Tim Johnson, Senior Arborist, City of Mitcham
Street tree trials have been a fundamental part of TREENET since its inception in 1997.Ten years since it was discussed at TREENET’s first symposium, the focus on tree species selection remains, and other factors which impact on the effective functioning of urban trees are also receiving considerable attention. Growing healthy trees in urban streets where conflicts involving space, soil and water are usual is as much about planning for, building and managing urban spaces, as it is about selecting tree species.
Much has been written on tree species selection and these related issues in papers presented at past symposia. Their full content can be viewed or downloaded at treenet.org. In recognition and celebration of the Tenth National Street Tree Symposium this paper reviews some of TREENET’s past works, introduces some current projects, and reports on some of the outstanding works of the last decade which provide a firm foundation for the next.
As TREENET was formed at the Waite Arboretum, it was inevitable that many of the original street tree trials focused on species which had proved themselves there. At TREENET’s Inaugural National Street Tree Symposium, Gardner (2000) introduced a range of the Arboretum’s trees as having potential for use in streets. These included the Wilga (Geijera parviflora) and Chinese pistachio (Pistachia chinensis).
Success Story #1: The Wilga (Geijera parviflora)
My research into the Wilga as a potential street tree species began in 1997 when considering species for use in the City of West Torrens. In reviewing the list of species which the Electricity (Principles of Vegetation Clearance) Regulations 1996 exempt from restrictions with regard to planting in proximity of overhead powerlines, I discounted the shrub species, noted some of the few tree species as having limited application in urban streets, and listed some species with which I was unfamiliar as needing further investigation. The Wilga was one of these species.
Examination of the specimens at the Waite Arboretum showed the species’ potential for street use, its structure appearing suited to appropriate formative pruning. A single specimen at the Adelaide Zoo showed the species had the capacity to reach a large enough size to be reasonably effective in residential streets and with appropriate maintenance could provide the necessary clearances. Several texts presented the Wilga as a hardy but slow growing species. But would they grow in streets?
A search for examples of the Wilga growing in streets in Adelaide during early 1997 proved futile. Colleagues in local government across South Australia were unfamiliar with the species; none could direct me to examples in streets or to a nursery where they were propagated. The substantial number of commercial nurseries which I contacted regarding the species were not familiar with it.
A few months after putting the quest for the Wilga on hold I happened to pass a display at a local government expo and noticed the species’ now familiar narrow foliage amongst some small nursery stock. The stall holder introduced himself as David Lawry. He seemed surprised that I knew the species, and more surprised that I wanted to buy his total stock. He agreed to sell me some trees, although he wanted to keep the majority to distribute to other councils. He wanted to do this to gain an understanding of how the species would performed under varying conditions.
The success of the Wilga under street conditions resulted in regular enquiries and requests for the species. Over time it has been more widely planted across South Australia, New South Wales, Victoria, and possibly even further afield. The fact that the Wilga is now propagated commercially and planted in streets across several states is testament to David Lawry’s vision and his networking abilities which have been instrumental to the success of TREENET.
Success Story #2: Chinese pistachio (Pistachia chinensis)
Following Dr Gardner’s suggestion that the Chinese pistachio may be a suitable species for planting as a street tree, a small number of isolated individual specimens were identified in streets and gardens around Adelaide. Few local government horticulturists were familiar with the species in cultivation at the time but interest in its potential was considerable. Though it was not widely available, small numbers were available as it was occasionally sought by enthusiastic gardeners looking for autumnal colour.
The first trial plantings of Chinese pistachio showed the species could thrive in Adelaide’s street environments. Obtaining stock in quantities remained difficult in the late 1990’s and into the new millennium, with local nurseries reporting difficulties in reliably sourcing or producing quality trees. As with the Wilga and a range of other species to date, TREENET’s initiation and reporting of street trials of Chinese pistachio resulted in increased interest and demand. As a result quality stock is now readily available. Chinese pistachio is proving itself as a reliable street tree across several councils in Adelaide and surrounding regions.
Dwarf apple-myrtle (Angophora hispida) and Japanese Zelkova (Zelkova serrata)
It is likely that production and use of other species which have been established as trials will also increase in coming years. It has been difficult to source stock of Dwarf apple-myrtle (Angophorahispida) in recent years, possibly due to the species’ propensity to shed seed over a very brief period whilst the fruits still hang on the tree. Or it could be due to local production difficulties associated with cold weather and frost. Trial stock at the time of planting has sometimes appeared less than promising, but within a few years the trees planted have developed into attractive specimens.
