The value of tree-lined routes and the threats they face were laid out in a Treework Environmental Practice conference at London's Kew Gardens on 25 November. Both highway and rail networks face combined problems of maturing tree stock, high public and governmental expectations and ongoing safety concerns, the gathering of 160 tree and landscape professionals heard.
Highways England senior principal environmental adviser Tony Sangwine said of the agency's 18,000ha soft estate: "It's a valuable national asset but at the moment it's not well managed." Having had an estate of 60 per cent grassland in the 1970s, the former Highways Agency "went for wall-to-wall planting, which was too many", he added.
"We now have a mature estate, with the problems that brings. We have the busiest road network in Europe and traffic management on trunk roads is really expensive, so we are looking at other methods of accessing the trees including way leaves on neighbouring land.
"We are also starting to use arisings for biomass, so generating revenue - we need to do more of this. There was no thinking of conservation until the 1980s. We use native species to reflect what's local and try to link up remnant habitats. We have a lot of sites of special scientific interest and all require some management. It can be delicate work and we aren't always very good at it."
From an amenity point of view, Sangwine said: "Integrating the road and the landscape is bloody difficult to do. You don't want a lot of planting in somewhere like East Anglia. But we have an opportunity to do more in peri-urban areas, such as avenues."
On "green" (vegetated) bridges, he added: "Mainland Europe is way ahead of us. There are more than 200 in France alone. We should be doing much more." Under the Government's recent five-year road investment strategy (RIS1), "these will double up as farm access roads to make them more cost-effective," he said. "But there will be none on a landscape scale, more's the pity."
The rail network faces many similar challenges, according to Network Rail environment manager Dr Neil Strong, who admitted: "Thirty per cent of our trees are probably too high if you are passing at 125mph (200kph)."
With around 40,000ha to manage - an area greater than the Isle of Wight - the network has 2.5 million trees above 6in (15cm) in diameter, he said, adding that of these the disease-prone ash is the most common in England at around 20 per cent. "There is a biosecurity issue there that we will have to manage safely."
In the stormy 2013-14 season 1,800 trees ended up on tracks, "around one-third of which we knew about because trains hit them," he added, though no train has been derailed by a tree since 2007. Yet the company is conscious of the social and environmental value of its trees, said Strong. "We have three times the national rate of tree cover and one-third of the UK population live within 500m of the network, so we have an impact."
Middlesex University professor of risk management David Ball said removing roadside trees on safety grounds is likely to lead to unintended consequences. "What's missing from the conventional risk-assessment process is any consideration of side-effects," he pointed out.
He explained the difference of hazard-based and risk-based approaches with the example of cycle helmets: "The benefits of wearing them are too modest to capture because cyclists and those around them change their behaviour as a result. The evidence shows this goes for roadside trees too. They are even being planted in Norfolk to reduce drivers' speed." Statutory road safety audits (RSAs) "only consider road safety matters" and "are an example of non-compensatory decision making - they don't consider all aspects in the round", said Ball.
Institute of Chartered Foresters development director Russell Horsey said of the Design Manual for Roads & Bridges, of which the RSA forms a part: "It's fine for highways but not for residential streets. You should demand evidence so its guidance is required." He added that Bristol, where he was previously senior arboricultural officer, "has a policy for build-outs (to calm traffic by restricting flow) to 'include a tree', which reduces damage".
Continent - European view on tree avenue protection and regulation
Katharina Brückmann, who leads Friends of the Earth Germany’s Tree & Avenue Protection project, said Germany’s trees have been hit by a federal regulation stipulating a 7.5m planting distance from the road for trees, "which isn’t an avenue any more, nor is it feasible as the road authorities don’t own that much land".
Meanwhile, at €30-€50 per metre, installing crash barriers "doubles the cost" of avenue planting while making maintenance such as verge mowing more difficult. "Planting will fall apart if there’s no allowance for trees," she said. "The biggest killers are alcohol, speed and distraction, so there are other more productive ways of reducing fatalities."
Nine of Germany’s 16 Länder (federal states) now give protection to existing avenues, she added. "If you remove an avenue tree you have to pay €400 into an avenue tree fund. Now for each tree felled, 1.7 are planted. But gaps in older avenues are getting bigger."
France faces similar regulatory pressures on its avenues, according to expert to the Council of Europe Chantal Pradines. "Since the 1960s trees in France are considered solely as obstacles," she said. "The dominant concept is a ‘forgiving road’ that gives the driver a margin of error. But the road should also prevent error by giving a perception of speed and showing where upcoming bends are. Several studies actually show that tree-lined streets are safer."
Kamil Witkos-Gnach, project manager for Polish non-government organisation Roads for Nature, pointed out some ways to make tree avenues earn their place. "Cherry tree avenues are part of the landscape in parts of Poland and the Czech Republic — the farmer who leases them also maintains them," he said. Beside such "avenue cropping", authorities in Poland, Germany and Spain have promoted their avenue networks as tourist attractions, he added.