The tree hunter
Claire Masset talks to Dr Henrik Sjöman about the future of urban forests
Self-confessed ‘tree geek’ Dr Henrik Sjöman is on a mission to save our urban forests. “More than half of the trees in many northern European cities are from just two or three species,” he explains. “If they get hit by pests and diseases, we are all in trouble. Helsinki, for example, relies on almost 45% of one species, Tilia europaea. This is stupid and very, very dangerous.”
He believes there is one golden rule when it comes to resilience: diversity. “We don’t know the future, so we need to use as many species with as great a diversity of traits and tolerance as we can. The more diverse your urban forest, the more resilient it is to pests, diseases and changes in climate.”
Sjöman divides his time between working as a researcher and teacher at the Department of Landscape Architecture, Planning and Management at the Swedish University of Agricultural Sciences, and acting as scientific curator for the Gothenburg Botanic Garden. He started his career as a landscaper at Gothenburg University and then joined its Landscape Laboratory as a research assistant in 2002.
As well as undertaking practical work at the university, his role has taken him all over the world, from North to Central America as well as China and Eastern Europe, to study plants in their native habitats. Today, his research is all about capturing how plants compete for resources and space, and how they tolerate different conditions. Once he has studied them in the wild, he brings them home for further evaluation.
“We push the plants in different directions. We monitor how they tolerate drought or flooding and how they do it. We see how far we can push them. We kill a lot of trees – but all this creates guidance and knowledge of where we can use them, where they will perform well.”
With the results of his research, Sjöman works with the nursery industry to increase the range of plants available to landscape gardeners, so they do not rely on the usual ‘safe’ options. As he says, gardeners need to “get out of their comfort zone” and try new things. “Liquidambar was a very exclusive tree 10 years ago, but now you see it everywhere.”
One tree he is keen to popularise is silver lime (Tilia tomentosa). “It’s more drought tolerant than other lime trees. In a dry and hot summer like our last one, our native limes get very stressed and suffered from aphids. Silver limes have hairs under their leaves that deter aphids – and they also will flower for longer, which is great for insects.”
He is keen to also point out, contrary to popular belief, that native species are not always best for increasing diversity. “There’s lots of evidence that the urban environment and diversity can be enhanced by exotic plants. Red oaks, for instance, have acorn crops every fourth year while our native oaks have eight or nine years between a real acorn crop, so birds are better off if you can add red oaks.”
At the moment, he says, the problem is that landscape architects and garden designers do not always see ecology as their number one tool when they start their designs. “The first thing to look at is: what kind of site and ecosystem do I have? And where do I want to move it to? Is this a new-built square? If so, then I would work with a pioneer species.”
Pioneer species, such as poplars, are the tough guys of the plant world. They are quick and easy to establish, their root system penetrates deep into the soil and they can take extreme conditions. In the wild, a pioneer forest produces a dense canopy, which decreases weed competition and creates a unique microclimate inside, ideal for nurturing late succession species, such as magnolias and fir trees, which can then take over.
“Some species that grow inside a forest are difficult to establish in an open situation,” Sjöman explains, “but we force them into these impossible situations. If you put a plant in a situation when it feels at home, it will grow relatively easily, but if you force it into the wrong situation, it’s going to perform really badly. We have a responsibility as designers that our plan will work. In Sweden, if a new plantation fails, 70% of the trees will never be replanted.”
Through his research, lecturing and campaign work, however, he sees that things are slowly changing for the better. “Landscape architects are getting invited onto projects earlier in the planning process. In Gothenburg and Malmö, spending on planting programmes has now increased fivefold. This was partly due to the severe loss of tree stock from Dutch elm disease. It was a minor catastrophe, but people took notice.”
Together with Dr Andrew Hirons, lecturer in arboriculture at Myerscough College in Reading, Sjöman recently launched an online tool that enables professional gardeners to select the right trees for gardens, built landscapes and streets. The guide offers a comprehensive, research-based, decision-making tool on selecting appropriate species for a range of contrasting planting scenarios, with information on more than 280 species.
He is under no illusion, however, that there is a lot more work to be done, particularly when it comes to green networks. “We rarely zoom out and look at how a scheme connects to its wider surroundings, but by doing that you can say ‘In this area, there is a lot of this and this, but this is missing’, so then you can focus on that. If we do that, we can make the whole area really resilient.”
With ecology and biodiversity now much higher on the agenda, knowledge of plant biology and behaviour is a must for designers, he believes. “The new kids on the block are the plantspeople. The more you know about plants, the better a designer you will be.”