Green cities are only possible with better tree knowledge

THERE is much talk in the environmental press – and to some extent elsewhere, including some arboricultural journals and magazines – about nature-based green cities. What such terminology actually means is less well defined, as are the links between nature-based green cities, urban forestry and indeed arboriculture which obviously overlap. What is certain is that each title and its achievement on the ground, will include trees.

If nature-based green cities and healthy and vibrant urban forests are to be achieved with all the benefits associated, then it is essential that there is a better knowledge of trees and tree species. The urban forest has many benefits to offer and delivers extensive ecosystem services. These have been quantified and valued in the many i-Tree studies and others 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.

In general, the studies have revealed that the urban forest is highly dependent on a relatively low number of tree species and faces many challenges which are exacerbated because of this reliance on relatively few species.

Koelreuteria paniculata. (Image: Nursery)

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

To meet the challenges which the future holds, species diversity needs to be increased, but the question of which species should be planted and in what numbers still needs to be answered. Making educated and informed decisions 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. Much of this information can be gleaned from a knowledge and understanding of the tree species’ natural range. Where does the species grow indigenously and what are the environmental conditions it is adapted to? These conditions will produce in the tree species natural tolerances and adaptations.

These can be matched to the planting environments for both current and future conditions at a particular planting site. Knowledge of a species’ natural environment is critical as the challenges of climate change are going to favour some species and not favour others. Some species are already present in the population and are already beginning to be threatened as change occurs.

There also has to be an understanding and knowledge 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 palette which is unlikely to enhance the possibility of nature-based green cities delivering. This limited palette is driven by a safety-first approach and a serious lack of knowledge of what else is available and the capacity of nurseries to produce what is required. Coupled with this there is the ever-present discussion about the use of ‘native’ trees in preference to so-called ‘aliens’. It has to be recognised that a truly resilient and diverse urban forest cannot be produced with ‘native’ trees alone, irrespective of how attractive the environmental and ecological arguments sound. The palette of so-called natives is just too small and the resilience of these species to the rigours of suggested climate change must be doubted.

Pterocarya fraxinifolia. (Image: Nursery)

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 that 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 a thousand years. Each species has the potential to deliver different ecosystem services at different rates 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 aesthetic objectives must be cast aside, and tree knowledge respected and valued.

Does the arboricultural industry have a role to play? Well of course it does, but it also is essential that we have an understanding of what nature-based green cities are and what urban forestry is, and how arboricultural and tree knowledge is critical. There has to be a wider understanding of the tree species and the extensive palette available if true diversity is to be achieved with a movement away from species selection in a comfort zone. Risks need to be taken and some species will not work, but some will. There also has to be a focus which is not entirely based on selling trees and plants irrespective of their capability to do the job required of them.