Bushfire recovery: damage has lessons for future responseResearch & Extension
As we approach the first anniversary of the devastating bushfires that impacted the Adelaide Hills, Bilpin and Batlow regions, NSW DPI Development Officer for Temperate Fruits, Kevin Dodds, shares his observations of post-fire response and recovery.
On the 20 December 2019 as I tuned into the radio news, I listened in horror to reports of bushfires impacting orchards and vineyards in the Adelaide Hills area of South Australia. The next day, the Gospers Mountain fire ripped through Bilpin and about ten days after that, the Dunns Road fire started its first of two runs on Batlow.
When the smoke cleared and safe road access was granted, emergency response agencies, landholders and NSW DPI staff were among the first allowed into the area to survey damage and to start the response and recovery process at Bilpin and Batlow. It quickly became apparent that there was a lack of information in the printed or digital media to guide fruit growers on damage assessment or strategies for orchard recovery.
From February through to leaf-fall, I focussed on capturing as many observations and learnings about the initial impact as possible, and tried to help growers with tough decisions, before the opportunity was lost.
This article presents a snapshot of some key observations that will help in the event of future fires. The observations have helped to inform a 3–5 year collaborative research project to be undertaken into bushfire impact and orchard recovery, with NSW DPI as the lead agency, PIRSA undertaking research in South Australia and Horticulture Innovation as a funding partner. A more detailed account of our post fire observations is due to be published by NSW DPI shortly. Watch this space.
Safety, water and crop load adjustment first
Natural disaster events like the 2019–20 fires can be overwhelming and making decisions about response and recovery priorities can be hard. Having said that, most growers were fairly quick to identify the most urgent issues and these included safety, water and crop stress.
Dangerous trees on orchard boundaries and some nettings structures were a major safety concern. Before staff and seasonal workers could safely return to work in the orchards, dangerous trees and structures needed to be identified and at least marked so they could be avoided and ultimately managed.
Damage to irrigation systems was extensive and included loss of power and fuel, burnt pumps, supply lines and drip lines. The damage was significant and, in many situations, would take weeks to repair. Knowing this, growers undertook secondary hand thinning to compensate for lost fruit growth. It was pleasing that in most blocks, this strategy proved successful and despite the fires, a significant amount of good fruit was still harvested in the affected regions.
Once the immediate challenges of safety, irrigation and crop load were addressed, growers and their advisers turned their thoughts to the damaged fruit trees and the prospect of their recovery or otherwise. Some responded by polling scorched trees while others took the wait-and-see approach.
Above: One of the first priorities after fire is re-establishing irrigation systems.
Blowtorch vs slow-cooker
Across the fire affected regions, there were two main types of fire that resulted in damage to orchard trees and infrastructure. To standardise the language about fire scenarios, I named these “blowtorch” and “slow-cooker”. The blowtorch was responsible for most of the damage, and it was the result of direct flame and/or scorching heat coming from native vegetation strips, wind breaks or heavy dry pasture adjacent to the orchards. The slow-cooker occurred where there was enough under-tree or interrow dry matter to sustain fire. Dry grass and/or mulch ignited randomly within blocks and fire persisted along the tree row assisted by wind. At Batlow and Bilpin, most of the damage in orchards was the result of blowtorch fires coming from boundary vegetation.
Left: The blowtorch. External fuel sources such as roadside vegetation strips produced wind-driven heat causing significant scorch damage to adjacent orchard blocks. (Photo taken: 18.02.2020, 45 days post fire). Right: A slow-cooker example. Internal fuels such as dry grass (unburnt, left) are important heat sources in some orchard blocks, resulting in significant fire damage to tree trunks and lower canopies (right). (Photo taken: 03.04.2020, 94 days post fire).
The two fire scenarios described identified the first key learning that might help us to better manage risk in the future. That is: potential damage can be significantly mitigated or reduced through boundary vegetation fuel management and minimisation of dry grass within or adjacent to the orchard (particularly in the tree row). Planting further from boundary vegetation would also reduce the potential for blowtorch burn but is not always possible with limited areas available for planting.
