Whatever benefits trees bring to towns and cities, large trees clearly provide them to a greater extent, but how much greater? Three new reports from Forest Research* aim to help tree managers understand how much, so enabling them to better make the case for planting and retaining large trees.
Head of the agency’s Urban Forest Research Group Dr Kieron Doick, who co-authored the reports, tells Horticulture Week: "Tree professionals ‘know’ that large-stature trees give the most benefit, but intuition and gut feeling aren’t enough — we need to put numbers behind it. Now that the iTree Eco survey tool has been in wide use for many years, we have a huge data bank on trees, and that resource gives us a lot of opportunities."
Drawing on 10 UK i-Tree Eco surveys from the past decade, Doick and colleagues modelled nearly 9,000 trees, of which more than one-third were classed as large species – that is, growing naturally to higher than 12m.
For tree managers, maximising benefits is a question of understanding age profiles of tree populations, he says. "How many are getting to maturity or over-maturity? And how do the benefits stack up over the tree’s life? What comes across clearly is the need to sustain trees to a good age, so you maximise the return on your initial investment in buying, siting and pruning them."
Doick and colleagues also drew on two dozen UK local authority tree strategies, to see to what extent they factored this in. "Some are based on service delivery and favour large-stature trees or insist on large-for-large replacements, and our research will give power to those writing such strategies," he says.
Preference for large trees was one of four common themes in tree selection policies in strategies, the others being preference for native species, diversity for forest resilience and ensuring the right tree is planted in the right place.
Earlier this year the Government consulted on possible new national standards to guide such strategies. Doick says: "Strategies need to respond to local needs and pressures. Our reports indicate some of the key areas that such national guidelines might feature, and that strategies might consider in responding to local requirements."
The total carbon stored by a tree increases largely in proportion to trunk diameter. But trees do not keep growing at the same rate forever and their rate of carbon sequestration declines once they reach about 1m in trunk diameter, according to the research. This tail-off is sharper for medium-size trees, while small trees never reach that size.
Similarly, rainfall and air pollution interception increases with the greater tree canopy size and total leaf area that larger trees offer. But size is not everything. The Scots pine, for example, "ranks below many medium-stature species" for these four ecosystem services, the researchers found. By contrast, the yew emerged as by some distance the highest ranking medium-sized tree for these services, while other medium-sized trees the alder, hazel and downy birch turn out to be relatively poor at run-off prevention and pollution removal.
"The difference in ecosystem services delivery between the first and last ranked species was considerable," the report notes. "As trees matured, oak species provided more than 70 times more carbon storage compared with plum species, and annually sequestered tenfold more carbon compared with elder. London plane delivered more than 20 times more avoided run-off and air pollution removal than elder."
But tree selection should not solely be guided by these criteria, he adds. "There’s a broad range of criteria to consider including climate change and pests and diseases."
The benefits they provide also depend on the subsequent care they are given. "Reducing mortality rates by only a few percent can significantly improve long-term total ecosystem services delivery and need not be expensive," the report points out.
More mature trees in poor condition, meanwhile, "provide lower ecosystem services delivery", because poorer growth slows carbon sequestration, and may also lead to canopy dieback, reducing capacity to intercept precipitation and airborne pollutants, it adds.
Most significant though is the threat of existing trees being removed altogether, of which "large and mature trees appear to be particularly at risk", due to health and safety concerns, subsidence claims, development pressures, services installation and demand for improved access, it states, concerns that have already led to "significant losses of large and mature trees in certain urban areas".
To address this, it concludes: "Capturing and reporting the benefits that urban trees provide to society has been shown to be a powerful method to justify improved tree management. Cost-benefit analyses have also demonstrated significantly greater benefits over the lifetime of a tree from planting large rather than small-stature trees."
It also notes: "Consideration of tree benefits that are not easily quantified or valued, such as cultural ecosystem services, lend further justification for management that aims to maximise tree longevity, size and health."
* Understanding the role of urban tree management on ecosystem services, Ecosystem services delivery by large stature urban trees and Ecosystem services delivery by small and medium stature urban trees, all published by Forest Research in June 2019 and available for free download.