Landscapes are becoming increasingly contrived as our lifestyles place more demands on land resources. Increasingly land use is prescribed and managed according to the needs of community in terms of conservation, agriculture, commercial, industry, residential and recreational uses. Apart from a few instances such as heavy industry and open cut mining, trees co-exist with these activities and have a function in most landscapes.
In the urban environment trees exist as natural elements that provide numerous social, environmental and economic benefits where they exist as a link to nature and previous land use, in green wedges, remnant pockets, through informal plantings in parks and public open space, gardens, in regular and managed plantings in urban streets and property perimeters to formal plantings in high profile parks, avenues and boulevards.
Probably the most contrived of these landscapes but also the one that makes the strongest visual and sensory impact is the formal avenue planting. These are characterised by trees of uniform species, age, size, spacing, ornamental characteristics and useful life expectancy.
One of the greatest problems facing the management of a formal avenue is that they are susceptible to disruption and fragmentation of the regular and uniform symmetry by losses of individual trees, reaching senescence all at the same time, build up of pest and disease due to the susceptibility of monocultures or by trees that are exposed to greater pressures of exposure to elements or urbanisation than other trees in the group.
Avenues can be on a grand and impressive scale as in the case of Memorial Avenues of Honour
They may be smaller but still notable such as Mont Albert Road in Canterbury, Melbourne.
They may be relatively insignificant suburban streets such as throughout Canberra suburbs.
Or may even be a planting bordering a residential driveway entry.
Almost any tree species could be used to achieve an avenue planting but species characteristics must be considered when choosing the right tree for the right site. Some of the species commonly observed include Elms (Ulmus spp.), Planes (Platanus spp.), Eucalypts (Eucalyptus spp. including Corymbia spp.), Oaks (Quercus spp.), Cypress (Cupressus spp.), Cedars (Cedrus spp.) to name only a few. Each species has a specific set of cultural requirements that would make them better suited to some situations than others. i.e. The Golden Monterey Cypress has performed well in some coastal situations where more salt sensitive species such as Elms would be impacted by salt laden winds and sandy soils.
On the other hand, Elms have proven to be relatively tough in the urban setting where there are increased levels of pollution and limitations on soil volume due to hard pavement and essential infrastructure.
Some species are particularly susceptible to specific stresses, pest and disease such as Elms which are susceptible to Elm-leaf Beetle here in Melbourne but have been all but eliminated from the northern hemisphere by Dutch Elm Disease. In some cases the relevant management authority has determined that the cost of ongoing management of an avenue to be disproportionate to the perceived amenity value of the group in comparison to the greater population of trees for which they are responsible and which can be managed over a greater cyclical timeframe. There are instances where treatment to manage Elm Leaf Beetle has been suspended in some smaller avenues of Elms because the species does not fall within the management objectives of some urban parks (Royal Park). Other instances where the original trees in a memorial avenue are difficult to identify from the profuse root suckers (Linton Avenue of Honor). Some avenues of water needy Poplar species have been allowed to succumb to drought stress because the resources required to sustain the avenue is disproportionate to budgetary constraints..
Many of the most notable and recognisable avenues are the mature ones that were planted in the early days of establishing cities and towns that can date back to the late 19th and early 20th Centuries and are approaching or in excess of 100 years old.
At the time of creating these avenues there were several prevailing factors that assisted in their successful establishment. They were generally planted in conjunction with establishing new suburbs and roads, or with strong emotional and commemorative impetus in terms of the memorial avenues after the First World War or, in the case of Canberra, as an integral component of the fabric of the entire city layout with each street and road having a specific planting theme. At the same time of these avenues being established there was a strong sense of civic pride and optimism, more of the population had an affinity with gardening or farming to some extent, as well as an interest in experimenting with the use of Australian native species and even mixing them with conventional species palettes of trees with exotic origins.
The trees were almost always planted into wide planting sites comprising native soils that had not been previously impacted by development, compaction or infrastructure. Many of the avenues of honour were planted into road verges that allowed surface runoff to flow to the root zones. There was also a sense of limitless space and so the avenues were generally spaced to provide adequate area and resources to provide for future growth.