The current local status of the Dwarf apple-myrtle is reminiscent of the Wilga in 2000. It has the potential to become a staple of semi-arid streetscapes. Discussions with commercial producers suggest that while propagation of Angophora hispida is still relatively low it is beginning to increase, with some minor plantings in recent years in South Australia and Victoria.
The excellent examples of Japanese zelkova (Zelkova serrata) in the Waite Arboretum suggested from the outset that the species had great potential for street planting, but stock quality and the progress of initial trials varied. The structure of Japanese zelkova both above and below the ground appears well suited to streetscape management regimes. Early trials of advanced trees performed poorly while small bare-root stock grew rapidly.
Recent Japanese zelkova trials in the City of Mitcham using 300mm container stock achieved acceptable growth rates. Several examples have survived extreme vandalism. Early maintenance requirements of Japanese zelkova are high, being similar to Celtis and Sophora. Ideally the trial planting of Dwarf apple-myrtle and Japanese zelkova will increase in the next few years so that the value of these species can be more widely assessed and reported.
Information on other tree species trials is reported in the proceedings of earlier symposia; see Watt (2005), Plant and See (2002), See (2003), Hay, Johnson & Kirwan (2002) and Johnson (2000, 2001, 2007). Beyond the snapshot information provided in these reports, information on the majority of established street tree trials is limited. Website “hits” and enquiries reflect the need for relevant information, and the number of trials planted by local councils has expanded over the last decade, but the rate at which information about street tree trials has been uploaded onto TREENET’s website is disappointing.
Some immediate benefits of the initial TREENET street tree trials are apparent:
- commercial production of a greater diversity of tree species
- greater diversity of species and an associated reduction in risk to the urban forest
- increased choices with regard to streetscaping resulting in greater diversity
- greater climate change adaptation potential
Expanding TREENET street tree trials in the future will sustain these community and commercial benefits. TREENET’s major challenges for the next decade include establishing systems to encourage and support more species trials and ensuring they are documented on the website and that their progress is reported.
Establishing and Monitoring TREENET Street Tree Trials
The rationale regarding establishing street tree trials has been detailed at previous symposia; see Watt (2005) and Hay, Johnson and Kirwan (2002), which includes the following practical suggestions with regard to establishing trials:
- Trials should utilise a small number of trees so that if issues or problems arise they will remain manageabl An ideal trial size is between five and ten trees.
- Locate trials such that if issues arise they will have little impact on neighbouring Reserve frontages make ideal trial sites for larger species. Avoid high profile sites.
- Species should not be excluded from trial for fear of potential problems, as problems may not eventuate under local condition
- Knowledge of species gained through experience with the seedling varieties or one selected form cannot be applied to other form One selection may thrive where other selections have failed. Ideally all selections should be tested.
- Trees are typically selected for a given location whereas the reverse may be more appropriate for tree tri Identify the species to be tested first, and then select an appropriate site where success is most likely.
- Learn about the species from all available sources including colleagues and nursery personnel
The methodology described in Plant and See (2002) expands on the practical approach needed to integrate trials into local government planting programs. The involvement of community volunteers in planning and implementing tree trials in Brisbane has considerable potential to benefit trials in other areas.
Monitoring and documenting TREENET trials requires resources. When TREENET began Internet speeds were slow, data storage was expensive, computer access and familiarity with the “information superhighway” was limited. It was anticipated that personnel involved in street tree trials might have difficulty uploading images and other data. Over the past decade technology has improved considerably and has become more affordable, but this increased access and familiarity with information systems has not increased the rate of information upload.
The value of street tree trials is substantially diminished if their progress cannot be readily documented and shared. TREENET must improve the effectiveness of data capture and sharing in relation to street tree trials. Opportunities to improve in this area are currently being investigated. A grant application has recently been lodged with the Local Government of South Australia’s Research and Development Scheme which, if successful, will enable a review of local government’s requirements with regard to tree trials, upgrading of species trial components of the website, and the collection of data on some established trials which are currently not documented. Similar funding opportunities may be available to assist with data collection in other states and territories.
