Despite death, decline and heat stress, Transport Canberra senior management say they remain confident that the eucalyptus mannifera specimens planted along Northbourne Avenue are the right choice for the light rail zone and will eventually thrive.
That claim has been disputed by an expert regional silviculturist who remains adamant that the trees are “doomed” and that planting advanced trees to satisfy public expectations demonstrates “a very basic lack of knowledge”.
Marcus Sainsbury, who is the senior manager design planning and environment for Transport Canberra wouldn’t put an exact number on the trees that are dying or likely to die along the Northbourne alignment although he conceded the long run of extremely high temperatures had been very stressful for the newly planted four-metre trees.
“There are two categories – the ones that are in permanent decline and ones that are under stress,” he said. “It’s fair to say that in general terms the industry would expect a 5 per cent loss of mature trees planted, and we’re confident that we are under that at the moment.
“Clearly some of the trees have died and by the same token, some of the trees under stress have made a recovery. We are monitoring the ones under stress and seeing how they go, maintaining a watering regime for the stressed trees over the past few weeks.”
Paradoxically, heavy surface watering can create shallow root systems and unsustainable growth because the trees aren’t anchored deeply. Mr Sainsbury says this is a fair observation but believes that the “bespoke” planting environment installed for each tree will solve any problems.
“We’ve used 30 cubic metres of growing medium for each tree, and that level of soil preparation was never undertaken in the past. Previous trees were sitting on a clay pan which meant that the roots spread out in shallow systems. But the manniferas have provision for deep-root watering, where the water is channelled down to the base of the tree pit to promote root growth.”
Mr Sainsbury says the growing environment should also prevent compression from construction traffic, parrying the suggestion that it’s a compromise to plant them out at this size rather than planting smaller trees.
It’s there that he differs considerably from independent silviculturist Peter Marshall, who runs TerraPreta truffles at Braidwood and has over 40 years of forestry experience. He believes more trees are doomed because of “this concept of growing a tree to be long and thin and whippy and putting it in to create an instant forest.
“It’s a construction zone with soil compaction and concrete in the soil so the PH will be very high from the leftover lime. The position is very exposed and there’s a massive swing in temperatures at ground level from day to night.
“Had they planted little dollar seedlings of the same species they would have grown a metre in every year, at an astonishingly lower cost. This nursery-grown stock is four metres tall now, but they’ll possibly die under the stress of exposure, while the tubestock would get there quickly, with a much better root architecture and stability.
“Where is the survey that says the public want tall saplings that then die? Have they made this up in their own heads?”
Marshall is also critical of the species choice, saying the eucalypts risk shedding limbs as a matter of course. “High-temperature limb breakage is an ecological adaptation. A water column in the trunk literally explodes and detaches itself, which creates a hollow nesting place for birds and animals, whose manure then fertilises the tree. That’s a great ecosystem adaptation, but a massive disadvantage in an urban environment.”
In stark contrast, the tree planting process is characterised as “careful and deliberate” by Marcus Sainsbury. “Back in May 2017, we established a landscape trial plot that replicated in exact detail the planting arrangements for Northbourne. We are not watering the trial plots trees at all now and the regime has shown that those trees are doing really well”
Peter Marshall argues that deciduous trees, perhaps Mexican oaks from a very similar climate but with a much stronger root and branch structure would have been a better choice amidst Canberra’s magnificent tapestry of trees. But Marcus Sainsbury is adamant that the manniferas are the right choice and will thrive.
Do you think mature eucalypts were the right choice for the Northbourne corridor?