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The National Arboretum of Doom

Peter Marshall, forester, truffle grower and expert on mycorrhizae gives his forthright view

An aerial view of the National Aboretum

A recent chat about the National Arboretum in Canberra puts me in mind of a lesson of 20 years past.

In winter 1995 I had a call from retired grazier John, he needed help. His problem had origins in WWII when John fought in the North African desert campaign. His young wife Joan nursed wounded soldiers in an English manor house, where she fell in love with the expansive woodland gardens.

Back in Australia in the 50s Joan suggested that they might plant such a garden around the family farmhouse. Initial response was unenthusiastic, ‘waste of good grass’, but she won him over. Forty years later her forest garden was one of the scenic gems of the district. One of Joan’s first plantings was a little Oak tree in a pot. She planned it to be the centrepiece of the garden but noticed it was forked with two tiny trunks. She sought opinions: should she clip one trunk off?

The response was along the lines of “I fought Rommel, I will not buy German secateurs, just plant the thing”. Memories were raw and hardware expensive. So in 1995 I went to help and found a vast, vigorous and beautiful 40-year-old oak tree towering above the house, with a huge split right down the middle.

Treatment was hard work, expensive and dangerous, but we owed it to Joan’s vision to try to save the tree. We hired a cherry picker and took several tonnes of limbs off the sides to reduce the stress. We drilled 1.4 metres through the trunks, inserted a huge threaded steel rod and bolted them back together. Good as new at the cost of $1,000 and two day's work for four people.

This story suggests the question that the ACT Government should be asking of the management of the Arboretum: What is $1,000 multiplied by 3,000 or 10,000 trees in 1995 adjusted to 2030 dollars?

Because that is the sum of money the ACT better be putting aside unless they snip off tens of thousands of forks very, very soon.

Management replied to Chief Minister Gallagher, after an inquiry from me a few years ago. They said “they are using an arboriculture model, not a silvicultural model”. Silviculture is the art and science of growing forests. It is the appropriate management model for the custodians of 100 forests, but they are deliberately ignoring it.

Errors start with the original concept of 100 single species block planted in forests. The idea of closely planted trees reaching for the sky like the pillars of a Gothic cathedral is inspiring, but the reality will be more like a windowless, dark ages blockhouse. This is rooted (intentional pun) in the growth habits of the tree species chosen. In general, Conifers grow away from gravity, broadleaf trees grow towards light. Radiata Pine will happily grow upwards away from the centre of the earth. Burr Oak will equally happily grow sideways towards the most accessible sunlight, unless pruned to encourage apical dominance.

Species also differ wildly in their love for light or tolerance for shade. Radiata, Beech and Redwood can grow in some shade. Closely planted light-demanding trees like Burr Oak, Mesa Oak and Giant Sequoia will shade each other, sulk and slowly die.

Sulking and dying will be the theme of the Arboretum for decades to come. Planting densities defy all silvicultural realities: Oaks replanted at 4-5 metres on a grid, when 10 -20 metres is appropriate; Gums at 2 metres along the rows when 7m would be a good start. A site that could accommodate perhaps 4,000 broadleaf trees is jam packed with ten times more.

People who find Haig Park to be overplanted and creepy might be shocked to learn that the Arboretum is much more crowded and will be far creepier much sooner.

By comparison the British National Arboretum (on much better soil) is stocked at only 40% of ours, the NZ National Arboretum at about one quarter and the stunning Hackfalls in NZ at even less.

Poor soil preparation adds to the disaster

Spacing mistakes are enough on their own to reduce the forests to moribund scrub, but a host of other odd decisions adds up to the perfect storm of forest failure.

Starting with poor soil preparation. The site is heavy clay, dramatically compacted by the bulldozers used to clear the site.

The underlying infrastructure of a healthy forest is porous, well-structured soil. A site of national significance should command the greatest diligence in developing soil structure before planting. Multiple passes with narrow tines over several years at the very least.

