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Nearly seven months on from when hundreds of new brittle gums had been carefully planted along Canberra's light rail corridor, greenery has turned to brown and branches have begun to snap. But the situation is nowhere near as troubling as some locals might think, the ACT government says. Of the 830 trees planted along Northbourne Avenue and the Federal Highway during the light rail's construction, about 25 have died and another 20 are ill. This amounts to less than 5 per cent of the total population and leaves the corridor with about 125 more than it had before the previous rotten and fungi-infested trees had been ripped out and replaced. Still, landscape architect Dr Andrew MacKenzie says the ACT government put the new trees at some risk of death, planting the same species along the corridor and creating a kind of "monoculture". "A monoculture is a single species and if you've got an abundance of one species of plant and it's prone to a pest or a disease attack, well it's kind of like a kid in a candy shop," Dr MacKenzie said. "If you combine that with the higher than average temperatures over the summer, and in fact the higher than average temperatures over the year ... it's whether the trees are able to adapt." While landscape uniformity, particularly along avenues, was typical for the national capital, the ACT government might better consider the health of trees in the future, Dr MacKenzie said. The eucalypts had reached maturity at their time of planting and stood at about five metres tall, creating an additional problem. Seedlings had the best chance at a long, healthy life. "I would suggest that the authorities becomes less preoccupied with how it looks and more interested in the long-term viability of the trees. They need to go back to their first principles," Dr MacKenzie said. "A natural forest has trees of different ages and varieties and they all grow at different times. What that does is it creates what's called a succession - different trees at different stages - and that would be a much better long-term strategy." The trees, having been nurtured and hand-irrigated at Yarralumla Nursery for the first part of their lives, would have also undergone "transplant shock", Dr MacKenzie said. This was particularly relevant given their concrete and construction site surrounds. An ACT government spokeswoman said each dead tree would be replaced with a like tree and they had been constantly monitored and managed. "The trees are planted into 30 cubic metre pits with high quality growing soil and incorporation of two watering and aeration tubes," the spokeswoman said. "The combination of soil and irrigation tube encourage deep root growth which in turn increases tree stability and better resilience to heat and drought." Many other streets in Canberra would soon face similar issues, having been planted in the avenue style some 50 to 80 years ago, forestry expert Dr Cris Brack said. As trees in those mini-monocultures came to the end of their natural lives, the death of a few would end in the death of all. The decision was whether to replant them uniformly, or adopt a healthier, diverse range of trees. "We've made that decision for Northbourne Avenue. It's going to be an avenue, we don't really have a choice. That's a heritage thing and a landscape thing," Dr Brack said. "For our street scape, are we making that decision ad hoc or is [diversity] going to happen?" The ACT government would not say how much the tree planting program along the light rail corridor cost.

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