What value an old tree?

Typical park with old tree in Duffy Canberra

Large, old trees are a familiar part of the suburban landscape, protected for their environmental and economic benefits.  But do they provide biodiversity benefits too?

It’s easy to get lost in Canberra’s suburbs: streets curve and dogleg in bewildering patterns that make a map a necessity in unfamiliar areas.  But what is the reason for this tangle?  Although I would not presume to understand what goes on in an urban planner’s mind, my guess is that part of the answer comes down to the presence of remnant eucalypts, i.e. large, old trees.

Large, old trees are protected in Canberra by ACT government law (see here).  The ACT Tree Register identifies “registered trees” that are of exceptional heritage, landscape or scientific value.  In addition, we have “regulated trees”: any tree that is > 12 m high, > 12 m wide at the crown, or is > 1.5 m in circumference at the trunk (this can be split between multiple trunks).  One purpose for this law is to prevent these trees being damaged or killed during the development of new housing estates – hence new streets being planned around remnant eucalypts, and the resulting confusion of kinks and bends.

A big reason for protecting old and large trees is to maintain Canberra’s urban forest (i.e. our suburban trees) for the environmental and economic benefits it provides.  These include wind reduction, shade, storm water management and landscape improvement.  Note there is no mention of any “biodiversity” benefit.  But does this mean that none exists?

In a recent study, I looked at what vegetation features make our little neighbourhood playground parks suitable habitat for a range of different birds1.  I found that the number of large eucalypts with a trunk diameter > 0.5 m (equivalent to >1.5 m circumference) was an important factor.  With increasing numbers of large eucalypts, parks supported more bird species, in particular woodland dependent species, had higher bird abundance and a greater probability of breeding birds.  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 biodiversity benefit, and further back up the ACT government’s decision to protect these trees².

 

However, trees (and in particular eucalypts) have a shelf-life in suburban areas.  Public safety fears about dropping branches can change old trees from benign providers of benefits to the community, to creaking, dangerous widow-makers.  If a tree poses a significant risk to life or property it may be removed under strictly controlled conditions (see here).  And fair enough – no one wants to wake to the sound of the backyard tree falling though the roof.  But the problem lies with the future of old trees in our suburbs.  Canberra has an urban forest renewal policy that ensures that we will not lose our nickname of the “bush capital” but there are no legal measures in place to protect younger trees – or to put it another way, there are no measures in place to protect future large trees.  Perhaps these measures are needed if we don’t want large old eucalypts to become another thing lost in our suburbs.

 

 

Suburban street with old trees, Turner, Canberra

¹ K Stagoll, AD Manning, E Knight, J Fischer and DB Lindenmayer (in prep.) Large trees are keystone structures in suburban parks.

² Note, however, that trees in parks (“public trees”; these include street trees) are protected by a different set of laws to other trees (Trespass on Territory Land Act 1932).  But I assume that the trees don’t know this.