Streets are "the best and worst places to plant a tree", Professor Peter Duincker of Dalhousie University, Canada, told the Institute of Chartered Foresters' Trees, People & The Built Environment 3 conference in Birmingham last week.
Having calibrated the "benefits and disamenities" of trees in a range of locations, his team found "they are both greatest on streets", where they are costliest to maintain but also most useful across 19 different social and environmental indices.
He also urged closer planting of street trees, saying: "What matters is how many leaves are in the sky, so more trees means the diversity of services they provide comes more quickly, even if the final canopy cover is the same."
Fellow Canadian Ian Buchanan, head of natural heritage and forestry for the municipality of York, Ontario, said of his bid to raise street trees higher up the local policy agenda: "The language of 'green infrastructure' is feeding into policy at all levels. We are investing in creating green streets," including in the region's £2.2bn integrated transport policy. "You need to make the business case. But it's a bloodbath at every stage."
Noting an 84% survival rate, Buchanan said: "We have standards on tree planting and won't budge on them. It's OK to say no to a tree if the site isn't suitable. Your street tree performance is your credibility and I want 100 years out of mine. We used to reject 60% of contractors' planting. There's no substitute for going out and touching them, especially in the early years." He admitted a "dog's breakfast" of tree planting technologies has been employed in the region - "though they all met the spec".
Having surveyed 26 recent studies of urban tree soils and planting techniques, US-based urban tree planting specialist James Urban said: "We need an awful lot more research on this. The whole area, particularly the biodiversity of urban soils, is still wide open." But soil volume is just one of the things "you need to get right" when planting street trees. The others are stock quality, room for trunk and canopy growth, water in and water out, he explained.
University of Birmingham reader in infrastructure monitoring Dr Nicole Metje said that while utilities under streets have traditionally been managed by open trenching "on a first come, first served basis", there are "smarter" techniques such as multi-utility ducts. Perth & Kinross Council principal engineer John Thomson said this approach means "you don't have to dig the road up each time, which compromises its lifespan - you just slide new (cables and pipes) through". He added: "They can go in deeper and can dramatically reduce street tree related concerns."
Having looked at data on 300,000 "single-vehicle" accidents, Florida attorney Jeffery van Treese found the percentage involving trees is similar in urban and rural areas, at around 15%, and they occur most commonly at night but in illuminated areas. Having graded each by severity from light injury to death, those involving trees, along with rollovers, came out as "most severe", he said.
However, roadside trees can aid safer driving, according to West 8 Urban Design & Landscape Architecture project manager Maarten Buijs, who cited the iconic avenues of the otherwise largely treeless Dutch polders. He said: "As well as giving depth and identity to the landscape, trees can guide the driver round bends, warn of upcoming junctions and screen off other drivers."
Trees - Helping reduce car reliance
Giving a wider view of how trees could fit into less car-reliant streetscapes in future, University of Birmingham professor of transport, energy and environment Miles Tight explained that his Visions 2030 project shows that not only are simulations of more integrated, pedestrianand cycle-friendly urban designs popular with the public, aspects of this have already been successfully implemented in some European cities.
Already more than 60% of journeys within Munster in western Germany are on foot or by bicycle, while the city's inner ring road is a car-free tree-lined promenade within which private car use is also banned, although public transport use "is still quite low", said Tight. He called for "a step change rather than unfocused, incremental change" in UK urban planning and design, but this should be "bottom-up" with the public "pushing for it". He added: "You need to have the discussion before you can bring about change."