Rot cut out in bid to save historic madrone tree

Press, 27 July 1989, Page 4

Rot cut out in bid to save historic madrone tree

By

DERRICK ROONEY

Dieter Steinegg, a Christchurch City Council tree surgeon, has been in a bit of a hole this month.

Mr Steinegg and an assistant have been burrowing down through the huge bole of one of New Zealand’s historic trees — the giant madrone in the Christchurch Botanic Gardens — in the hope of stopping the rot that has been slowly eating its heart out. The tree, more than a century old and reputed to be the biggest, and certainly the finest, example of its species in cultivation anywhere in the world, shows few outward signs of decay. It is still growing slowly but steadily, and in the last 20 years its bole diameter has increased about 20cm and its height more than two metres.

But when Mr Steinegg was called to attend to it, the tree was apparently on the point of collapse through hidden decay under the root crown.

The problem was uncovered this winter when gardens staff began clearing away accumulated debris from the base of the trunk.

Mr Steinegg dug down through the rotting interior of the bole and spent nine days excavating a

mini-cavern about a metre below the root plate. Working by electric lighting, powered by a portable generator, Mr Steinegg and an assistant then spent several more days carefully scraping away rotten material from the undersides of the main roots. The “cavern,” entered by ladder through the middle of the tree, has plenty of room for four people. It puts a whole new perspective on tree viewing, according to Mr Steinegg, who has encouraged members of the gardens staff to take this rare chance to "go underground” for a look at the processes and structure that go on underneath trees.

The experience, he hopes, will give them an eye for potential “dangers from below” in other trees.

When he began digging, Mr Steinegg was pessimistic about what he might find. However, he has now satisfied himself that the old tree has sufficient healthy root tissue to sustain and support it — for a while.

Before refilling the “cavern” with gravel he will install perforated plastic pipes to circulate air to the roots and discourage further rotting. He used a similar technique

recently to install a network of permanent “underground airways” before planting trees in the remodelled Victoria Square. Oxygen, says Mr Steinegg, is as important to the roots of trees as it is to their foliage, and the blocking of air through soil compaction is a major cause of failure in urban trees. He blames the trampling feet of generations of garden visitors for the madrone’s present problems.

Even after this round of surgery and the restoration of air to its roots, the long-term outlook for the madrone is uncertain. Like a late Hemingway hero, it is old and tired.

At intervals since the early 1960 s it has undergone several bouts of tree surgery. For many years its main trunks have been braced by criss-crossing wire ropes, and about 20 years ago its bole was reinforced with concrete.

A further, complication revealed by Mr Steinegg’s excavations is that the tree has no tap-root. When he has finished clearing away the rot, he .will drive a pole, 30cm to 40cm in diameter, down through the middle of the bole to serve as a surrogate taproot, and will wire the main

stems to it, for extra security. The brdnches will be thinned, also, to ease the burden on the ageing bole. Each of the five main trunks weighs probably several tonnes, and with the crown spread of 24 metres creating a huge sail area, the wind-loading on the bole is enormous. Ideally, says Mr Steinegg, the tree should be relieved of onethird of this burden. However, he will settle for about 15 per cent, and he says that if the thinning is done sensitively there will be no diminishing of the beauty of the tree. All this work will extend the , life of the madrone for probably ' 15 years, at a cost to Christchurch ratepayers running well into four figures. Mr Steinegg believes it will be money well spent: because of the size and beauty of the tree, because of its historical value (planted probably in the 1860 s, it is one of the oldest trees in the gardens), and because of the rarity in cultivation of mature examples of the species. Next time round, however, he has no plans to repeat the operation. By then, the old tree will have run out of lifetime. It will be too dangerous to leave standing.