A recently planted group of Styphnolobium japonicum in a town square in eastern France.
Keith Sacre, Barcham Trees
Diversity within tree populations is now a recognised and well-documented goal of urban forest management.
Styphnolobium in full flower in Prague.
The most often quoted guidance is that of Frank Santamour which suggests that no urban tree population should consist of no more than 10% of any given species, no more than 20% of any given genus and no more than 30% of any given family. This guidance dates back to 1990.
Other guidance is available. Miller and Miller suggested in 1991 that no proven species should exceed more than 10% of any population; Moll in 1989 suggested that no species should be more than 5% of a city’s population and no genus more than 10%; Grey and Denbe in 1986 suggested that one species should not amount to more than 10-15% of the total population; and Barber in 1975 suggested that communities should establish maximum population densities for each species as a percentage of the entire street tree population and no more than 5% should be any one species used. More recently, in 2020 the Nature Based Solutions Institute from Barcelona suggested another form of guidance which relates trees to the urban population they serve. This is the 3-30-300 rule which suggests that every person should be able to see 3 trees from their window, there should be 30% canopy cover and the distance from any home to the nearest park should be 300 metres or less.
The benefits of diversity in creating and maintaining population resilience and the vulnerability of populations dependent on one or two species have been widely accepted by tree managers and landowners alike, yet from a nursery perspective the species range selected by clients – whether tree managers in the public sector, landscape architects, garden designers or other professionals involved with tree selection – remains relatively small.
There are of course notable exceptions, such as Rupert Bentley Walls and his work in the London Borough of Hackney, and Oliver Stutter and his work with colleagues in the London Borough of Southwark. I am sure there are others, but I have had the benefit of walking their areas with them recently so they are fresh in my memory. I would love to visit more areas where diversity is a key objective and am open to invitations.
Reticence
Why is this reticence to embrace diversity so apparently entrenched? There are probably many reasons, and I cannot guarantee that I will cover them all here.
In my experience there is a tendency for those in the public sector to have a preference for known species which are successful in their own locations, and they work with a limited palette. This is perfectly understandable as they are working with public money and are accountable for the successes they achieve, so the incentive to take risks is missing.
There is also a general lack of knowledge about tree species and the number of alternatives which are available. This is particularly true among landscape architects and many garden designers who work with a limited palette of species, more often than not related purely to aesthetics and compatibility with a design brief focused on the hard landscape. This is coupled often with a woeful knowledge of trees in general, an impression that has been reinforced over the years at many CPD sessions I have conducted at the offices of landscape architects and garden designers.
Then nurseries have a role, and they often limit their range to those trees which will sell, with a reluctance to gamble and widen their offer to include less popular, more difficult to grow species. It is also true that certain species seem to gather a negative mythological reputation which carries and spreads to potential users, deterring them from experimenting and trying that new species.
There are certainly more reasons than those outlined here, but it is interesting to consider one species and examine in an unscientific way the reasons why, although it is used extensively throughout Europe, barriers appear to be present which limit its use in the UK. The species in question is Styphnolobium japonicum (previously known as Sophora japonica), the Japanese pagoda tree.
Mature Styphnolobium japonicum showing autumn colour in Dresden, Germany.
Styphnolobium newly planted autumn 2021, pruned at planting.
The case of Styphnolobium japonicum
I have seen this tree growing very successfully across Europe including in major cities such as Berlin, Paris, Prague, Dresden and León, often as a street tree (photographs included here).
The tree, which is a native of China and not Japan, was first introduced into England in 1753. At Kew gardens in London there is a wonderful example of the species’ longevity. It was planted in 1753 by the famous nursery man James Gordon. The tree was one of the first of the species planted in this country and is now classified as one of Kew’s ‘Old Lions’ Styphnolobium japonicum becomes a significant tree, reaching some 25 metres in ideal conditions. It is usually upright and spreading with a broadly rounded crown at maturity. It is a relatively fast grower with a crown which produces light shade not dissimilar to that generated by ash. It is a member of the pea and bean family (Leguminosae), and must be one of its most beautiful representatives.
When the tree is in full flower, it is spectacular, with the entire canopy covered with creamy-white flowers creating a glazed, almost haunting effect. These flowers do not fade on the tree but drop off quite fresh, creating a mirror image between ground and canopy. This effect is quite stunning.
Styphnolobium does not flower until it has reached 30–40 years of age, with blossoms developing in September – and in cold wet summers they do not develop at all. I have seen the tree in Grange Gardens, Lewes (East Sussex) in full flower and it was a joy. As that tree ages and some of the forecasted impacts of climate change become reality, it will, perhaps, flower annually.
As with all tree species, there are elements of folklore associated with Styphnolobium. It is infamous in Chinese culture because demons are said to have been drawn to it. Even more sinister is the story that the last Ming emperor, the Chongzhen emperor Zhu Youjian, hanged himself from a pagoda tree after peasants stormed the Forbidden City in 1644. It is also claimed that well water into which autumn leaves fall becomes a laxative, and it is said that turners of the fresh green wood could be immediately affected by colic.
There is also a great deal of modern negative association attached to the species. It is said to produce significant amounts of dead wood in the canopy which is a potential risk. It is said to have a very demanding and greedy root system which, if true, would be a serious disadvantage in the urban environment. It is said to die back significantly both on the nursery and subsequently after planting, and indeed the young shoots can die back and look unsightly; and it is also the case that in its early development the crown can appear scruffy and untidy.
How much of the negativity is based in truth and how much is mythology passed by word of mouth? Can all those European cities where the tree is successfully used be wrong? I don’t know all the answers but would offer a couple of thoughts. The tree’s canopy needs to be developed slowly, which involves pruning both on the nursery and subsequently after planting. In my experience, allowing the leader and lateral branches to develop three to four buds at a time proves successful not only in reducing die back to almost zero but also in shaping and developing the crown in a way which removes the potential for future die back. Any concerns about the demands of the root system can be avoided by careful site selection; I am not suggesting that the species can be planted everywhere.
Open-minded approach
What the above does call into question is the failure to understand that the nursery tree when planted in the landscape is not the finished article. The nursery is a stage in the development of crown structure appropriate for the urban environment. Formative and structural pruning needs to be continued for four to five years after planting, dependent on the species being planted. It has to be recognised that all tree species are different and require modified and subtle variations in their maintenance and management after planting. This is obviously relevant to species choice for any site.
If true diversity in the urban environment is to be achieved then there needs to be a wider knowledge of tree species, both in their variety and individual requirements, coupled with a detailed understanding of the existing tree population and a quantification of what is there now. There also needs to be a more open-minded approach and a willingness to experiment, casting aside some of the mythologies which surround some of the tree species not being used at the moment.
Further information about Styphnolobium japonicum is available in The Plantsman’s Choice article by Dr Henrik Sjöman and Dr Andy Hirons in issue 182 of the ARB Magazine (autumn 2018).
This article was taken from Issue 198 Autumn 2022 of the ARB Magazine, which is available to view free to members by simply logging in to the website and viewing your profile area.