May..A blog by Andy Hirons, Building resilience into the urban forest

One of the most important strategic issues facing those managing our urban forest is how to build resilience within the urban tree population. The issues at play include factors relating to biosecurity, a changing climate and urban planning. As a result, safeguarding the future of the urban forest is a highly complex task.

In this month’s blog, I have identified some general principles that I think are useful for informing strategies to build resilience into our urban forest. It is based on a small contribution I made to the work the Arboricultural Association’s upcoming guidance on the ‘Application of Biosecurity at Work’, please look out for the full guidance over the next few months.

Predictions for climate change suggest that conditions for tree growth are likely to become even more challenging. Trees under stress are less able to perform key ecosystem services and are more vulnerable to pests and pathogens. Therefore, the first principle of building a sustainable and resilient urban forest should be to promote the health and vitality of existing urban trees.

There is general agreement that increasing species diversity within the urban tree population will increase the future resilience of the urban forest. This is based on the assumption that the greater the range of species, the more likely it is that the health of fewer trees will be compromised by any single threat. Urban forests, or sectors of the urban forest, become more vulnerable if they are comprised of only a few dominant species: a significant climatic event, pest or pathogen outbreak may make it necessary to remove a high percentage of the urban tree canopy. Therefore, the second principle for building resilience into the urban forest should be the strategic diversification of the urban tree population.

The term strategic is particularly relevant because simply increasing the range of species planted within our urban areas is not sufficient to build resilience within the tree population. There are many species that will never be appropriate to plant in streets or courtyards; they do not have the traits or strategies required to cope with the conditions often associated with these locations. A diverse urban forest has little value if it leads to high mortality rates during tree establishment. Diversification should be strategic in that it should expand the range of species used from a species pool that has long-term growth potential.

Studies (e.g. Trees in Towns II and iTree projects) on the diversity of current urban trees can justifiably present urban forests as diverse according to widely accepted indices of biodiversity. However, these studies also indicate that whilst a broad range of species may be found in urban landscapes, a narrow range of dominant large-crowned species provide most of the ecosystem services. Consequently, threats to a relatively few species disproportionately compromises the value of the urban forest as a whole. Indeed at the local scale (e.g. street or park), the potential loss of a single species may remove the vast majority of tree cover in that location. Since the provision of ecosystem services scales with tree size, diversification should be strategic so sites capable of supporting large specimens also support a diverse range of large species.

Prescriptive quotas for diversity are not always helpful as they often fail to adequately consider scale. It is the diversity at the landscape scale that is most important for the building resilience in our urban forests. Consequently, it is not necessary for every planting project to have the widest variety of species possible. There are legitimate elements of urban landscape design that, for aesthetic effect, are more appropriately achieved with a narrow range of species. It may be that significant site constraints also limit the species pool from which to select. However, where design intentions or site constraints do not limit the range of species used, new planting should be diverse. Indeed, the future resilience of a planting scheme is a meritorious design goal in itself. Vulnerabilities to the urban forest come from plantings with a narrow range of species multiplied across a landscape. Diversification should be strategic so that the governance of tree populations takes place at the largest scale possible.

Where a diverse urban forest has been identified it is vital that complacency is not allowed to supress the pursuit of species diversity. Those responsible for urban tree populations must be forward looking. Whilst diversity within the tree population should a fundamental objective in urban forest management, care should be taken to anticipate future threats. All diversity is equal, but some diversity is more equal than others. It is possible that a seemingly diverse urban forest is more vulnerable to polyphagous insects than a less diverse urban forest made up of less susceptible species. For example, Asian longhorn beetle is a threat to many key genera used in UK urban landscapes (e.g. Acer, Aesculus, Alnus, Betula, Carpinus, Corylus, Fagus, Fraxinus, Platanus, Populus, Prunus, Salix and Ulmus) so it is possible to have a fairly diverse urban tree population that is still threatened by certain scenarios. Diversification should be strategic so that future plantings are designed to extend species diversity beyond the known hosts of significant biotic threats.

When selecting trees for new planting sites it is important to remember that trees have no foresight. They cannot anticipate the likely future stresses inherent to the planting location and they cannot anticipate the potential threat from pests and pathogens. As a result, consideration of such matters must be the responsibility of those tasked with selecting trees. Tree selection is, therefore, of strategic importance to building a resilient urban forest and warrants attention within continued professional development schemes.

Finally, the strategic diversification of urban forests must be achieved in a sustainable way that does not compromise the biosecurity of the urban forest by irresponsibly importing trees directly to site. Those responsible to planting trees in urban environments must follow good practice with regards to the procurement of trees so that the new pests and pathogens are not introduced into our urban forests as a consequence of well-intended goals for species diversity.

The maidenhair tree Ginkgo biloba is a good example of a species that could be used to help build the resilience of our urban forest. It is hardy, drought tolerant and free from major pests and diseases. It also has great autumn colour.