Green cities: Why more variety in tree species and better horticultural knowledge are essential

IF nature-based green cities are to be achieved, with all the benefits associated, then it is essential there is a better knowledge of plants – and trees in particular. The urban forest has many benefits to offer and delivers extensive ecosystem services. These have been quantified and valued in the many i-Tree and other studies which have been carried out in the UK up until the present date. The studies underestimate the real value, because services such as health and well-being are not currently included.

The studies have revealed, in general, that the urban forest is highly dependent on a relatively low number of tree species and faces many challenges. 

Of these challenges two are particularly important, if the contribution trees make to nature-based green cities is not only to be sustained but enhanced into the future. These are climate change and the threat of alien pest and disease in the UK. An urban forest heavily dependent on relatively few species is vulnerable and lacks the resilience necessary to cope with either of these challenges.

To meet the challenges which the future holds, the species diversity needs to be increased, but the question of which species should be planted and in what numbers needs to be answered. This depends on a knowledge of trees and their characteristics.

The tolerances of individual species must be known and understood so that appropriate selections are made. Knowledge of their natural environment is critical as the challenges of climate change are going to favour some species and not others. 

Some of these species are already present in the population and threatened as change occurs. There also has to be an understanding of the growth characteristics of individual species and how they might impact on the environment in which they are planted. It remains true that most landscape architects and others involved in the planting of trees work from a very limited palatte which is unlikely to enhance the possibility of nature-based green cities delivering. This limited palatte is driven by a safety-first approach, a serious lack of knowledge of what else is available and the capacity of nurseries to produce what is required. 

Many tree-planting programmes are driven by numbers and percentage increases in canopy cover which are ill-informed and not strategically planned beyond one planting season. Yet there is an unwritten expectation that planting will ensure the ecosystem services required will automatically follow. Tree species have different characteristics.

Their age potential varies enormously with some approaching the end of their useful life after 60-or-so years, while others will live beyond 1,000 years. Each species has the potential to deliver different ecosystem services at a different rate over a different period. 

If the potential of nature-based green cities is to be realised, then it is essential that greater tree and plant knowledge is utilised and put into practice.

There is much knowledge already in the public domain, but it is not widely utilised.

Examples such as the Trees and Design Action Group’s Tree Species Selection Guide, authored by Dr Andrew Hirons from Lancaster University and Myerscough College, need to be more widely promoted, but this must be coupled with a desire and intention to use different species and understand why. The lazy approach to species selection, which is fuelled by a lack of knowledge and the need to meet narrow  objectives must be cast aside, and tree knowledge respected and valued.

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 fairly small. 

There are, of course, notable exceptions such as Rupert Bentley Walls and his work in the London Borough of Hackney, or 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.

So why is this reticence to embrace diversity apparently so 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 work in their own locations. 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 available. This is particularly true with 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 often coupled with a woeful knowledge of trees in general, an opinion 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 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 above, but it is interesting to consider one species and examine the reasons why, though used extensively throughout Europe, barriers appear to limit its use in the UK.

The species in question is Styphnolobium japonicum, previously known as Sophora japonica (the Japanese pagoda tree). I have seen this tree growing very successfully across Europe, including major cities such as Berlin, Paris, Prague, and Dresden, often as a street tree. 

The tree, a native of China and not Japan, was 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 produced by ash.

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. 
Styphnolobium does not flower until it has reached 30–40 years of age, with blossoms developing in September or, in cold, wet summers, not developing at all. I have seen the tree in Grange Gardens in full flower and it was a joy. As the tree ages and some of the forecasted impacts of climate change become reality it will, perhaps, flower annually. 

Yet there is a great deal of 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 it is true that young shoots can die back and look unseemly. It is also true that, in its early development, the crown can appear scruffy and untidy. How much of this is true or 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 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 in not only reducing die to almost zero, but shapes and develops the crown in a way which removes the potential for future dieback. Any concerns about the demands of the root system can be avoided by careful site selection. It is not suggested that the species can be planted everywhere.

Styphnolobium japonicum in Dresden Germany. (Image: EA)

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 the maintenance and management after planting.

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.

Does the horticultural industry, of which arboriculture is a part, have a role to play? Well of course it does, but it also has to develop an understanding of what nature-based green cities are and how horticultural and plant knowledge is critical to develop the ability to set specific plant knowledge in context. It also requires a focus which is not entirely based on selling trees and plants, irrespective of their capability to do the job required.