With the pressures of urbanisation have come many constraints that are imposed on the existing avenues such as road widening, installation of kerb and channel, introduction of vehicle parking beneath the trees, installation of overhead and underground services along with numerous targets that elevate risk potential. As a consequence structural pruning is often required, all of which cumulatively results in health, vigour, structural integrity, visual amenity and ultimately useful life expectancy being diminished. These pressures can be manifest as poor canopy management and inappropriate pruning, damage to trunks and disruption of the root system. Once they have occurred most of these impacts are irreparable and irreversible.
With these cumulative constraints management options for extending the life of the avenue becomes increasingly limited as well as seriously restricting the potential for successful re-establishment of any new plantings.
Example of cumulative pressures of urbanisation impacting on the growing conditions of avenue of trees in Thompson Avenue including paved surfaces, kerb & channel, infrastructure such as seats, bins bollards and buildings. Spacing has increased due to the loss of trees. Repeated pruning events has resulted in poor weight distribution on over-extended limbs.
There are fewer options to extend the life of the avenue when the trees are reaching the end of their mature life and moving towards senescence. As trees age their ability to adapt to dramatic changes to growing conditions is diminished and with cumulative pressures they become increasingly susceptible to disease, decay and ultimately a spiral of decline.
Management plans must be prepared in advance to plan for the eventual time when removal of an avenue becomes unavoidable. Given the longevity and familiarity of these landscape features there is likely to be an emotional response to wholesale removal of these features if the community are not educated and notified beforehand about the problems associated with sustaining ageing avenues of trees, especially if it is directly associated with their home or neighbourhood.
Options for renewal may include;
Replacing trees only when they die or fail.
There is a likelihood that eventually this strategy will fragment the avenue until it is no longer recognisable due to various age and size classes. It may also lead the public to query what management strategy is in place to extend the life and quality of the avenue.
Remove and replant the trees as an entire group.
This will result in a dramatic change in the established landscape which may be unacceptable to the public. It does permit the re-establishment of a new tree population of uniform species, age and size as well as allowing the site to be prepared in advance for anticipated new landscape use including installing underground services, new carpark and road alignments as well as water sensitive urban design (WSUD) to assist in drainage and passive irrigation of the tree population. Undertaking such works in advance is future proofing the new trees from some of the pressures associated with performing infrastructure works on a haphazard basis once trees are established.
Removal and replanting of every second or third tree.
This strategy diminishes the uniform quality of the avenue and if spacing is inadequate could reduce the success of the new plantings due to competition for resources of available soil water, nutrients and light. This approach does not allow for large scale redesign of the landscape.
Planting a new row of trees adjacent to the existing stands.
With available land being increasingly at a premium it is unlikely that this option would be available in most urban settings. Problems associated with competition for resources could also limit successful re-establishment. Landscape upgrade and renewal may also be difficult to achieve under this option.
Remove and Replant in smaller manageable sections over a specified timeframe.
This option permits elements of the avenue to remain intact while providing an opportunity to successfully re-establish a new avenue of uniform age, size and species at the same time as working progressively towards the goal of long term landscape renewal. It also permits upgrade and renewal of infrastructure such as road widening, carparking, overhead and underground services as well as WSUD for drainage and irrigation. Based on scheduled visual tree assessment methodology, specific groups of trees can be identified based on deficiencies identified in health, safety, useful life expectancy or the need to upgrade infrastructure.
The keys to successful re-establishment of avenue plantings will be dependent on;
- appropriate species selection for the site with consideration to exposure and orientation to prevailing weather and topography,
- provision of adequate spacing for future growth requirements with minimal competition from adjacent established trees,
- good planting site preparation with consideration of future constraints / opportunities
- a scheduled establishment maintenance program that includes watering and formative pruning with scheduled re-inspections for the life of the trees.
One of the key problems for the relevant authorities charged with managing these aging avenues into the future is allocation of resources within budgetary constraints. It is not practical to keep expending resources on retaining trees if there is not a high probability of the tree remaining viable in the medium to long term, especially when the costs associated with maintenance of aging trees grows exponentially the older they become whilst the perceived amenity value of the tree is on the decline.
i.e. Trees that are reaching the mature stage require more intensive tree management which becomes more costly and less effective the longer they are retained.
When there are large numbers of trees within a given avenue then the cost of management is generally much higher than the typical street tree management program where uniformity is not an integral part of the management program. For each significant avenue that is actively managed there are many examples where the relevant authority has, for one reason or another, allowed a lesser avenue to go without appropriate maintenance to reduce costs and drain on their finite resources.