Geelong’s trees are a valuable, vibrant part of the city providing a vast array of environmental, social and economic benefits to the local community. The urban forest promotes Geelong’s liveability and influences neighbourhood characters. It sequesters and stores carbon, enhances our local biodiversity, reduces stormwater run-off into rivers and the bay, absorbs air pollution and shades the city and its hard surfaces during hot summers.
More than this, Geelong’s trees help characterise the City into what it is today: from the heritage values of the majestic Elm trees to the beauty of the lemon scented gums planted along Brougham Street, the Palms along Moorabool Street and the Norfolk Island Pines lining Geelong’s waterfront. Without our trees, Geelong would be a very different place.
Geelong’s urban forest is the sum of all vegetation across the City of Greater Geelong, excluding rural land. Council itself is responsible for managing around 120,000 urban trees: 75,268 trees in streets and around 45,000 trees in parks. Council also manages trees in conservation reserves and those along rural roadways, waterways and bike trails. Conservation areas in particular are addressed in Council’s Biodiversity Strategy. Whilst rural parkland trees and rural conservation areas are an important component of Geelong’s natural landscapes, urban trees have greater monetary values and more direct and positive health impacts in areas of greater human density and are therefore worth strategically managing and investing in. Council will continue to encourage private landholders to revegetate rural landholdings and Council will manage rural conservation reserves to complement the Urban Forest Strategy.
This Strategy will provide a framework in which to actively manage the urban tree population including urban streets, urban parkland and urban conservation reserves for improved environmental, social and economic outcomes for Geelong.
An audit of the City's existing street tree population was completed in 2014 providing information on their current status. This data only pertains to street trees and does not include park or conservation reserve trees. Whilst open space is an important provider of tree coverage, street trees provide greater design and maintenance challenges and are also more valuable for providing streetscape liveability.
To obtain an overall understanding of this status certain qualitative and quantitative criteria are analysed: species diversity, tree health and structure, useful life expectancy, tree age and canopy cover.
Certain tree attributes were collected as part of the audit to model the dollar value of the environmental benefits that Geelong’s urban forest provides. Vacant tree sites were also measured to provide an indication of the opportunity available for increasing the tree resource.
The City of Greater Geelong is responsible for managing the following;
|120,268 Urban trees =
||75,268 street trees +
||45,000 park trees
Tree species diversity
Geelong’s streets house around 230 different tree species with the most common being Callistemon. The top 10 most common species are:
||% of population
|| Queensland Brush Box
||Callistemon Kings Park Special
||Kings Park Special
||Red Flowering Gum
||Prickly Leaved Paperbark
||Snow In Summer
All top ten species are Australian natives, demonstrating a past trend and preference for planting native trees from the 1960’s to the 1980’s such as Callistemons, Melaleucas and Lophostemons.
Street tree health, structure and age
Street tree health
Figure 1: Tree health across Geelong's street tree population.
Figure 3: Tree age across Geelong's street tree population
98.7% of Geelong’s public urban trees are healthy or are in fair condition and 97% are structurally sound. Best practice suggests that 90% of urban trees should be healthy so these figures demonstrate an active and effective maintenance program.
The poorly performing trees are actively being managed as part of the City's tree maintenance program.
Ideally, within an urban forest there is a good mix of age ranges to ensure a dynamic population. 70% of Geelong’s street trees are still in their growing phase and 30% are at mature stage which represents a sound diversity of ages.
Mature trees will need active monitoring and management for their eventual decline until their time of removal.
Street tree structure
Figure 2: Tree structure across Geelong's street tree population
Useful life expectancy
Useful Life Expectancy (ULE) is an important indicator of urban trees as it provides an opportunity for the City to manage tree loss with a succession plan. ULE is a measure of how long a specific tree will remain functional in the landscape before it will need to be actively managed with a view to being removed. Many factors influence ULE such as tree age, tree health, structure and surrounding conflicts with infrastructure and ideally there is a spread of ULE’s across the population minimising the risk of mass tree removal in any given period.
Figure 4: Useful Life Expectancy for Geelong’s street tree population.
