In the previous issue, we looked at climate change and how scientists are testing different plants in simulated drought and heatwave conditions.
This month we look at the Dubbo’s Heat Island Amelioration Project.
Solutions to keeping cities cool will involve not only choosing the right plants but in many cases enhancing growing conditions beneath the surface.
In the past, street tree planting methodology has often been based on digging a hole into the sub-base of the road, planting the tree, and then having the unrealistic expectation that the tree will thrive and persist long term in this environment. The result being that many trees planted decades ago are now in poor health and serious decline.
Such was the case in Dubbo NSW when the Regional Council (DRC), in 2015, took the plunge and removed failing trees in its Street Tree Masterplan to trial the Stockholm Method of planting. Dubbo’s Heat Island Amelioration Project will lower street surface temperatures by up to 20 degrees and recently won the Council a top award at the 2019 Local Government NSW Excellence in the Environment Awards. Ian McAlister, who led the project from planning to completion, said the new landscape, overwhelmingly, has the approval of the Dubbo community.
“My expectation is that any methodology that provides a tree with sufficient room for root extension – and a growing medium to provide support and moisture and nutrient holding capacity – will ultimately produce a better performing longer lived tree for our urban environments,” said Ian.
“Our first Stockholm Method plantings were undertaken as part of the Darling Street Beautification project. We used only two tree species – the endemic Angophora floribunda (Rough-barked Apple) and Agathis robusta (Kauri Pine). Both of these tree species are performing exceptionally well, reinforcing the importance of improving the planting conditions of the trees if you want them to thrive in the urban environment.
“This project had a multifaceted approach and considerable cost efficiencies were achieved. It involved the upgrading of the stormwater system in this area, replacement of potable water mains, and improving the connectivity of the City’s cycleway and pedestrian walkway. The Stockholm Method (modified) is comparatively quick to install and low in resource demand.
“For Dubbo, it was extremely cost effective as we used waste rock generated from a sub-division, and compost that we produced at our own facility. The biochar is generally the only external material that we purchase.
“The rock matrix, when securely locked together, resists subsidence in roadways even under heavy loads and the biochar has extremely good moisture holding capacity. This is extremely important in the hotter climes of Australia as it reduces moisture stress on the trees and, even if you require to provide supplementary water, reduces the watering frequency and thus costs.
“The Stockholm Method (modified) to date appears to be an extremely sound planting method. One of the things that I really like about it is the flexibility it offers in terms of adding additional unplanned underground services. It allows for the rock matrix to be excavated, being careful of the tree protection zones. New services can then be laid and the rock matrix replaced allowing for continued, uninterrupted root growth through the vault. Other systems are not quite so flexible but this can be overcome, to some extent, at installation stage through the addition of spare conduits through the vault.
“The major way that I see climate change impacting on the arboricultural industry will be through the tree species selection for our urban areas. As our climate becomes hotter and drier, the rainfall more erratic, and higher intensity storms, the tree species that we use will need to shift to survive and thrive in these changing environmental conditions. This issue will potentially impact all areas of the arboricultural industry. As a professional industry we need to be taking the lead in assisting in the identification of what trees will be suitable for planting into the future for specific regions of Australia.”
Ian said the Stockholm Method – modified or otherwise – is an effective tree planting system that can be used to improve tree planting success throughout Australia.
The three components – rock (250mm – 300mm), biochar and compost/ soil mix are blended off site and then brought in as required. The pits are excavated using a backhoe, then lined with a geotech fabric before the blended material is installed in layers of approximately 300mm.
Between layers a small robotic mini vibrating roller is dropped into the pit. The vibrating roller helps to ensure that the rock matrix is firmly locked together, and the vibrating action helps to fill the voids with biochar/soil/compost between the rocks.
The geofabric is then brought across the top of the rock matrix. This geofabric helps to reduce root penetration into the road base.
The subgrade of the road can then be built to normal specifications across the top of the root vault.
Depending on where the pit is within the streetscape an open-ended concrete vault is installed on top of the rock matrix and backfilled with the rock matrix. This serves a number of purposes – it prevents asphalting against the trunk of the tree, provides additional water / air exchange, allows easy access for supplementary watering to occur in drought / dry conditions and provides the opportunity to install a decorative grate to further enhance the streetscape.