Last week's Arboricultural Association amenity conference heard a wide range of international perspectives on its theme of sustainability and the urban forest.
British-born Matthew Wells, who for the past year-and-a-half has looked after the 90,000 trees in wealthy Santa Monica, Los Angeles, said California's ongoing drought has thrown existing challenges into sharper focus. "Homeowners are being told to switch off irrigation. Once plants get stressed they are more susceptible to pests and diseases. It's a problem as 55 per cent of my trees are classed as having moderate water demand."
Meanwhile, 19 out of 20 US cities in a recent survey are losing tree canopy cover, said Wells. "'Risk' is itself a risk. You have residents, lawyers, insurers and consultants saying you need to do something because of risk or you will be to blame, which leads to excessive pruning and removal. That consumes resources that could be spent on planting and is a disincentive to plant."
He listed his own metrics for urban forest management as net tree gain or loss; percentage canopy cover; tree diversity and hence resilience; and public benefit, calculated by the iTree software package, which the city first used in 2001. "The main benefit of trees has been to property values - a value that has since doubled," said Wells.
"Diversity is about age as well as species. London is quite uniform in both these senses. You don't want too many senescent trees as they require more management. Also, the urban forest isn't a botanical garden. I don't need 200 species - maybe 40 that are low-maintenance. The community goes nuts if you plant a load of trees and they die. Are there ones that do well but which we haven't taken advantage of? But we have to do our homework first, even looking at trees that have worked in the past but gone out of fashion."
On the subject of expanding the range of suitable tree species, Myerscough College senior lecturer Dr Andrew Hirons pointed out that urban trees are effectively under drought stress already due to their reduced root capacity and lack of surface permeability. "Trees have avoidance mechanisms such as stomatal closure or leaf phenology, but we want trees to tolerate lack of water, not avoid it. Some are avoiders, some are tolerators."
In a recently published study with Swedish and US colleagues, Hirons assessed two variables - loss of leaf turgor and loss of conductivity - in a range of Acer species. "This throws up species at the negative end of the scale that are underused," he said. "We'd like this information in specifiers' manuals. It's no good having a diverse tree population if they won't survive drought stress."
Like Santa Monica, Oakville in Ontario was also an early adopter of iTree, according to its urban forest supervisor of 30 years John McNeil. "The biggest changes have been in the decade since our first iTree survey in 2005," he said. "Before that we didn't have an inventory, our department had a low profile, yet we had a high-value urban forest facing threats such as emerald ash borer and Asian longhorn beetle."
Here, the study found that trees' greatest contribution was in combating air pollution, valued at C$2.1 million (£1m) a year. The tulip tree (Liriodendron tulipifera) came out as scoring highest in such ecosystem services and "has been very successful for us", he added.
Another benefit was that when the highly destructive emerald ash borer was confirmed in 2008 "we were ready for it", said McNeil. With a £2m budget for this alone, a policy of prioritising deployment of bio-insecticides on the largest specimens "has been successful" in retaining ash trees' share of the canopy of around 10 per cent.
In terms of public perception, reducing the "complicated matrix of criteria" in the city's 30-year urban forest management plan to a single variable of maintaining canopy cover "is a saleable, understandable, visual metric that resonated with the community", he added.
This year Oakville became the first municipality to redo an iTree study, with a further revision planned in 2020. "It is being combined with other iTree studies in the greater Toronto area to show the cumulative benefit of the urban forest for around five million people," McNeil explained.
Session chair Dr Mark Johnston added: "iTree is a means to an end. You need to know what you'll do with the data when you have it."
Paul Barber, a tree health consultant based in Perth, Australia, said: "A proactive approach to managing your tree population will save you money. Get planting right at the start rather than getting people like me in to work miracles later on." He pointed out: "Mycorrhizal fungi can be of great benefit but many are host-specific and need to be sourced from the tree's endemic habitat."