Large urban trees require strategy to avoid felling: study

The study also found that smaller trees in Canberra's urban parks could still have a ''strong positive effect'' on bird diversity but were not subject to any formal approval process before they were removed.

The report further warns ''the loss of large trees from urban settings may have far-reaching ecological consequences''.

The study, led by Australian National University PhD candidate Karen Stagoll, looked at 3300 eucalypt trees in more than 100 neighbourhood parks in Canberra and found 44 species of birds across the area, most of them native.

Ms Stagoll said the team's study was the first in the world to explicitly demonstrate that large trees in urban parks were ''keystone structures'' that helped provide important habitat and contribute to the richness, abundance and breeding of birds.

She agreed that might seem like common sense but previous studies had focused more on agricultural and forestry landscapes and did not look at the value of urban trees in terms of promoting bird diversity.

''We found that parks with more large eucalypts had more bird species and higher bird numbers, including more native woodland-dependent birds, some of which are declining in Canberra and the surrounding region. Birds were also more likely to breed in parks with large trees,'' she said.

''We also found that if parks had really large, old trees they had more birds than parks with only smaller trees.''

The study said in urban areas, management policies ''often cause trees to be felled or extensively pruned before they reach their full biological potential, thereby limiting their value to wildlife''.

It argued for the preservation of very large trees - with trunk diameters more than 100cm - in urban areas. Any risk they posed should be managed by measures other than removal such as fencing and landscaping, for example by putting shrubs instead of grass under trees to keep people away.

''Of course they have to manage the public safety risk but if there are ways of trying to keep some of these trees in the landscape, it's really important,'' Ms Stagoll said.

The study found in the ACT no approval was needed to remove trees less than 50cm in trunk diameter but park trees ''as small as 40cm in diameter can have a strong positive effect on bird diversity''. It also found 457 trees - or 14per cent - of the park trees examined were 40-49cm in trunk diameter so they did not receive formal protection.

Ms Stagoll said that might be one area of the law that needed to be reviewed.

''I think the legislation needs to be evidence-based and regularly reviewed and the value of these trees for biodiversity needs to be explicitly acknowledged because, at the moment, it's not mentioned in the legislation,'' she said. ''If we want to maintain the Bush Capital and have birds in our cities, we need to be thinking about their habitat.

''We're having so much growth in Canberra at the moment, we need to think ahead to what sort of city we want in the future. If you're planting a sapling now you're looking ahead to 60, 70, 80 years. And it's good that they're planting those trees but if we're losing those little trees, we could have decades where we're losing the birds.''