The City of Brisbane’s initiative to involve volunteers in establishing street tree trials may be a key to long term monitoring and provision of data and images. Many Councils own and manage land which is leased to community groups such as Scouts and Guides, Kindergartens and sporting groups. These tenants often request tree planting on or near their facilities. Trees planted could include species trials, which the groups might then monitor on behalf of the council. Establishing and monitoring TREENET trials might also be integrated into school lessons, which could provide additional long-term benefits. For instance a teacher may champion a project and then go on to establish trials at other sites during their career.
Street Tree Trials for the Next Decade
Gardner (2000) introduced over 30 species with potential as street trees. Of these, not less than 12 have so far been included in street tree trials. Over the next decade TREENET plans to further investigate some of those which have not yet been established in trials, including:
- Rough-barked broad-leaved apple-myrtle (Angophora subvelutina)
- Whitewood (Atalaya hemiglauca)
- Large-fruited yellow jacket (Corymbia watsoniana)
- Caley’s ironbark (Eucalyptus caleyi)
- Forest elder (Nuxia floribunda)
- Mt Atlas pistachio (Pistachia atlantica)
- Field oak (Quercus agrifolia)
- Blue oak (Quercus douglasii)
- Englemann oak (Quercus englemannii)
- Vallonea oak (Quercus ithaburensis)
- Valley oak (Quercus lobata)
- Interior live oak (Quercus wislizenii)
- White ironwood (Vepris lanceolata)
- Puriri (Vitex lucens)
Nicolle (2002) listed some recently discovered species with potential for street planting as well as some relatively unknown ones with desirable characteristics. They included:
- Bandalup silver mallet (Eucalyptus purpurata)
- Newdegate mallett (Eucalyptus mimica)
- Cup gum (Eucalyptus cosmophylla, pink-flowering form)
- Quoin Head marlock (Eucalyptus mcquoidii)
- Apricot-flowered mallee Eucalyptus x stoaptera
- Merrit (Eucalyptus urna)
- Victoria Range stringybark (Eucalyptus victoriana)
- Smooth-barked apple (Angophora leiocarpa)
- Coastal brown mallet (Eucalyptus astringens su redacta)
- Wing-fruited mallee (Eucalyptus kingsmillii subsp. alatissima)
It is anticipated that trial plantings of these species will be established in the next decade, though it is acknowledged that some of these species may provide particular challenges in propagation, production and establishment. Anyone with knowledge of the existence of any of these species in streetscapes is encouraged to contact TREENET with this information.
Street Tree Trials Go Underground
Issues with tree root systems, and particularly misconceptions about tree root growth, were instrumental in the formation of TREENET. A species’ suitability for planting in urban streets, in terms of both tree health and impacts on infrastructure and underground utilities, is dependent on its root system. Moore (2002) introduced TREENET to tree root management through soil management. The title of his presentation, ‘Tree Root Networks – A Vital Ingredient of TREENET’ remains as valid today.
Early collaboration between TREENET and the civil engineering profession provided infrastructure to irrigate street trees with stormwater: see Porch, Zanker and Pezzaniti (2003). This preliminary work suggested that street trees might benefit through irrigation with stormwater without compromising infrastructure integrity. Ongoing collaboration with the engineering profession has increased opportunities to improve the sustainability of the urban forest and to avoid or minimize conflicts with infrastructure: see Wettenhall (2006), Argue (2006), Denman (2006), Plant (2002), O’Malley and Cameron (2001).
Prolonged drought, climate change and pollution issues have provided additional incentives to advance this work. A prototype ‘TREENET Inlet’ was demonstrated at the TREENET Symposium in 2008. Over the past year the ‘TREENET Inlet’ has developed into a device to extract stormwater from the street, to detain it temporarily until it can infiltrate soils beneath the road verges. An application for a provisional patent has been lodged to protect the current design and so enable field testing over the coming year.
Civil Engineering and the Urban Forest
The City of Mitcham’s initial investigations of stormwater infiltration infrastructure in 2003 were followed by construction of soakage trenches in association with planting of river red gum (Eucalyptus camaldulensis) saplings in Doncaster Avenue at Colonel Light Gardens: see Johnson (2007). This project continues to function as planned, with no issues or problems observed in the two years since construction. The health and vigour of the young trees is excellent. Surface porosity appears to have remained high, with surface flows visibly diminishing while traversing the infiltration trench sites.
