This article relates to the ongoing discussion on Australian Rural & Regional News: Open for Debate – Bushfires, Logging, Burns & Forest Management

Frank Batini 

I have observed patterns of foliage scorch, some tree deaths and then recovery in the northern jarrah forest for 22 years. These can be readily explained in terms of soil depth, water-holding capacity, winter rainfall and summer drought. Winter rainfall at Jarrahdale in 2023 was 884 mm, about 15 per cent below the 10 year average. This was followed by the driest period on record, 22 mm over six months, with several days of 40 degrees or above. Despite this, the southwest forests are predominantly healthy.

Academic publications and the ABC have reported stressed vegetation in the jarrah forest, on the Swan coastal plain, in heathlands from Geraldton to Busselton and raised public concern about the future health of the forests. Comments such as” “It has happened before in 2010-2011, but this time it is worse” and “it is likely that the iconic wet forests will also be hit” conflict with the statement “it is limited to pockets of dead trees on shallow soil and some loss of canopy”.

Some scientists and the IPCC predict that WA will experience a “catastrophe”, a “devastation of the forests and woodlands” and that “climate change induced dieback, drought, transition and collapse”.  Their solution is for Australia and the world to achieve “net zero by 2050”. They also propose “collecting more data to improve climate modelling”, suggesting that they are aware that current climate models are deficient.

Over the 64 years since 1960, carbon dioxide (CO2) levels in the atmosphere have risen from 320 to 420 ppm. Even if the whole world achieves net zero by 2050 (a really big ask) CO2 will still continue to be pumped into the atmosphere (albeit at levels where it can be assimilated). To reduce levels back to 320 ppm the world would need to decarbonise to ”well below net zero”.  This would take a considerable time, perhaps decades and even if this herculean task can be achieved there is no guarantee that the climate will revert back to that of the 1960’s.

How can our forested ecosystems survive until CO2 levels reach acceptable limits? In contrast to the dire predictions coming from Academia, my surveys over 22 years of crown scorch, some drought deaths and then recovery show these impacts are most noticeable on sites with shallow soil, often near exposed rock, and in areas that were mined for bauxite where 3-6 metres of soil has been removed. Areas of higher quality old-growth and re-growth forest and sites that have been thinned show much less stress, but may have thinner crowns in a drought year.

Substantial changes in rainfall, hydrology, fire intervals, forest structure and ecology have occurred in the jarrah forest over the past 150 years. Change will continue to occur. Data from tree ring studies by the University of WA indicate that that several, decade-long, dry periods have occurred in the last 600 years.  In the most recent, from 1880-1910 (BOM  data), the rainfall average in Perth and Jarrahdale was very similar to the present.

Rainfall data also show that the five decades between 1911 and 1965 were much wetter, with the UWA tree ring study defining this as the wettest period in 600 years. Parts of the jarrah forest died through water-logging and Phytophthora disease, whereas water-loving species migrated uphill. A major concern is that most of the research and monitoring of ecosystems in the south-west and forests of WA has been done since the 1960’s and has only measured the “receding tide” associated with a period of lower rainfall. Great care must be taken when interpreting short-term data on ecological and climatic change. Unfortunately, this reality is ignored in recent scientific publications.

Several trials have shown that reducing tree canopy by thinning will raise watertable, increase soil moisture and streamflow and improve ecosystem health. The current Forest Management Plan proposes to ecologically thin 80000 hectares of forest (four per cent) in the next 10 years, mostly in the karri forest.  The bulk of the jarrah forest, where the current drought is more noticeable, will need to survive without the benefit of thinning.

However, the jarrah forest has experienced many long dry periods before. The ability of jarrah to grow in much lower rainfall supports my view that it will survive.  Maps of site-vegetation are available and provide a good insight on the likely changes in understorey and overstorey composition and structure.

My hypothesis is that this forest has repeatedly shown resilience to drought. Despite a decline in rainfall, streamflow and groundwater it will continue to survive as a functional ecosystem. This is the “good news”, as opposed to the alarmism of other commentators.

Frank Batini MSc (Oxon), BSc (UWA), Dip For (AFS), previously an Adjunct Professor of Environmental Science at Murdoch University has 60 years of  experience as a forester, environmental manager and consultant in the management of natural resources.