances were recorded in the category of historic/cultural/utility features which were extracted from either point-based or line-based datasets. These included sawmill sites, mining sites and settlements together with timber, mining and construction-work tramways and railways. Such areas were not included in the initial GIS analysis to identify old-growth forest. Rather, they were used to cross reference with identified old-growth forest and provided the focus for field checking. Such areas were usually classified as being 'significantly disturbed'.