To maintain a healthy, vibrant and sustainable urban tree population, there is an understanding that communities should plant a variety of tree species. There is a history of events where a major pest or disease has devastated a population of urban trees (primarily in the Northern Hemisphere), which should make the requirement to state the importance of diversity as redundant. Tree diversity within an urban forest landscape provides functional, aesthetic benefits as well as biological and ecological advantages. “A common tenet of popular ecology is that high species diversity contributes to the stability of ecosystems by reducing hazards of catastrophic loss of a particular species” (Richards, 1983). However, there is a significant amount of evidence from plant ecological studies that relationships between diversity and stability cannot be as simply expressed as this proposition suggests.
As Clark, Matheny, Cross and Wake (1997) pointed out we know that urban forests act to provide a multitude of functions to communities, but we do not yet know how to design those forests to maximise that function. Clark et al. (1997) go on to indicate that the main criteria of urban forest sustainability are canopy cover, age distribution, species mix and the preservation of native vegetation.
Achieving an appropriate diversity of tree species is one important factor in achieving a sustainable urban forest. However, there should be a suite of management tools utilised to manage and sustain a healthy, vibrant urban forest that responds to a community’s requirements.
Street tree species do not occur as a monoculture to the extent found in agricultural crops or forest plantations; nor are monocultures suitable over the range of street conditions encountered over a given municipality. Most urban tree populations around the world, however, are dominated by relatively few species that have predominately proven adaptable and useful under fairly austere conditions.
There are guidelines or rules that aim to set target levels for taxon diversity within a street tree population. Santamour (1990) suggests that there should be no more than 30% of any one family, 20% of any one genus, or 10% of one species in an urban tree population.
This rule was predicated on the significant losses of elm to Dutch Elm Disease (DED) during the 1950’s and 60’s, however the implementation of contemporary approaches to pest and disease management, sanitation cutting and appropriate plant spacing, would have reduced the losses and impact on the landscape (Richards, 1983). In a study of Greater Melbourne’s street trees (Frank, Waters, Beer & May, 2006), the Myrtaceae Family comprised 52% of the total plants (street trees across Greater Melbourne). This significantly disagrees with the 10-20-30 rule. Myrtaceae is one of the most characteristic Australian families, providing the dominant taxa in most Australian vegetation communities except for treeless areas, rainforests and those communities of arid and semi-arid regions; dominated instead by Acacia (Mimosaceae). The high percentage of this family in streets could be seen as being analogous to natural Australian plant communities, especially in the southern part of the continent; consequently, it may not be a point for concern.
Miller and Miller (1991) recommended that “liberal use” of a species should not exceed 10%. Jaenson et al. (1992) suggested that tree managers should use species percentages derived from rapid, sample surveys to “reassess their recommended species lists to achieve a 5%-10% ceiling on any one tree species”. These simple numerical limits have no scientific basis. “Street tree diversity should relate to the range of conditions and objectives in a community rather than to simple numerical standards” (Richards, 1993). Regardless of percentage, a species might be considered overused if it is often planted where other proven species are likely to be better suited (Richards, 1993).
It should not be considered a criticism to use a proven species for critical or high profile planting sites; particularly where certain species form iconic avenues within a city. A logical process is to use proven species where they are believed to provide the most benefit and to encourage suitable alternatives elsewhere. This would result in a reduced planting rate for the more common species, but its relatively high success rate will maintain its prominence in the population (Richards, 1993).
A level of species diversity will evolve as a dynamic balance based on the continuing removal and replacement of trees based on natural tree attrition, as well as changing social, aesthetic, design, environmental and economic factors. Species diversity should also be related to the diversity of site conditions and functional requirements on a community’s streets (Richards, 1983). This dynamic process will result in a diversity of proven species that meet design objectives and match existing conditions rather than diversity being a product of a predetermined species richness standard or diversification formula (Simons and Johnson, 2008).
Another facet of street tree diversity lies in the available planting sites throughout the city. Improving planting sites that can support more species can increase species diversity. It may also include the development of new planting sites that allow planting of different species within an established avenue or landscape that could provide a highlight, such as roundabouts, medians, or curb outstands, while also providing diversity within the stand.
It is important that the managing authority of public trees stipulate the diversity objectives or targets that would be more applicable (specific) to that area. The potential risks presented, perceived or real, should be clarified as being chronic or acute with appropriate contemporary management techniques described to control them. History suggests that it is impossible to predict when the next devastating pathogen will strike (Gilman, 1997). Developing a diverse species population does provide some risk mitigation, however the concept of species diversity should not be idealized above issues of species prominence, urban adaptability, canopy coverage and management resource allocation.
Richards (1983) goes further to state,
“The stability of natural tree populations depends on the long-term success, or longevity, of individuals and also on the success of species reproduction. In street tree populations, stability depends primarily on the longevity of individual trees and sufficient numbers of successful planted replacements”, also, “The few species predominating in the older population have proven their adaptation, and thus are logical species to choose for replacement plantings wherever they are appropriate”.
