That Mulberry Tree

Amongst all the beautiful images of the 2021 Permaculture Principles calendar, is Talia Davis’ stunning aerial shot of a mudbrick house and lush green tree amongst the devastation of bushfires on the south coast of NSW.

As with all the calendar images, there is a brief story that provides context: “Despite drought conditions this 40 year old mulberry tree had been deeply watered in the months before the fire went through in December 2019, and was on the fire side of the house. This tree in combination with cleared space, well-sealed and strong construction, appear to be what saved the house. Brett and Wendy are now installing water harvesting structures, aiming to increase soil water absorption and reduce the flammability of the forest around the house

The image and story are used to illustrate the permaculture design principle Apply Self-Regulation and Accept Feedback. It does so in a variety of ways:

  • Firstly, the timely allocation of limited water to mature trees instead of annual gardens in drought shows prioritising what is important for the long term, as well as being conscious of the elevated risk of catastrophic bushfire.
  • Secondly, careful design and placement passively contributes to many functions (including in this case, bushfire protection). The mulberry tree is a passive (but growing) and largely self-regulating fire defense element. This can be contrasted with an active element such as a large firefighting pump, which depends on fuel, maintenance and an operator to be useful, and instead of growing, it depreciates over time, accelerated by lack of timely maintenance and testing.
  • Thirdly, the experience of the drought and bushfire has led to further action to more effective harvest water in the landscape through passive water harvesting structures to increase soil moisture, and through active management to reduce the flammability of the forest around the house.

Behind this dramatic illustration, is the complex subject of bushfire resilient house and landscape design. A story by ABC journalist Kate Aubrey provides more detail on the observations of the owners and comments by forest tree expert from ANU that touch on some of the complexities of bushfire resilient design, including the role of vegetation as an asset and/or a hazard.

The idea that plants and especially trees might be an asset rather than just a hazard in bushfires was highlighted by our research during the mid-1970s for Permaculture One (at the house and property that Bill Mollison defended from the 1967 bushfires that devastated the mountain fringing suburbs of Hobart).

Over the decades since, I have looked closely at the role of species selection and vegetation management in contributing to bushfire protection. I agree with the owners about the likely role of the Mulberry in helping to protect the house. The ABC report of the quoted tree expert gave the impression that the species of tree was less important than the form and condition. Rather than disagreeing with the valid points conveyed by this reporting, I thought it might be useful to those inspired by this story to add some of my own observations.

It is true that any tree upwind of a house can act as an ember trap that can significantly reduce the likelihood of ember infiltration, which is the primary way houses are destroyed in bushfires, especially when they get into the roof space.

Secondly, a tree can catch large flying debris from a firestorm that might otherwise break windows and allow entering embers to destroy the building.

Thirdly, a tree can absorb radiant heat, so reducing the overheating of the house (and protect active house defenders from potentially lethal radiation levels).

In addition, an actively transpiring tree with abundant moisture can transpire so much water when heated by the radiation from the fire front that it steams water vapour which further attenuates radiate heat.

On the other hand, if the tree catches alight or breaks in the windstorm then these potential benefits turn into greater threats.

What determines whether sheltering trees or shrubs are a benefit or a hazard is affected by many factors including a fair dose of chance. Fine foliage, retained dry leaves and dead twigs, flaky, ribbon and fibrous bark are all downsides while large leaves that primarily shed in winter and smooth bark, such as a mulberry are an asset. Retention of low branches such as by most conifers is a disadvantage compared with the “self-pruning” nature of most eucalypts, although this is something that can be easily changed (by pruning!).

Volatile oils, resins and waxes in species such as eucalypts, many other Australian natives, and conifers are widely recognised as a downside but few understand the linkage between these flammable compounds and soil infertility that these species are adapted to. I have yet to find a comprehensive published explanation, but this is my understanding based on decades of observation, reading between the lines of lots different sources and some resultant hypothesising.

In geologically young regions with minerally rich and deep, free-draining soils underlaid by permanent sweet groundwater (much of the temperate and continental northern hemisphere), predominantly winter deciduous trees have access to essential minerals, especially calcium, boron, copper, manganese and possibly silica that contribute to strong cell walls allowing plants to retain water.

In geologically old regions where leached and compacted poorly drained soils with saline or absent water tables predominate (much of Australia), the vegetation has evolved to create organic compounds that to some degree are metabolic substitutes for minerals in the critical function of water retention.

Unfortunately these substitutes are as flammable as petrochemicals, so when vegetation does dry out due to drought and fire, they contribute to the intensity of combustion.

On the other hand, if plants evolved to mineral rich soils, and have access to balanced nutrition, they will have higher level of minerals that act as fire retardants within foliage. The ash remaining from burning any biomass is the total mineral content. Low ash content is one of the characteristics that make for good firewood, but the high ash content in our garden vegetation is a crude sign of fertility, the ability to hold moisture and low flammability. Consequently, fertile and balanced garden soils that retain water and are growing well-managed and productive food plants and trees are an asset rather than a liability in bushfire.

Beyond the transformation possible at the garden farming scale to create a fire safe zone around our homes, we have to be more circumspect about ways to make our broadacre farmlands and forests firesafe. In Bushfire Resilient Land and Climate Care I canvased diverse strategies including thinning, grazing, accelerated decomposition with or without the benefit of earthwork to rehydrating soils as well as appropriate patterns of ecological/cultural burning.

This focus on just a few of the factors in bushfire resilient landscape design inspired by this photo would be incomplete without at least mentioning the tricky issue of staying and defending vs early and safe evacuation. Research over many decades has shown that the presence of one “able-bodied” person is between two and four times more important than any other design factor or site feature in determining whether a house burns down (see Joan Webster 2000, pp 77-78).

In the absence of a stay and defend plan against the worst of fires, it is important to ensure as many the other factors are working in our favour. Passive and self-regulating elements of our property designs are critical in this situation.

While researching The Flywire House case study in 1983, one of our conclusions was that bushfire resilient house and landscape design works at two levels:

  • Property design and management covering all bases collectively increasing the chance that passive design will lead to survival of houses and other critical assets.
  • Having these bases covered increases the confidence that the house and property is a haven that can be actively defended to survive the worst of firestorms. Whether we are psychologically and physically prepared for such an experience is one of those tough questions that no one else can answer for us. However, thoughtful design, fine tuning, careful maintenance, good kit, and fire plan testing in the worst of weather, all contribute to empowering that most potential element in bushfire resilient design: the human element.

Finally, for many of us facing the relief of a La Nina benign summer in some of the world’s most bushfire prone regions, this is the opportunity to seriously consider our choices and move to less challenging locales or double down with long term resilience strategies to make our place the best safe haven we can for challenging futures, from whatever quarter.

Here’s to a safe and prosperous way down in 2021.

David Holmgren
Melliodora
November 2020

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