“Carbon sequestration is a co-benefit of the precipitation-recycling and cooling power of trees. As trees process and redistribute water, they simultaneously cool planetary surfaces.”
– Dr David Ellison *
COMMENT by PIP McMANUS
Last updated Thursday, 5 December 2019, 4.20pm. Links added.
Most days I walk along the shaded footpath which runs from the Wills Terrace foot bridge to Schwarz Cres along the eastern edge of the river bed. For many years now this lovely walkway (pictured above) has been becoming ever less shady. One morning this week I counted 42 dead or dying big trees along this short stretch.
Fortunately the native trees are doing well; the non-natives, particularly the Rain Trees and Pepper Trees all seem destined to die. It is not clear whether this creeping necrosis, which is signalled by a pale grey change in bark colour moving up the trunk, is due to disease or lack of water.
Many locals have been dismayed too at the sudden loss, all around town, of magnificent mature Jacaranda trees (pictured below) which have been unable to cope with prolonged drought. Several beautiful specimens in the mall near the Flynn Church demonstrate this telling change in the bark along with shedding of dead branches.
So are we to abandon non-native trees and the dense shade canopies they afford in the face of long summers which appear destined to be hotter and drier? There is no argument that naturally adapted desert plants survive well in our arid environment but just as we choose to seek air conditioned interiors during the heat of the day, when we venture outside we seek dense shade and green lawn areas which are provided predominantly by exotic plantings.
The Stuart Highway features a significant and welcoming avenue of mature native trees. One has to wonder if the recent culling of a large number of these fabulous trees near the northern entrance to town was really a necessity. No warning, no discussion just suddenly they were decimated.
We live in a town where many ancient trees and rocks have sacred site status. On a global scale some countries are granting legal rights to natural ecosystems (lakes, rivers), rights to exist and flourish. The destruction of these magnificent trees along the highway resonates with the attitude of a consumer oriented society in which the natural world is automatically subordinate to the demands of urban efficiency. Could there not have been a compromise by widening the road from one side only, keeping a very narrow median strip and moving the eastern footpath further east, meandering perhaps through sections of bush?
From what I have observed there is ample space to do this, but it would mean a more considered and unconventional approach to standard road widening policy – God forbid! The Alice Springs Garden Cemetery in contrast is a refreshing oasis of flourishing greenery, a testament to considered plantings and observant care.
A request to meet directly with any relevant staff at the Town Council to discuss their tree policy was unsuccessful, though I was provided with a number of links to policy documents – most of which concentrate on statistical data.
While the Alice Springs Town Council’s Climate Action Plan 2018- 2021 has formally recommended 42 actions for the mitigation of and adaptation to climate change, these are largely in the area of renewable energy, lighting and power use efficiency. There is little if any mention of the role trees and gardens play in heat mitigation.
It is great to see inclusion of passive energy saving measures such as pool blankets, and an action ensuring new building are appropriately insulated and shaded. Local and NT governments must lead the way in respect to passive cooling measures but one only has to look at the ongoing construction of wildly inappropriate hotboxes (minimal or no eaves or verandahs , dark metal cladding, basic insulation, poor orientation, glass cladding – The Supreme Court building!) to know that meaningful passive cooling has a long way to go.
It will only arrive when there is a mandated change in building codes which actually address our specific climatic conditions. Of relevance: one of the listed “actions” in council’s Climate Action Plan , no 6.5.37, states: Lobby the NT Government to provide strong leadership on climate action.
The NT government has committed $15 million dollars to Mparntwe/Alice Springs for CBD revitalisation which one would hope will include significant expenditure for greening our town. The design and documentation works for this project were due to be completed last month and include shade structures and cooling initiatives, identified in the design tender awarded to GHD Ltd engineering for the first phase.