Richards (1983) also suggests that there is little ecological basis in the premise that replacing the proven adapted species with a high diversity of other species can enhance the future stability of a street tree population.
Risks of catastrophic disruption of the population by disease or insects, often unduly attributed to species, are best reduced directly by appropriate site selection, good planting and establishment maintenance, and good tree and pest management techniques to mitigate damage or stress to trees (Richards, 1983).
This should not preclude the continued research into what additional tree species or cultivated varieties could be suitable for a given area, because at least a moderate diversity of species can provide greater benefits over the diverse street tree situations in a community. A conscious effort should always be made to prevent an over-dependence on a few tree species.
Most additional species are unlikely to prove as widely adapted to the range of physical and climatic conditions as the existing older trees. Richards (1983) states that the most logical candidates for a moderate increase in the number of tree species in a population are the less common species among the older trees in the population that are performing well and have proven relatively adapted to a city’s street tree conditions. There is also an appropriate, but probably limited, place for short-lived species that have proven useful in particular street tree planting situations, such as commercial areas. Researched species or new cultivated varieties that on the surface would appear to be factor matched to a particularly area, but are as yet unproven should be treated as tests, or used in low profile areas to ascertain their suitability before broadly introducing them into the main tree population.
Certainly climate change requires some consideration in developing a sustainable urban forest. Climate model projections for the coming decades indicate an increasing risk of below average rainfall for southern and eastern mainland Australia, higher temperatures and evaporation, and below average runoff. In particular there is a significant projected increase in frequency of extremely hot years and extremely dry years. The selection of species more suited to extended dry periods would be beneficial.
Another focus of more recent urban forest renewal programs seems to be the development of age diversity rather than species diversity. Trees ultimately die, and their expected longevity can be estimated from experience for any given species and growth situation.
Good age diversity is essential for future population stability. Richards (1983) states that inadequate replacement of the dominant species that are proven adapted in the older age classes is a more certain threat to future stability than is low species diversity among the older trees. Dominance of the major species in mature age classes guarantees a destabilising effect on the population when many of them begin to decline and die in the near future.
According to Richards (1983), if emphasis on species diversity among replacements causes inadequate replacement to maintain proven adapted species; it is a misuse of ecological concepts. Species diversity contributes to the stability of a street tree population only to the extent that individual species or cultivars prove successful.
Diminishing budgets demands efficiencies in tree management. Successful urban tree plantings requires, carefully selected, sited and maintained trees to assure a high success rate. Even with good management, the probability of some uncontrollable losses requires a replacement rate that taxes most street tree management programs. Misdirected emphasis on species diversity by using untried species in replacement plantings can add further stress by reducing replacement success over the long run. Urban tree populations require human intervention in order to sustain them. The maintenance of stability in natural forest stands by large numbers of replacement trees, compensating for high mortality, is not an option for street tree populations.
Diversity of age provides a greater ability to normalise budgetary requirements or to help regulate annual expenditure. In human terms it would be similar to an aging population being supported by less and less young in the workforce over time. By maintaining a mixture of age classes, tree removal and replacement programs become a more evenly paced process. Extremes, such as those associated with sudden mass senescence, are minimised, allowing for budgets to be more easily managed and regulated.
Richards (1983) suggests that a good age distribution for population stability would be about 40% trees under 20cm diameter, 30% 20 to 40cm trees in the early functional stage, 20% 40 to 60cm functionally mature trees, and 10% older trees with most of their functional life behind them.
A healthy mixture of young, medium, and old trees provides a nearly constant turnover of generations over time as new trees replace the old. In addition, trees of different sizes provide more complex habitat for wildlife and can support a greater number of species.
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Frank, S., Waters, G., Beer, R. & May, P. (2006). An analysis of the street tree population of Greater Melbourne at the beginning of the 21st century. Arboriculture & Urban Forestry 32(4) July 2006.
Gilman, E. F. (1997) Trees for urban and suburban landscapes. Delmar Jaenson, R., Bassuk, N., Schwager, S., and Headley, D. (1992). A statistical method for the accurate and rapid sampling of urban street tree populations. Journal of Arboriculture 18: 171-183
Miller, R.H. and Miller, R.W. (1991). Planting survival of selected street tree taxa. Journal of Arboriculture 17:185-191
Richards, N.A., (1983). Diversity and stability in a street tree population. Urban Ecology. 7: 159. 171.
Richards, N.A., (1993). Reasonable guidelines for street tree diversity. Journal of Arboriculture 19(6). 344-350.
Santamour, Jr. F. S. (1990). Trees for Urban Planting: Diversity, Uniformity, and Common Sense. Proceedings of the Seventh Conference of the Metropolitan Tree Improvement Alliance
Simons, K. and Johnson, G. R (2008). The road to a thoughtful street tree master plan. A practical guide to systematic planning and design. Minnesota Local Road Research Board U.S.D.A. Forest Service – Northeastern Area.