Yet photo after photo on the Friends website shows sultans, generals, keen school kids and rock stars trying to hammer baby trees into unprepared heavily compacted soil. The Buchan Wattles are the first to drown in oxygen starved soil, next will be the Dawn Redwoods.

I wonder if the Arboretum owns a penetrometer? Ten minutes work with such an instrument would show exactly why the forests are failing.

Irrigation failures

Next the irrigation. Hugely expensive, very difficult to maintain and anathema to the health of the trees. In poor draining and poorly prepared soils, the roots just hang around the sprinklers and have no incentive to range far and deep in search of water. The roots stay near the surface, to be damaged by hot days, and become irrigation addicts. One day a big wind will blow them over in swathes. The absence of purpose designed windbreaks hardly helps. Planting one tenth the number of trees and burying a watering tube under each one would have given drought security with proper root structure at a tiny fraction of the capital and maintenance costs.

Pruning is paramount

Next issue is form pruning, corrective pruning, pre-emptive pruning. Or more correctly the absence of these vital procedures, replaced by the charmingly whimsical pruning style of the Chief Arborist. I have never met an expert silviculturist bold enough to be deliberately creating a “mysterious forest” which visitors “can get lost in” (Canberra Times, June 28th 2015).

I visited his Persian Ironwood forest convinced he was misquoted. He is as good as his word, it really is pruned so that children can get lost, run up branches, fall and break their arms. Distraught parents will need to tiger walk under chest high limbs to find their Hansel and Gretels. Note to ACT Govt: Your 100 forests do not meet Australian Standards for adventure playgrounds, please increase your public liability cover!

The article also gives an insight into the astonishing labour requirements of managing overplanting. Our arborist used 5,000 cuts of his electric pruners to form up 400 Persian Ironwood. That’s 12.5 cuts per tree, a fairly light treatment. Multiply 12.5 by 48,000 makes 600,000 cuts required per year, for the next 6-10 years. Better start training more arborists, perhaps fifty or so, and budget $150,000 for a set of pruners each.

In a thoughtful editorial (March 15, 2017) the Canberra Times mused that “Since nothing of this scale had been attempted before in Canberra, there was always an element of trial and error with the arboretum”. That's not quite right. Trials have been run many times before, successes and errors are plainly evident:

Lessons to be learnt from other arboretums

Example 1 — Westbourne Woods. Charles Weston prepared planting holes with explosives on a wind exposed site, put in wind breaks, planted wide, form pruned and taught us what works.

Example 2 — The Deodar and Cork Oak forests hard against the arboretum. Planted quincunx with a silvicultural plan to heavily thin when best genetics became evident. War and depression intervened thus the thinning never happened. Resulting in: locked up forest, little annual growth increment, little understory and shortened tree life.

Example 3 — The mighty Professor Lindsay Pryor’s little arboretum just down the hill. Lessons to be learned for those with the skills to see.

Example 4 — The magnificent street trees of Canberra, the biggest arboretum in the world. Planted wide, form and lift pruned to 5-6 metres in the first twenty years, they stand as the best examples a planner of public access forests could wish for.

Arboretum management must learn from these examples to plan in time and in three dimensions, above and below ground. If they don't do so very soon, we are indeed looking at the Arboretum of Doom.

Note from the Editor: We seek articles from opinion leaders like Peter Marshall on important garden issues — organics, climate change, soil health, ornamental and food gardening.

The Marshalls won champion truffle producer with their Terra Preta (dark earth) because of its superior deep-black colour, and also because their truffles are grown organically. They don't use fungicides obviously, nor do they use weedicides or pesticides which taint the flavour. Truffels are bio-accumulators, picking up chemicals in the soil, which is why their truffles are sought by epicurean restaurants in Australia and overseas.
Truffles are fungi, like mushrooms. Their spores are like the seeds of plants and their method of reproduction is from these spores.
They evolved at the same time as flowering plants and need chilling hours at -6°C , therefore high altitude, but they also need hot summers and oxygen. With sugar from plant hosts and moist soil, it takes 4-6 years for the first truffles to emerge, which starts as a pinhead and grows up to 400gm in 3-12 weeks.


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