Written by: Ed Scull OAM
Banner image: Sheep In Wooded Landscape by Hans Heysen
How strange it is that we almost universally revere the iconic eucalypt depicted in a Heysen painting but seemingly have few qualms about removing its like when it comes to land use, urban development, road planning and maintenance. If there are misgivings these appear to be often subsumed by the economic rationale applied to such development.
It says something about our community that trees, like other assets, need to be defined by their monetary equivalent before they can be truly valued.
Dr. Eddy Wajon (EW), writing on the economic value of trees, concludes that “much more work is required to come to an agreement as to what trees are worth, but no matter what, it needs to be acknowledged that trees and other plant species are valuable, and once lost, are almost impossible to replace.
The Amenity Value
A number of methods have been developed to determine the value of trees and these (for example, the Helliwell and CAVAT systems) have been used successfully to prosecute the case for retaining the “offending” tree by adopting an alternative roadway solution, despite the reduced cost implicit in simply removing it.
We are talking about what are referred to as “amenity trees” and a number of methods have been used, notably in the UK, to assign such trees a “value”.
Amenity trees are typically those that are not grown or managed for their commodity value as a timber or other crop but which provide other and less tangible (ie. less marketable) benefits to the community. They have value because “someone” expects them to provide an ongoing or future benefit or to make a difference to the way we live. They are valuable because they are!
[The] amenity value for a 40 year old Eucalypt… could… be in the range of $40,000 to $60,000.
The CAVAT system
The CAVAT system utilises the Base Value (BV) calculated using trunk area as an index of tree size. This is multiplied by the Unit Value Factor (UVF) which represents the full cost of a newly planted tree based on a typical nursery gate price and planting costs including transport, labour, materials and intermediate care which for the UK is an agreed average price of approximately A$28 per sq.cm.
Without making an exhaustive calculation for Australian eucalypts this would represent a value of approximately $55,000 for an established tree with a base diameter of 500 mm at 1 to 1.5 metre from the ground.
To determine the Amenity Value (AV) the Base Value is adjusted by a range of considerations (Community Tree Indexes (CTIs) related to accessibility, exposure to the public, possible heritage value and projected longevity taking into account health and structure etc.
It is suggested that for the relatively few iconic trees remaining in the remnant vegetation within existing road verges around the wheatbelt in WA, the amenity value would be only marginally changed from Base Value under the prevailing CTIs.
Iconic trees close to population centres or tourist precincts could be subject to CTIs in the range of 100 – 250%, adding significantly to the Base Value of the tree.
Studies carried out in the UK using a quick method derived from the CAVAT indicated that for trees with a trunk diameter of 70 – 100 cm and based on 2017 figures the BV and hence the amenity value for an established tree could be in the order of $100,000 to $200,000.
Studies in WA (EW) indicated that amenity value for a 40 year old Eucalypt may not be this high but could nevertheless be in the range of $40,000 to $60,000.
While the Amenity Value, imprecise as it is, probably represents the best estimate of value that community can place on a tree there are a number of environmental values that are more tangible but difficult to express in dollar terms.
These become of increasing importance as the effects of global warming and extreme climate variation are recognised and include the following:
The tree through the process of photosynthesis, absorbs carbon dioxide and produces the sugars essential for tree growth. The carbon is permanently stored in the tree’s biomass (EW) comprising the wood, roots (50% & 40% dry weight accumulated over the life of the tree ) and foliage. Oxygen is “breathed” back into the atmosphere.
Based on data relating to the biomass of the tree and its rate of growth, the amount of stored carbon can be estimated. Estimates vary according to the size, maturity and variety of tree but it is calculable that a large tree such as a Jarrah or Marri will absorb around 167 kg of carbon dioxide (CO2) per year. This accounts for about 90 cubic metres of CO2 containing 45kg of carbon.
Over a 40 year lifetime (for example) it is expected that a single tree will sequester about 1.8 tonnes of carbon, having removed approximately 3600 cubic metres of CO2 from the atmosphere.
In purely monetary terms this is the equivalent of only $19 in carbon credits at current market rates and it would be interesting to compare the cost of maintaining a verge side tree in the environment with the cost of industrially storing the equivalent volume of CO2 by geological sequestration, often promoted as a preferred response to climate control.
Trees play an essential role in cleansing the atmosphere of pollutants such as those derived from vehicles and industry and in replenishing oxygen through the process of photosynthesis.
Continental oxygen depletion is discussed as a major problem in relation to the clearing of forest and rainforest resources, notably in areas such as those in SE Asia and the Amazon basin.
The value of a tree as an oxygen generator can be calculated from the level of carbon sequestrated. An estimate (EW) based on current market price of the equivalent amount of oxygen separated from the atmosphere for industrial purposes (approx. $1420 per tonne) would indicate that a mature Jarrah contributes oxygen worth about $14,000 to the atmosphere over its lifetime.
Photo: Saline area in the Western Australian wheat belt near Bannister, WA. 1981, © CSIRO Land and Water. Source, Wikimedia Commons.
Dryland salinity is highly detrimental to agriculture, infrastructure, water resources and biodiversity with total costs estimated to be in excess of $200 per hectare of affected land per annum. It is estimated that between 1 and 2 million hectares of otherwise highly productive land in WA is salt affected.
The removal of trees and native vegetation from salinity prone dryland allows the water table to rise carrying salt to the surface. Salinity, introduced to the shallow root zone of crops, pastures and trees interferes with nitrogen uptake reducing growth and production capacity.
Surface salinity also has a detrimental effect on infrastructure such as roads as a result of chemical degradation of road base material leading to poor road quality, reduced longevity and high maintenance costs. Salinity also contributes to poor water quality in the water catchment, reserves and streams fed by surface and groundwater run-off. These are significant costs familiar to WA and in particular to the central wheatbelt.
