Two dead street trees right next to each other in Marrickville – notable because they are beside each other.

My last post was about the NSW government’s ‘Five Million Trees’ initiative.  See – https://bit.ly/2H2IEkz    The aim is to increase Sydney’s existing tree canopy from 16.8% to 40% over the next 12 years at a cost of $37.5 million over four years.

Fortune had it that I came across a long & detailed article from Scenario Journal titled, ‘How Many Trees Are Enough? Tree Death and the Urban Canopy’ written by Lara A. Roman.  https://bit.ly/2K1G1gC

The article looked at tree survival across US cities.  Research has found that, despite concentrated campaigns to increase the urban canopy in many major US cities, the canopy cover is actually declining. This is of major concern considering the extra pressure we have from increased development, a changing climate & its associated urban heat island impacts.

Three questions were asked –

  • “With major new planting campaigns, how many of those trees will survive for decades, reaching a mature size at which their environmental and socioeconomic benefits are greatest?
  • How many trees are enough – that is, how many need to be planted to make a lasting impact, and meet a city’s canopy cover goals?
  • What are the implications of future tree death for managing the urban forest, in terms of cycles of tree removal and replacement?”

“For street trees in New York City, eight to nine years after planting, 26.2% were dead.  For a yard tree give-away program in Sacramento,  five years after planting, 29.1% had died, on top of 15.1% that were never planted by residents.  For these yard and street tree examples, over a quarter of the trees planted died within the first five to nine years, and furthermore, for the tree give-away, some trees never made it into the ground.”                                                 

As part of their 5 Million Trees initiative the NSW government plans to give away 15,000 trees to people who are building homes in new land release areas in Western Sydney.  I wonder how many of these trees will actually be planted.  I don’t know about you, but I have bought plants from the nursery & left it too long before I put them into the ground.   One can realistically expect that high numbers of these giveaway trees will not be planted in western Sydney.  Will these be acknowledged, or will they just be written up as a successful addition to the urban canopy simply because they were given away?

The Inner West Council has given away trees for the past two National Tree Days & I applaud them for this.  It would be interesting to know whether Council has gathered information about how many of these trees made it into the ground & how many are still alive today?

The article notes that in the US there is very little in regards to monitoring & follow-up with tree planting campaigns & calls for a nationally coordinated monitoring network.

The author then cites the common belief that street trees live on average for only 7-years & a maximum of 13-years. Apparently, this belief was formed decades ago where a questionnaire was sent to urban foresters “asking the local experts to estimate the typical tree lifespans in their cities.”  The author describes this as “overly pessimistic.”

One only need to look around our own suburbs to know that there are plenty of streets with street trees that are 70-100 years of age.  Generally, these are QLD Brushbox or Fig trees.

A study to understand the change in the street tree population of West Oakland in the US looking at annual tree planting & tree deaths found that the annual mortality rate was 3.7%.   Most of the tree loss came from newly planted trees with a trunk of 7.6cms (3 inches).

“Extra vigilance during the establishment phase, in terms of maintenance and stewardship, might have the most payoff for ensuring planting survival, and thus achieving larger canopy objectives.”

“Planting a few hundred trees, or even a million, does not automatically translate into an increase in the overall tree population over the long-term. To increase population levels, the survival and planting rates have to out-weigh losses from tree death and removal, including both old and young individuals.”

I think this article brings up serious issues concerning increasing the urban forest.  I’ve noticed that street tree removals post Marrickville Council’s Tree Inventory meant that for at least 3-4 years the urban forest numbers stayed relatively the same.  With 1,590 streettrees to be removed from 2013, planting around 400 new trees a year meant that making up ground was slow.

I have noticed that a lot of street trees have been removed in the last couple of years.  Since Council does not report the removal of trees 5-metres or under & since 49% of our street trees were found to be 5-metres or under, we have no idea where street tree numbers are at now & no ability to calculate this ourselves.  The Tree Inventory was costly, so I doubt we will have a new one done for many years.

Increasing the canopy means not the number of trees planted, but the “percentage of total ground area; for example, at 50% canopy cover, half of the total ground area is covered by the vertical projection of tree crowns.”  

This has an impact on the way Council prune street trees as well. Continuing to remove side branches that serve to make the tree canopy fuller & produce longer periods of shade is counterproductive – unless it is required for safety reasons like branches over the road affecting passing trucks as an example.  However, you only need to look at the street trees around you to know that many street trees are pruned to have no side branches.

Council’s policy of no tree branches 2.5-metres (8.2 feet) above a footpath pretty much ensures that the footpath will have no shade except for perhaps an hour when the sun is in the right position.

The Five Million Trees initiative may result in significant changes with tree planting & reporting.  It would be good if this information is available to the community should they be interested.  Hopefully, outdated ways to manage street trees get dumped & our urban landscape becomes green & flourishing.

A Marrickville streetscape – a good one because it has street trees 2 storeys tall.  Photo June 2016.