Burns ain’t burns
Closer inspection of blocks and individual trees in the two fire scenarios of blowtorch and slow-cooker revealed another important learning which has a very significant bearing on the assessment of the block viability and recovery of the trees. Under the blowtorch scenario, the flame and heat entered the block from the edge, killing trees up to a certain distance in from the boundary until the heat began to dissipate and tree damage declined. In most situations this resulted in a fairly well-defined area of impact and an intensity gradient. Given some time for post-fire re-growth, it is relatively easy to determine the area that will need to be replanted in a blowtorch block.
In the slow-cooker scenario flames and heat from dry grass ran through blocks, along and across rows, around tree trunks and up into the tree canopies. The area of impact was random and the intensity of the burning varied from one tree to the next depending on fuel load. Unlike the relatively defined blowtorch, the random nature of the slow-cooker means it is much harder to predict the location and number of trees that will ultimately die and need to be replaced.
Bark windows can mislead
Small, randomly cut bark windows like this one will give localised information on conductive tissue damage but can be misleading. (Photo taken: 15.01.2020, 15 days post fire).
After a few days of cutting small bark windows in tree trunks (and watching others do the same) in order to assess damage to conductive tissues, it became clear that this approach does not tell the whole story and is likely to be quite misleading. The result of a bark window assessment can vary greatly according to height and side chosen on the trunk or branch. To gain greater clarity of the degree of phloem and xylem damage, I conducted a significant number of destructive tree trunk debarkings in both blowtorch and slow-cooker sites. This revealed the unpredictable distribution of conductive tissue damage from one tree to the next, particularly in slow-cooker sites. The height and radius of the conductive tissue damage was likely influenced by factors including fuel distribution and wind direction and this means random bark windows can give random results. A better strategy might be to destructively debark a sample of trees with a range of different impact levels to better understand the visual recovery ques provided by the trees several months after the fire.
Ringbarking at three heights (left) and the same tree trunk fully debarked (right). In this case the tree sustained phloem damage around 360° of the lower trunk. (Photos taken : 09.04.2020, 100 days post fire).
Understanding phloem damage
Plant physiology 101 (of tree trunks) tells us that xylem carries water and nutrients from the roots up the tree to the canopy and phloem carries carbohydrates down to the roots. The phloem is just under the bark layer and the xylem is the woody part (further in the trunk) comprised of the annual growth rings. So, what happens if fire kills the phloem around 360° of the trunk, but the xylem survives? The answer is that trees can (at least initially) produce new growth and fill existing fruit, but there will be no transfer of surplus carbohydrates to the root system. This is exactly what we observed at Batlow and Bilpin. Growers were encouraged by the appearance of new growth on some quite seriously burnt slow-cooker trees and some even managed to harvest fruit from the same trees (anecdotally noting elevated sugar levels in this fruit, another indicator that surplus carbohydrates were most likely pooling in the canopy due to the phloem being cut-off). The long-term prognosis however, for phloem ringbarked trees is not good, so it is important to try to understand the degree of phloem loss soon after the fire rather than waiting a season only to find the trees go into decline.
Summary – initial lessons
My early observations generated some learnings that may help growers to better understand, prepare for and respond to bushfire damage in their orchard in the future. These lessons include;
- Be aware of the potential sources of fire and heat surrounding and within your orchard. To assess your risk, consider the potential for blowtorch and or slow-cooker fires in and around your orchard.
- Consider if there are strategies you can employ to mitigate the risks. For example, weed-free under tree strips, maintenance of green inter-row and under tree species, hazard reduction of boundary vegetation and dry pastures, proximity of bin sheds and other material stockpiles to orchard blocks and use of sprinkler systems.
- Small bark windows can be a misleading diagnosis tool. Consider sacrificing some trees by de-barking several in each block to properly understand the fire impact on conductive tissues. If this is done after the trees have had a chance to begin recovering, you may be able to relate the level of visual canopy recovery to the degree of conductive tissue damage observed, giving you a tool to better assess the remaining trees based on vegetative re-growth.
- In the case of blowtorch impact, there will be a reasonably definable distance from the fire source where trees will be recoverable.
- In the case of slow-cooker impact, tree damage will be in random isolated patches or throughout the entire block and the intensity of damage to conductive tissues will vary from tree to tree, making this very difficult to assess.
Further reading: A five-year joint NSW Department of Primary Industries and Hort Innovation national project – Developing management strategies to enhance the recovery of horticulture from bushfires – to assess short and long-term effects of bushfires in apple orchards and other tree crops in New South Wales and South Australia was announced in August 2020. The early work and observations reported above predate this project.