Speaking at a Trees & Design Action Group (TDAG) seminar in Birmingham today (15 March), Myerscough College senior arboriculture lecturer Dr Andy Hirons and Gothenburg Botanic Garden scientific curator Dr Henrik Sjöman presented their new guide, Species Selection Guide for Green Infrastructure, the results of two years of Natural Environment Research Council-funded collaboration. It details 280 tree species and will be made freely available online shortly.
"Before you even think about ecosystem services or aesthetic considerations, you need to consider the suitability of the tree, because a dead or underperforming tree won’t achieve those things," said Hirons.
The UK may not have a dry season, but drought tolerance is still an important consideration, he added. "Individual trees can take up 100 litres of water a day, though there is great variation. That can be challenging in an urban environment."
He noted: "There are plenty of drought-tolerant species to choose from, like Pinus nigra or Eucalyptus. But they may have different strategies — they may tolerate water deficit, or avoid it, such as by losing leaves in summer — and by understanding this we can make better choices. You don’t want them shedding leaves all the time."
Hirons and Sjöman have sought to develop an objective measure of drought tolerance, specifically the leaf turgor loss point — "the point after which it won’t recover", Hirons explained. Combining this with a further measure of the tree's capacity to conduct water through the xylem when under stress: "We can begin to make robust recommendations."
Sjöman, who has worked with Hirons on these topics for more than five years, said: "As a landscape architect 20 years ago, I was called in last and asked to fill a space. But that has changed a lot. The science now gives us the evidence for why we plant trees. But it’s crucial that the trees we plant today become the large and healthy specimens of tomorrow, so selection is vital."
As well as the individual circumstances, tree managers need to consider the species diversity of the wider estate, he told the seminar. "More than half of the tree population in many northern European cities is made up of just two or three species. Yet we don’t know which pests and diseases will come and how badly they will hit us, so we have to be more methodical."
Specifiers need to also consider where in the natural plant succession a tree species falls, said Sjöman. "Pioneer species are likely to invest in deep roots because their competitors root at the surface. They are also subject to more wind and temperature variation and so more likely to be drought-tolerant." He gave as an example the Chinese poplar, Populus simonii, which "looks good, with shiny leaves that turn yellow in autumn, and moves nicely in the wind".
He also urged selectors to "go outside your comfort zone", adding: "We all have five or so species that we know work." He pointed out: "Even in parks, trees need aftercare. Some trees invest early in their own development and you may need to water them for three or even five years. They aren’t drought-tolerant until they’re established."
He rejected the argument that a native-only tree policy best promotes biodiversity, pointing to the Chinese lilac tree, Syringa reticulata. "As well as being among our most drought-tolerant trees, [it] flowers and provides nectar in July and August. How many native trees do that?"
Sjöman also suggested that the natural variability in species can be sidestepped by selecting named cultivars, saying: "Acer rubrum has a huge distribution and if that’s all you ask for it could be from anywhere."
On this point, Hirons also cited the popular parkland tree Liriodendron tulipifera "whose range extends from the Canadian border to northern Florida". He added: "Historically, nurseries haven’t been very good at saying where within those ranges their trees are from."
There "are almost too many" additional factors to consider, including the site, the rooting area, biosecurity and the simple availability of trees on the market, said Hirons. "We need to engage more with the UK nursery sector to provide the kind of trees we want, rather than bringing them in from Italy or Hungary. They have to anticipate what we will want in five or 10 years."
TDAG is asking those interested to pre-register to receive the free guide, which is due to be released next month. By doing so, users will also receive future updates for free.