The success of the Doncaster Avenue project has resulted in several new trials involving infiltration trenches and, more recently, permeable brick paving. Council’s support for these works is indicated by the inclusion of a new budget item in the City’s 2009/2010 budget for investigation and construction of water sensitive urban design projects (WSUD) in streets and on reserves. The funding provides for a cautious approach to WSUD, enabling investigation of issues surrounding soil stability and contamination as well as management of stormwater quantity and quality.
Risks associated with working with the limited information available are managed by selecting suitable locations and incorporating ‘fail-safes’ into designs to ensure that stormwater can be diverted to bypass the interception system, or to allow the quantity of stormwater harvested to be reduced if problems arise. Some of Mitcham’s trials have been established on reserve areas and, in the case of street infrastructure, in areas of Colonel Light Gardens where nature strips are relatively broad. Level sites are favoured for stormwater interception trials.
As Doncaster Avenue does not have kerbs and water tables, infiltration was achieved at the road shoulder through the porous soil surface. Achieving a similar result in an urban street with existing stormwater management infrastructure including kerbs and water tables required a different approach. A side entry pit was designed to feed a soakage trench in Dorset Avenue in Colonel Light Gardens, at a site where a single tree had to be removed from a mature avenue of white cedar (Melia azedarach): see Figures 1 and 2.
Voids between the crushed rock in the soakage trench provide storage for approximately two kilolitres of stormwater which quickly infiltrates surrounding soils. When the system’s capacity is reached the pit simply fills to the top and all subsequent flow bypasses it and continues downstream through the pre-existing stormwater system.
The capacity of soil to absorb and store water is a limiting factor in the design of infiltration systems. Trees and other vegetation have the potential to enhance the capacity of such systems with soil stabilization being a secondary benefit. Eamus (2007) describes soil stabilization as just one of a range of poorly understood and poorly acknowledged economic benefits of the ecosystem services provided by vegetation. Eamus, Hatton, Cook and Colvin (2006) provide further detail of the relationship between soil, plant and atmosphere, stating:
“Generally, as the availability of water at a site increases, the leaf area index and rate of water use by vegetation increases, and the vegetation will use almost all of the water that arrives as rainfall”.
Incorporating well vegetated stormwater infiltration infrastructure into streetscape design may therefore be an effective means of managing a considerable portion of all stormwater.
The Potential of Permeable Pavements
With financial support from the Adelaide and Mount Lofty Ranges Natural Resource Management Board and academic supervision by Dr Don Cameron (University of SA) and Dr Greg Moore (University of Melbourne), the City of Mitcham has begun an investigation of the interrelationships between permeable brick paving, soil moisture levels, soil oxygen levels and the resulting growth of street trees including root system development.
Six field trials of two permeable footpath designs were constructed in July and August 2009. They will be monitored and analysed against a control of non-permeable concrete block paving to determine the influence of pavement construction on the establishment and growth of Pyrus calleryana ‘Glen’s form’ Chanticleer. Bores to three metres deep were installed across the field trials to allow regular soil moisture and oxygen monitoring.
It is anticipated that increased rainfall infiltration and higher oxygen levels in the soil beneath permeable pavements may provide for more rapid tree growth and development. The porosity of the base material beneath permeable paving is also expected to desiccate rapidly following rainfall events, thus limiting root development near the paved surface which might result in pavement damage (Randrup et. al 2001). Root growth and development will be assessed following the three year monitoring period through non-injurious excavation of root zones.
The Future: Increased Integration of Urban Forestry, Civil Engineering and Water Sensitive Urban Design.
Evapotranspiration of moisture by trees has the potential to significantly enhance the stormwater management capacity of permeable paving and other water-sensitive design features. ‘Hydraulic lift’ described in Eamus et. al. (2006) may also work in reverse, allowing increased infiltration rates at depth and increasing the speed and volume of soil moisture recharge in areas surrounding water sensitive design features.
Deep rooted perennial native grasses may also be adept at ‘hydraulic shift’; redistributing moisture during the night when evapotranspiration is minimal to equalise water potential throughout the organism. If moisture is in abundance at a point of root contact, given adequate moisture and time, this high moisture potential might by osmosis and entropy be distributed throughout the plant. In situations where time and moisture permit, this moisture might also recharge the soils around drier roots, as described by Eamus et. al. (2006).