Whilst the current population is healthy and diverse, around 14% of Council’s street trees will reach the end of their useful lives within a ten year period, meaning they will require active maintenance and eventual removal. This figure is above normal, which is anecdotally a 10% loss each decade, and is primarily due to the prevalence of Callistemons, Melaleucas and Eucalypts. Callistemons are a relatively short lived tree species and having been planted en mass in the 1980’s, they are now reaching the natural end of their useful lives and will need removing in a relatively short period of time. Melaleucas and Eucalypts have vigorous root systems and many are outgrowing their locations.
Around 14% of the City's street trees will reach the end of their useful lives within a ten year period.
Whilst tree height is not a key criteria in measuring urban forest performance, it is of great interest to note that Geelong’s tree sizes are generally quite small. 93% of the population are only up to 10 metres in height and over half of the trees are only 5 metres in height.
Given that only 22% of the street tree population are young, this suggests that the public urban forest consists of a very high percentage of small statured trees and is potentially not functioning at its most efficient capacity. This means that there is a large opportunity cost of missing environmental and economic values such as shading, air pollution and carbon sequestering and stormwater interception.
Large canopied trees are favoured for maximising these benefits and the results clearly show that Geelong has a small percentage of large trees.
Distribution of tree heights amongst Geelong’s street tree population.
Arguably, of most concern regarding Geelong’s urban forest is the lower than average tree canopy cover. Tree canopy cover measures the amount of tree canopy that spreads over the urban environ, particularly over hard impervious surfaces. The greater the canopy cover, the greater the benefits derived from the urban forest. A simplified canopy cover analysis conducted by the Institute of Sustainable Futures across Australia found that Geelong had a municipal wide canopy cover of 10.9%, compared to other Victorian municipalities as detailed in Figure 6 below.
Figure 6: Canopy cover measures for municipalities across Victoria (Jacobs et al, 2012).
Using the same methodology, canopy cover was calculated for urban Geelong which excluded all rural land, giving more relevance to the figure for use in an urban forest analysis. Urban Geelong has a tree canopy cover of 14%, with approximately 7% on private land and 7% on Council land. Peer reviewed literature recommends an optimum urban land use canopy cover of 40% because it delivers the maximum benefits to an urban environment. Geelong’s urban canopy cover is well below this figure and is exacerbated by two factors: extensive industrial areas across the city with very few trees and a legacy of small statured, small canopied trees across residential areas. Canopy cover over specific suburbs is provided in Figure 7 below.
Urban density is likely to affect the percentage of canopy cover on private land in future developments as the average house blocks are smaller and dwelling footprints are larger leaving less room for vegetation. This places greater emphasis and responsibility on Council to establish and maintain canopy cover on public land as well as investigating the possibility of mandating minimum lot sizes to accommodate one canopy tree per allotment and encouraging sufficient space for trees in new developments.
Wandana Heights, whilst displaying the lowest canopy cover has been the recipient of a targeted tree planting program so has a large number of small juvenile trees.
||City owned land
|Percentage of canopy cover
Figure 7: Canopy cover measures for the City of Greater Geelong.
The economic value of Geelong's street trees
Specific data was collected in order to model the dollar value of the urban forests environmental benefits. A United States Forestry Service model called i-Tree Eco was used to formulate an economic value of Geelong’s street trees.
A value is placed on the capacity of the urban forest as a whole to ameliorate air pollution, reduce stormwater flows and save energy use in buildings by shading north and western walls.
The model also produces a structural value for the urban forest, namely how much it would cost to replace all of the trees to their original size. The results are as follows:
|Number of trees analysed
||1,112,375 square metres
||Avoided stormwater run-off
||Stormwater run-off value
||8,455.74 cubic metres per annum
||$19,210.43 per annum
||Amount of air pollution removed
||1,387 tonnes per annum
||73 tonnes per annum
|Air pollution removed
|$3,311 per annum
Due to their size, stature and populations, the most valuable species in Geelong are the Lophostemon confertus (Queensland brushbox), Melaleuca and Yellow gums.
Geelong’s street trees absorb 73 tonnes of air pollution each year and stem stormwater flows equivalent to the size of four Olympic swimming pools annually.
Geelong’s street trees alone store 19,000 tonnes and sequester 1,300 tonnes of carbon per year.
“It is estimated that Geelong's street and park trees are worth over $370 million in structural asset value”