Richard Harper

The Forest Management Plan (FMP) 2014-2023 commits to a review of silvicultural practices by a panel of independent experts to inform the development of the next FMP (2024-2033). In addition, in September 2021 the WA government announced that south-west native forests are to be protected from logging from 2024. The announcement noted that from 2024, timber taken from native forests will be limited to forest management that improves forest health, and clearing for approved mining operations. Further, a Department of Biodiversity, Conservation and Attractions (DBCA) Explanatory Note (2022) (Appendix 1) specifies that harvesting in native forests will be restricted to 'ecological' thinning.

The current FMP (2014-2023) covers 1,935,030 hectares (ha) of forest across all tenures. Much of this is in tenures in which timber harvesting or thinning, is not permitted (e.g., parks and reserves). The primary focus of this report is on the ~ 220,000 ha of regrowth forests within the 960,000 ha of forests that are available for timber harvesting under the current FMP. These regrowth forests are the forests that are potentially available for, and would most likely benefit from, ecological thinning. Management recommendations specific to ecological thinning are made for three broad forest communities (the jarrah, karri, and wandoo forests). A fourth category, mining rehabilitation sites, is included because mining is a major and ongoing activity that impacts areas of these forests and results in a completely reconstituted forest.

South-west Western Australia has a long history of timber harvesting in native forests. For decades the standard silvicultural systems have primarily focused on long-term timber production in an ecologically sustainable forest management (ESFM) framework. As a result, there are now 144,770 ha of even-aged jarrah regrowth and 74,820 ha of even-aged karri regrowth. Nearly 80% of the jarrah regrowth has been established since 1970, with the majority in the southernjarrah forests. In addition, 24,540 ha of forest has been rehabilitated after mining using non-native species (3,760 ha) or with jarrah and marri (20,780 ha). An estimated 7,000 ha of further mining and rehabilitation in State forests is planned for 2024-2033.

Many of these regrowth stands are young stands with very high levels of stocking and poor vigour. However, they use large amounts of water, which reduces the water available for groundwater recharge or streamflows. Regional reductions in rainfall and global increases in temperatures mean that the amount of water available to terrestrial and aquatic ecosystems in south-west WA is likely to decline over the next century. The combination of a drying climate and large areas of heavily stocked regrowth will predispose these forests to acute moisture stress. As water availability declines, tree vigour will decline and the potential for increased mortality rates within stands and forested landscapes will increase and riparian areas will continue to dry. A drying climate and changed forest water balance may also increase the severity of bushfires within these landscapes.

Over the past 20 years, forest scientists and hydrologists around the world have demonstrated that thinning in stands with high levels of stocking can improve soil moisture balance and improve tree vigour. This can make stands more resistant to the impacts of drought and heat waves and landscapes more resilient to changing disturbance regimes. Thinning also has other potential benefits. It can improve the water balance in riparian ecosystems and in some circumstances enhance streamflow, and increase soil and groundwater storage. Thinning also increases the growth rates of trees, which may accelerate the development of habitat features such as tree hollows, and reduce susceptibility to fire induced mortality. However, thinning to achieve these ecological outcomes has not been widely practiced. Most (but not all) thinning activity in south-west WA has focused on improving tree growth for timber production rather than for specific ecological and environmental outcomes…

Rising ground-waters and resultant salinity threaten agricultural land, conservation reserves and water resources in southern Australia. Although revegetation with woody plants is often considered as a strategy to restore catchment water balances, farm forestry has not been adopted in low-rainfall environments to the extent of that in high rainfall zones. Similarly, there is some conjecture that the proportion of revegetation needed to restore catchment water balances may be as high as 80%. The JVAP publication Trees, Water and Salt provides a set of guidelines for revegetation of farmland, however these have not been tested at the catchment scale in drier (<400 mm annual rainfall) environments that are representative of the wheat and wool-belt of much of southern Australia. Reforestation in these regions is often of limited scale, and thus at an inappropriate scale to assess catchment scale responses. This study measured the hydrologic response of an 80 ha catchment to partial reforestation, near Wickepin, Western Australia. This region, which has around 300 mm annual rainfall, has agriculture that comprises rotations of cropping and pastures. These trees were established using the procedures outlined in Trees, Water and Salt. An issue with dryland reforestation has been the lack of clear economic drivers. The emergence of markets for carbon sequestration and bioenergy from trees, in response to national climate change policies may increase the future rate of reforestation. Key issues include understanding the rates of both sequestration and biomass production in drier environments such as Wickepin and also how best to integrate reforestation with agricultural production. Of particular interest are the interaction of belts of trees with agriculture and the utilization of land that is poorly productive, such as that which has been affected by salinity.

The site evaluation procedures outlined in this report are intended to be applicable across the range of stages of development of farm forestry enterprises. The procedures described can be applied to areas where there is no specific information on the requirements of tree species—in these cases, site evaluations focus on identifying conditions likely to limit tree growth. But the procedures are also applicable in areas where there is a large body of knowledge on tree performance—in these cases, the data collected during the site evaluation can be interpreted with a high degree of confidence and can be used to estimate tree growth rates. The scientific underpinnings of site evaluation for Australian forestry are reviewed by Ryan et al. (2002), as another output of this project.

This report presents the results of a field trial using phase farming with trees, a system designed to grow tree crops in short rotations of 3-5 years to intentionally deplete soil water as a buffer for agriculture in areas prone to dryland salinity. The trial showed that on suitable soils, a short tree phase will deplete soil water sufficiently to allow a subsequent return to agriculture for a 11-20 year rotation. Biomass production from the tree phase was also measured, with a view to emerging biomass energy and biofuel production systems. Some of the technical and policy issues that need resolution before this farming system can be introduced in southern Australia are discussed.