In addition to the potential soil moisture benefits of native grasses some species may provide valuable pollution remediation services. Research conducted at Flinders University suggests that the roots of several species of native grasses support bacteria which can contribute to the rhizoremediation of soil- borne hydrocarbons. We await the publication of this research by Prof. Richard Bentham and Sharyn Gaskin with interest, as it may have application in the design of sustainable stormwater management devices which directly support improved urban forestry.
In conclusion, climate change and the predicted progressive drying of southeast Australia present serious challenges and corresponding opportunities for urban forestry. As communities focus more clearly on these challenges we will be presented with opportunities to highlight the benefits of urban trees and the services they deliver. Urban trees will in the future contribute more to reducing greenhouse gas emissions, managing urban climates, purifying and recycling stormwater, managing groundwater and bioremediating pollution. By working closely with research institutions and related professions in delivering these services, TREENET is continuing to help build a sustainable urban forest and a sustainable future. TREENET’s work over the past decade has given it the credibility and respect through which it will influence urban forest development in the years to come. TREENET’s results have been achieved largely by its volunteers. The challenges and results of the past decade were many; those of the next will be greater.
References & Bibliography
- Argue, J., 2006. Going With the Flow: Trees, Habitat and Urban Waterways. TREENET Proceedings of the Seventh National Street Tree Symposium, 7th and 8th September 2006
- Denman, L., 2003. An Investigation of the Potential to Use Street Trees and Designed Soils to Treat Urban Stormwater. TREENET Proceedings of the Fourth National Street Tree Symposium, 4th and 5th September 2003.
- Eamus, D., 2007. Carbon and Water Fluxes of Trees. TREENET Proceedings of the Eighth National Street Tree Symposium, 6th and 7th September 2007
- Eamus, D., Hatton, T., Cook, P., and Colvin, C. (2006) Ecohydrology: Vegetation Function, Water and Resource Management. CSIRO Publishing. 348pp.
- Gardner, J., 2000. TREENET and the Waite Arboretum. TREENET Proceedings of the Inaugural Street Tree Symposium, 7th – 8th September 2000.
- Hay, J., Johnson, T., and Kirwan, L. 2002. Street Tree Trials in the City of West Torrens. TREENET Proceedings of the Third National Street Tree Symposium, 5th and 6th September 2002
- Johnson, T. 2000. Greening the City of West Torrens. TREENET Proceedings of the Inaugural Street Tree Symposium, 7th – 8th September 2000.
- Johnson, T., 2001. Establishing TREENET Trial Sites. TREENET Proceedings of the Second National Street Tree Symposium, 6th and 7th September 2001.
- Johnson, T., 2007. TREENET Trials. TREENET Proceedings of the Eighth National Street Tree Symposium, 6th and 7th September 2007
- Moore, G., 2002. Tree Root Networks: A Vital Ingredient of TREENET. TREENET Proceedings of the Third National Street Tree Symposium, 5th and 6th September 2002
- O’Malley, A., and Cameron, D., 2001. Street Trees Influence on Soil Moisture, Urban Dwellings and Pavements and Walkley Heights as a UniSA-TREENET Research Site. TREENET Proceedings of the Second National Street Tree Symposium, 6th and 7th September 2001
- Plant, L., and See, M. 2002. Street Tree Species Trials in Brisbane. TREENET Proceedings of the Third National Street Tree Symposium, 5th and 6th September 2002.
- Plant, L., 2002. Constructing Root Space for Trees in Australian Cities. TREENET Proceedings of the Third National Street Tree Symposium, 5th and 6th September 2002.
- Porch, S., Zanker, J. and Pezzaniti, D., 2003. Stormwater Harvesting Trials for “Irrigation” of Street Trees and Water Quality and Quantity Improvement. TREENET Proceedings of the Fourth National Street Tree Symposium, 4th and 5th September 2003.
- Randrup T. B., E.G. McPherson and L.R.Costello, 2001. A review of tree root conflicts with sidewalks, curbs, and roads. Urban Ecosystems, 5: 2001, pp 209–225
- See, M. 2003. Street Tree Trials – Avenues of Interest. TREENET Proceedings of the Fourth National Street Tree Symposium, 4th & 5th September 2003.
- Watt, S. 2005. TREENET: New Tree Variety Assessment. TREENET Proceedings of the Sixth National Street Tree Symposium, 1st and 2nd September 2005.
- Wettenhall, G., 2006. Green Streets: Creative Stormwater Design. TREENET Proceedings of the Seventh National Street Tree Symposium, 7th and 8th September 2006.