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Collaboration with nurseries delivers effective urban forest strategies for cities

The word ‘forest’ evokes the feeling of being immersed in a richness of nature and, in an urban context, promises the multidimensional experience of trees in a potentially otherwise harsh landscape.

Planting trees is widely seen as a strategy to reduce the negative impacts of climate change in cities, with city authorities worldwide introducing plans and policies to plant millions of trees within and around the city environment. When making a commitment to meaningful change, it is useful to present some form of measurement that can show evidence of progress and success. The term ‘canopy cover’, presented as a % of the total city area, refers to the layer of leaves, branches, and stems of trees that cover the ground when viewed from above. The strength of using canopy cover targets as a commitment to increasing the vegetation in cities is that it voices a simple and powerful ambition. In complex political agendas, this provides a common and comparable narrative. The problem, however, is that canopy targets are a commitment to quantity, with no in-built measure of quality.

‘WE NEED A MORE NUANCED APPROACH’

“We need a more nuanced approach to canopy cover targets set by city authorities,” says Cecil Konijnendijk, professor of Urban Forestry at the University of British Columbia. “Having well stated goals, objectives, and clarity of vision are as important as a numerical target. It is also important to show that decision making is data-driven.” It is unrealistic to suggest a global norm for canopy cover targets, because cities vary so much in their typology and continuum of infrastructural elements – they have histories of nature, culture, community – and these elements all define what level of canopy cover is possible and optimal. Canopy cover targets should also narrate what the canopy really brings to the city, and what is the story behind the number that is pitched as the target. A well-informed and well-debated target has real power, and motivates the set of benefits delivered by an urban forest beyond responding to climate change.

DIVERSITY IS CRITICAL

Making a commitment to a canopy target means nothing without successful tree planting and survival to maturity, and diversity is critical to creating a resilient urban forest and delivering on quality at the same time as meeting the target. Reports on tree inventories around the world are alarming. Only 30 species contribute to the 1.2 million trees in Shanghai. More than three quarters of Barcelona’s trees come from only 3 species. Ideally, no more than 10 per cent of an urban tree population should consist of any one species, no more than 20 per cent of a particular genus, and no more than 30 per cent of any given family.

NURSERIES SHOULD BE BROUGHT IN EARLY ON IN URBAN FOREST DISCUSSIONS

To support the need for a more diverse supply of urban trees, nurseries will need to expand their production range. Some nurseries are taking the lead on expanding their catalogues, looking to tree selection guides that describe the morphological and physiological characteristics of species that define their suitability for and contribution to the urban setting. “Nurseries should be brought in early on in discussions to manage supply and demand dynamics for urban forests,” suggests Cecil. “City authorities, landscape architects, and other disciplines in the green supply chain must collaborate in these initiatives and share the risk of defining, selecting and supplying trees for urban forests.”


Dr Audrey Timm, Technical Advisor, United Kingdom, [email protected]