Urban trees need rigorous criteria

Trees should be selected for urban sites using more rigorous criteria to ensure that they survive and so deliver ecosystem services, Dr Henrik Sjoman told last week's Arboricultural Association Amenity Conference at Keele University in Staffordshire.

Sjöman: researcher at the Swedish University of Agricultural Sciences
Sjöman: researcher at the Swedish University of Agricultural Sciences

"Tree selection is key to the whole chain, and heat and drought are the biggest limitations on trees, so you have to look at how they handle these things," said Sjoman, who is lecturer and researcher at the Swedish University of Agricultural Sciences and curator at Gothenburg Botanic Garden. "Ecosystem services are related to tree size and health, and if we don't care about selection, we won't get the benefits."

In collaboration with Nina Bassuk of Cornell University in the USA and Andrew Hirons of Myerscough College "we have been trying to understand trees' background", he said. "They have to compete for resources and tolerate different conditions. In nature, some trees are really smart, while others are quite stupid in this regard - if one plan fails, they have no other."

On drought, he added: "They can avoid it by developing a big, deep root system, but in an urban environment they can't usually do that. Some trees such as birch aren't that clever - they more or less rely on their root system, so they panic and lose leaves, which is not what you want in July."

Similarly, the katsura (Cercidiphyllum japonicum) "is a riverside tree that doesn't have a drought-tolerating strategy". By contrast, Tilia tomentosa (the silver lime) "has leaf hairs that mean it loses less water", while the silver maple (Acer saccharinum) "twists its leaves round to reflect the sunlight so they don't get too warm".

Sjoman's research has included measuring "leaf turgor loss point" as an indicator of drought tolerance. "Under drought stress the (leaf) cell shrinks until eventually the structure will collapse," he explained. "The amount of negative pressure required for this varies, which we can measure and so rank trees from sensitive to tolerant."

So far Sjoman and colleagues have ranked 27 acer species and varieties. "Some at the bottom end should never be planted - they are the ones that typically grow near water in the understorey, such as A. truncatum (Shandong maple)," he said. By contrast, A. tataricum and A. grandidentatum "grow in mountains, where you need a good (drought-tolerating) strategy".

This approach means challenging some cherished ideas about amenity trees. "Local authorities so badly want magnolias as street trees and will spend thousands of kronor on them. They don't die straight away. If they did, we would learn the lesson. We tested nine and they were all at the low end of the scale, which makes sense as in the wild they grow in the best sites. They should never be planted unless the conditions are spot-on."

Sjoman is currently testing "untraditional" trees at Kew Gardens, Ness Botanic Garden and Hilliers to assess their suitability for UK planting. "In the next few years we will have a huge body of data. I hope we'll see it in nursery catalogues. It makes it possible to argue for these trees. We can have a more grown-up conversation."

Dr Andrew Koeser of the University of Florida gave another key reason for ensuring trees in urban environments reach maturity: the carbon footprint incurred in growing, transporting and establishing them. Having conducted life-cycle assessments on US trees, he has calculated that for a tree to "pay back the environmental debt it has accumulated" it has to live for 33 years to reach its "point of carbon neutrality".

"There are few papers on trees' environmental costs. It's the gross benefits of trees that are cited, not net," he said. "There is nursery maintenance, spraying and weeding, mechanical harvesting, wire and burlap, and in the US may be shipped long distances. If they don't make it to establishment, you are doing an ecological disservice."

However, a tree's carbon footprint can be reduced by more than half if a manual approach is taken to transplanting and maintenance, he said, reducing the payback period to 26 years. "Some cities still dig tree holes with a shovel," he added. The results were published in the Journal of Environmental Horticulture this month.

Former USDA Forest Service research forester Professor Francisco Escobedo also urged tree managers to be sure they have buy-in from political masters and the public before investing in i-Tree software-based studies of their tree populations. "Are you using the most convincing information to persuade them?" he asked delegates.

"i-Tree Streets and Eco are mostly focused on carbon sequestration and air quality, but if communities aren't interested in those, you won't get very far with them. They may put more value on shade or aesthetics, or effect on house prices, as we found in a residents' survey in Florida. This sort of data is resource-intensive to gather. Is it answering a question that communities are asking? If not, you have to question the value of the exercise."

Tree choice Henrik Sjoman's recommendations for urban planting

- Acer x zoeschense

"A cross between A. campestre and A. cappadocicum that has the autumn colour of the field maple."

- Tilia tomentosa (silver lime)

"With hairs on the underside of its leaves, its turgor loss point is better than other limes though still pretty low. It also doesn't drop honeydew."

- Eucommia ulmoides (Chinese rubber tree)

"Very tolerant of a warm, dry climate and has no (pest and disease) threats, though it has no flashy flowering or autumn colour."

- Quercus frainetto (Hungarian oak)

"Invest in a big one because small ones take so long to establish."

- Ginkgo biloba

"Its drought tolerance is amazing. People are gaining confidence in it - it has become trendy."

- Koelreuteria paniculata

"It grows on bare rock and can take almost anything. Yellow flowers give late-season interest."

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