Managing the impacts of urban development on old trees

How green is my backyard? Three quarters of Australia’s population lives in urban areas so for most of us our ‘backyard’ is an urban space. And most of our urban spaces aren’t that wildlife friendly. In a study undertaken by Darren Le Roux and colleagues in Canberra (Le Roux et al., 2014), urban green space supported fewer mature trees, tree hollows, dead trees, logs, shrubs and litter than adjacent nature reserves. Each of these features represents important habitat for native species of birds, bats and invertebrates. As long as these habitat resources are absent from our urban green spaces, then so too will much of our native wildlife. What’s more, this wildlife will be pushed out of our growing urban areas.

As an example, iconic old eucalypts that are scattered through Australia’s suburbs (most of which pre-date European settlement) provide many habitat resources for native wildlife, including hollows where birds, bats and possums nest or roost; large volumes of nectar that support many of our invertebrates, birds and mammals; large dead branches that fall to the ground and large ribbons or slabs of peeling bark under which many unique invertebrates live (and which subsequently support many birds and lizards). In fact, the researchers found that a third of all native bird species in Canberra ONLY use large, mature eucalypts over 150 years old.

In contrast to conventional thinking that connectivity is critical for wildlife, they found that isolated large trees in urban areas support more species and individuals of birds than equivalent large trees in intact areas such as nature reserves. That is, removing a large tree in an urban area is more detrimental to biodiversity than removing a large tree from a nature reserve!

However, large old eucalypts have a bad reputation in urban areas: they have dropped their limbs on cars, houses and even people; they can have an expansive root system that destroy footpaths and drains; they block solar access; and they can represent a hazard in areas with high bushfire risk.

Large eucalypts are therefore often removed to make way for new suburbs. And where eucalypts are planted in suburbs, they are rarely permitted to get to an age when they provide suitable habitat (eg, tree hollows) for many native species. For example, in the suburbs of Canberra, planted eucalypts are generally permitted to reach 60-80 years of age before they are removed.

Establishing more trees, extending the standing life of existing trees and increasing the rate at which eucalypts develop habitat features—will reduce the rate at which large, old eucalypts will decline in urban green spaces, no feasible combination of these will totally reverse a decline in large old eucalypts in Canberra’s suburbs. Can we offset this loss in some other way? What about the use of nest boxes? The researchers are currently examining these issues.

See Decision Point #82 for the complete story

Figure 2. Over two-thirds of the mature eucalypts at this site (left) were removed for this section of the new suburb of Gungahlin in Canberra (right) (Source: Google Earth).

Figure 2. Over two-thirds of the mature eucalypts at this site (left) were removed for this section of the new suburb of Gungahlin in Canberra (right) (Source: Google Earth).


Reference

Le Roux DS, K Ikin, DB Lindenmayer, W Blanchard, AD Manning & P Gibbons (2014). Reduced availability of habitat structures in urban landscapes: Implications for policy and practice. Landscape and Urban Planning 125: 57-64.

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