Despite a number of direct inquiries to GHD and to the project supervisor and media manager at DIPL (Department of Infrastructure, Planning and Logistics), I have not been able to find any more detail regarding this major proposal. The Katherine CBD revitalisation plan by Ashford Group Architects (Darwin) on the other hand, reveals a detailed vision (a six-minute video is easily found) following extensive consultation with community groups, and includes a transformative focus on re-greening the town.
This newspaper has reported on a number of occasions on the loss of nearly 1000 trees last summer. Are we to expect a similar lamentable loss by the end of this summer? The cost in dollar terms, not to mention the decades of growth time required to nurture a replacement young tree to maturity, could be readily avoided by a more vigilant attitude towards those trees which are well established but may manifest signs of sudden or gradual heat stress.
Right: The vines were removed from this walkway many years ago and have never been replaced. The tree died last summer and is still standing.
Local gardening expert Geoff Miers urges us to consider the key value afforded by trees with big canopies because of their multiple benefits – shade, colour, coolness. Whether Australian natives like the Burdekin plum or exotic favourites like the Jacaranda, the Ilawarra Flame Tree or the Poinciana, their spectacular displays in bloom can transform the feel of a town.
Dense groupings of native shade trees such as Whitewood and Bottlebrush can provide windbreaks and refreshing colour and shade. Native tree canopies are less dense than exotics but an intelligent mixture of both makes sense. Corridors of dense tree shade would encourage people to walk, lower air temperature, reduce people’s exposure to UV radiation and give direct shade to buildings.
The NT government commissioned a heat mapping and heat mitigation study carried out in 2018 by a team from the Faculty of Built Environment at the University of NSW. As result of this study’s recommendations the City of Darwin is undertaking measures to mitigate urban heat sink in Darwin with a trial project in Cavenagh St. A 55 meter shade structure across the entire street is projected to be covered with green vines. Other cooling recommendations include lightening surface treatments, and installing instant planter boxes of medium sized trees and vertical green walls.
The same UNSW team also carried out a heat mapping study in Mparntwe/Alice Springs. This study revealed that the average difference in surface temperature between street pavements and green areas (or reflective roofs) is of the order of 22-24 ºC. Trees produce lower reflected solar radiation than areas like pavements, dark asphalt or metal shade structures, combining the beneficial effect of vegetation with shading.
The specific plantings and approach will certainly require relevant changes for our desert climate but the fundamental proposition is the same – smart environmentally sustainable heat mitigation initiatives which do not rely on closing people away in energy guzzling concrete boxes, but rather create naturally appealing spaces where passive cooling is paramount.
In the late afternoon I often take the river walks to the Telegraph Station. On these blazing days when the hills are still reflecting unwelcome heat I am constantly surprised by how noticeably cooler it is to suddenly dip down into a narrow dry creek crossing. Even half a meter beneath baking ground level there is a refreshing difference.
It’s ludicrously simple. Hot air rises. We all know that. But why do we not create parks and public spaces with sunken gardens. Within traditional urban desert cultures, in places such as Iran, Dubai, the Middle East and North Africa, vernacular and public landscaping has employed central courtyards, water elements and sunken gardens in order to create cool, green living spaces protected from winds and sandstorms.
Paddington Reservoir Gardens (pictured) in Sydney is an award-winning re-imagining of a public industrial site which features a delightful sunken garden, a calm, cool space in the middle of a huge city.
For many of us who call Mparntwe/Alice Springs home the prospect of innovative and sustainable urban greenery measures could be a key factor in our decision to stay.
So imagine if you will parks, gardens and pedestrian linkages in Mparntwe/Alice which feature dedicated channels or gardens, gently sloped grassy banks, oriented to catch winter sun but also to funnel a cooling breeze in summer, to capture runoff and to water sheltering clusters of healthy trees – places to stroll comfortably or to sit a while in deep shade.
* “Trees, forests and water: cool insights for a hot world” by Dr David Ellison, Swedish University of Agricultural Sciences, in Global Environmental Change, vol. 43, pp. 51-61.