Australasian Science: Australia's authority on science since 1938

The Value of an Old Tree in the City

By Karen Ikin

Large old trees provide a significant biodiversity benefit that should be factored in by governments when managing biodiversity.

Large old trees are valued and protected in many of Australia’s city and suburban landscapes because of the environmental and economic benefits they provide. These include wind reduction, shade, storm water management and landscape improvement. The biodiversity benefit of old trees, however, is often forgotten and rarely quantified.

In a recent study I looked at what vegetation features make our neighbourhood parks suitable habitat for a range of different birds. I found that the number of large eucalypts with a trunk diameter larger than 0.5 metres (equivalent to a circumference of 1.5 metres or greater) was an important factor.

With increasing numbers of large eucalypts, parks supported more bird species – in particular woodland-dependent species, some of which are declining in south-eastern Australia – had higher bird abundance and a greater probability of breeding birds. Parks with the largest trees (with a trunk diameter more than 1 metre) had up to three times the number of woodland-dependent species than parks with only smaller trees. Moreover, large eucalypts affected the mix of species found at the parks, and parks with low numbers of large eucalypts harboured more exotic species.

Clearly, these findings show that large old trees provide a significant biodiversity benefit, and this value should be factored in by governments when protecting this resource.

In agricultural and forestry landscapes, large trees are considered “keystone structures” that disproportionately provide crucial resources like food, nest sites and shelter for wildlife. However, research projects demonstrating this in urban landscapes are few and far between.

If cities are to maintain a diversity of wildlife it’s vital that large trees in urban environments are managed properly. Losing large trees from cities may have far-reaching ecological consequences that may undermine other wildlife conservation measures.

Large eucalypts are protected in the Australian Capital Territory, but existing measures may not go far enough. In our study we found that trees as small as 40 cm in diameter can have a strong positive effect on bird diversity. However, this is smaller than minimum sizes prescribed by many managing authorities, including those in North America, Europe, Asia and Australia.

In our study area, government law regulates only the removal of trees over 50 cm in diameter. This means 457 park trees measuring 40–49 cm in diameter (14% of all trees that we measured) do not receive formal protection. Tree preservation laws, therefore, may not be providing adequate protection for a large number of important trees.

It is likely that similar trees are at risk in other cities in Australia and the rest of the world.

Of course, retaining old trees in public spaces has many challenges. Public safety from falling branches is one issue, but this can be managed by strategies other than tree removal, such as fencing off risky areas or using landscaping to minimise the risk.

We also need to plan for the future if we want to keep large trees and our native bird species within the city limits. It takes decades for a newly planted sapling to grow into a large tree, so we need to think and act early.

Karen Ikin is a researcher with the Environmental Decisions Group. She is based at the Australian National University.