Urban trees are a breath of fresh air

Trees are crucial features in our urban streetscapes. They keep us cooler, clean the air, contribute to the biodiversity of the urban landscape, provide habitat, help maintain soil moisture levels, aid in managing storm water runoff, provide amenity and appeal. Many urban trees connect us to our historic and cultural roots; trees provide a sense of place.

Urban tree cover is recognised world-wide as an important tool for combating climate change. Trees absorb carbon and deliver oxygen. Open grassland spaces and parkland gardens are attractive, but trees are much, much better at providing relief from the urban heat island effect and sequestering carbon.

In 2018, Melbourne’s tree canopy cover was measured at 15.3%, one of the lowest rates of Australian cities, (Sydney and Adelaide each have 23%, Brisbane has 44% tree canopy cover).

I wonder how the urban areas of Baw Baw are measuring up? Can you imagine some of Drouin’s beautiful tree-lined streets without their trees? 

Urban tree cover requires proper planning and robust policies. Streetscapes don’t have to be ‘hardscapes’ that exacerbate the urban heat island effect, create storm water runoff issues and negatively impact on human and ecological health. 

Roads and footpaths make up the largest area of public space in any urban context. It is obligatory of planners, engineers and developers to be fully cognisant of the benefits of urban trees and how to incorporate them into urban design.

We need urban trees more than ever.

Bell Miner associated dieback

Urban Drouin and many surrounding locations are renown for the ‘tinkling’ call of the ‘Bellbird’ or more properly, the Bell Miner. Bell Miners are native honeyeaters. They survive in colonies in eucalypt woodlands – Thornell’s Reserve at Longwarry North, Roberts Ct bush, Bellbird Park wetlands, the golf course and John Lardner Reserve at Crystal Waters in Drouin and many other places in the district.

Bellbird Park in Drouin is named after the bird of course and an early settler in the area, Will Rumble, was inspired to write many poems in which the Bellbird is celebrated:

    "Oh weary traveller, rest a while

    In a Gum-trees (sic) welcome shade

    Out where the Bell-birds (sic) lovely calls

    Go ringing through the Glade"

    Call of the Bell-birds - Bushland Thoughts - Will Rumble

Bellbird or Bell Miner

Mr Rumble might today be aghast at the viewpoint among many avian researchers of the negative impact that the Bell Miner has on the environment.

Like many other honeyeaters, Bell Miners aggressively protect their feeding zones. They chase away other honeyeaters and numerous other birds. Experienced bird watchers will often decry the lack of diversity of birds within the proximity of a Bell Miner population.

Bell Miners are even identified as one of the causes of eucalypt dieback in some woodland areas, a process that even has its own name, ‘Bell Miner associated dieback’. 

Is this thinning of the canopy an example of the beginnings of Bell Miner associated dieback at Bellbird Park in Drouin?

Eucalypt woodlands normally support a large variety of bird, mammal, reptile and invertebrate species. One particular invertebrate species, a member of the psyllid genera, is a sap-sucking insect that survives by sucking the sap from leaves and shoots.

The winged, adult psyllid lays it eggs on the eucalypt leaves. Both the adult psyllid and the hatched nymphs are sap suckers. The nymph psyllids secrete lerp, a protective sugary layer over themselves. It is the sugary lerp that attracts the Bell Miners.

Sugary lerp - the exudate of a psyllid nymph

Researchers have discovered that the Bell Miner virtually ‘farms’ the psyllid population by consuming just the lerp, leaving the alive nymph underneath to continue its life cycle on the eucalypt foliage – making more lerp, eventually turning into an adult psyllid, laying eggs, more nymphs, more lerp, and so on.

Bell Miners acrobatically collecting lerp from eucalypt foliage

A point is sometimes reached where the psyllid population is too large for the Bell Miners to keep up and the trees begin to lose their foliage and start to die. As a result, the Bell Miners end up having a reduced food source and often move on to another location. Sometimes the birds will leave in time for the trees to recover – and sometimes not. Many bird watchers have records of Bell Miners being present, then disappearing and many seasons later reappearing. 

Enlarged view of lerp in the beak of a Bell Miner

 

Many other honeyeaters and other birds use lerp as a food source. It has been discovered that most other birds consume the lerp and the psyllid nymph underneath at the same time and this has the effect of keeping the psyllid infestation in check to some degree.

The Bell Miner is a bit of a paradox – loved and loathed at the same time.