Lowering the watertable reduces surface salinity through leaching. The maintenance of roadside reserves with deep rooted trees and native vegetation provides high levels of transpiration of water to the atmosphere.
As a preventative measure these may complement or provide an alternative to costly drainage and pumping systems commonly used to protect arable land and main road infrastructure. Ironically, where roadside trees may be considered to be an impact risk to the motorist, deep drainage at the roadside as an alternative to trees presents its own hazard in the event of a vehicle leaving the road at speed.
Photo: Saline area in the Western Australian Wheatbelt near Bannister, WA. 1981, © CSIRO Land and Water. Source, Wikimedia Commons.
Trees and other native vegetation act as an important environmental cleanser reducing salt levels and by taking up and absorbing contaminants, waste and excess nutrients in the soil and storing them or converting them to less hazardous forms. This reduces the leaching of contaminants improving quality in the water catchment, reserves and streams fed by surface and groundwater run-off.
Established trees also play an important role in stabilising soil and reducing erosion in and around run-off areas, surface drains and streams.
The passive role played by trees needs to be contrasted with what can be a high recurrent cost to otherwise treat contaminated water and to clear and maintain essential waterways. In extreme cases costs due to flooding, algae blooms, infrastructure damage, fish kills etc. can be added to this ledger.
The tree and other native vegetation is an important support for native flora and fauna, much of which is endangered in both urban and rural settings.
Mature trees provide essential nesting sites for parrots and cockatoos and flowering species such as the eucalypts and woodland shrubs provide both food and a refuge for honeyeaters and wrens, etc.
While healthy trees are essential it is also important that dead and fallen stock within remnant bushland is maintained in support of reptiles and the smaller marsupials and rodents.
If the risks that local and international smugglers are prepared to take and the prices their clientele are reported to pay are any indication, the value of the tree that nurtures this “commodity” will be considerable.
This raises the issue as to what is viable remnant bushland and to what extent the pressure on the remaining bush at the roadside due to transport and road improvement demands is rendering this essential resource as marginal, if not unviable.
The need to retain habitat is a recognised factor in road design but with the demand for wider roads, improved hard shoulders, barriers, fire breaks and frequent passing lanes to accommodate light vehicles with heavy haulage, it is apparent that the strip, once revered as the “long paddock”, between the road and the fence line is shrinking.
Surely there is a case to be made for a requirement that rural roadway funding should include the cost of resuming land, where necessary, to ensure that a ratio of width of roadside vegetation to total road surface should be sustained – the wider the road the more deep the vegetation.
Importantly such a resource must be well managed with the maintenance of undergrowth and firebreaks to prevent extreme bushfire events and the protection of infrastructure such as fences and waterways etc.
It is essential that island remnants of bush are reconnected to sustain wildlife corridors to promote diversity and a healthy genetic population and that established amenity trees continue to be accommodated within these remnant areas.
Trees also provide essential shade, windbreaks and shelter that contribute to habitat and the liveability of urban areas. This will become more essential as the extremes of climate change and increasing environmental temperatures are experienced. In this regard, the shelter naturally sought by livestock and the well documented effect trees have on streetscape temperatures and humidity in urban areas are testament to the need to retain trees as a planning imperative.
The tree as a commodity
If the benefit of carbon sequestration is to be maintained the wood content must be preserved by extending the life of the tree or by using its timber prudently, for such as construction or furniture production. Use or destruction of timber by fire releases the carbon back into the atmosphere as CO2 and while the process is slowed, timber converted to fuel, mulch or to woodchips for paper suffers a similar fate.
Depending on timber quality and milling efficiency a 500mm diameter jarrah log will yield 30 – 40 timber sections 50×100 mm. At current retail prices this would value the log at approximately $600 to $800 per linear metre with a substantial tree yielding in the order of $10,000 as a once off opportunity, somewhat less than the amenity value of the same tree.
To value our iconic tree is to not consider them as museum items but to recognise their ongoing contribution to the environment and to value the positive role they play in the communities interaction with and enjoyment of that environment.
To include a notional value of the tree on the benefit side of the cost-benefit analysis and to not just consider it as an impediment and therefore a cost, will go a long way toward including the established tree as a respected element of good planning and in retaining its rightful place as a valued amenity.
- “Putting a Value on Trees: an economist’s perspective” C.Price; Presented to the Ann.Conf. of the Arboricultural Association, York 2006 (subsequently published)
- “CAVAT (Capital Asset Value for Amenity Trees): valuing amenity trees as public assets”, Kieron J.Doick etal; Aboricultural Journal 2018, Vol40, No 2, 67-91
- “Considerations concerning CAVAT: What does its “Tree Amenity Value” actually measure” Colin Price; Aboricultural Journal 2020
- Letter to the Editor, Aboricultural Journal 2020, Christopher Neilan MICFor, MA (Cantab), (for and on behalf of the CAVAT Executive Group
- Letter to the Editor in reply to C.Neilan’s letter, Aboricultural Journal 2021
- “Trees have a Economic Value” Dr.Eddy Wajon. Correspondence dated February 2012.
- “Tree Valuation” Dr.Eddy Wajon, Director, Wajon & Associates. Correspondence dated May 2011.
- Celebration of a special tree: the Powder-bark wandoo
- Dr. Eddy Wajon
- WA Wildlife Society
- Significant Tree Register
- Urban Trees: Worth More Than They Cost
- ABC Fact Check: Tasmanian Liberals say younger forests are better at sequestering carbon than old-growth forests.
- Old forests three times less flammable