The question of ‘the right tree in the right place’ in our urban environments becomes ever-more complex when we begin to think about the future.

WHEN discussing diversity in the urban forest, two questions always asked are: which species should be planted and will species selected be available from the nursery when needed? These questions are almost always underpinned by the seemingly never-ending discussion about the value of natives vs exotics.

There are many publications and catalogues which offer advice and suggestions. The most recent at the time of writing – and certainly one of the most comprehensive – is the Trees and Design Action Group’s Tree Species Selection for Green Infrastructure, A Guide for Specifiers, researched and authored by Dr Andrew Hirons and Dr Henrik Sjoman. This document, which is freely available as a download from the TDAG website, provides profiles for over 280 species supported with explanatory guidance (www.tdag.co.uk).

Much can be learned from Dr Sjoman’s work. His research has included the selection of natural habitats across the world which currently replicate, as far as is possible, the environmental conditions likely to be found in Scandinavian cities in the 2050s, as a result of climate change.

In his doctoral thesis published in 2012 he lists tree species identified from case studies as specialists for warm, dry habitats which have never been grown or grown to a limited extent, in the CNE region. This list is reproduced as follows: Carpinus orientalis, Carpinus turczaninowii, Celtis bungeana, Fraxinus chinensis, Morus mongolica, Ostrya japonica, Quercus aliena var acuteserrata, Quercus baronii, Quercus dalechampii, Quercus pubescens, Quercus wutaishanica, Sorbus folgneri, Syringa pekinensis, Ulmus glaucescens and Ulmus pumila.

Forestry Journal: Acer campestre, a true native.Acer campestre, a true native.

In Germany, the Institute of Forest Botany and Forest Zoology has produced a Climate-Species-Matrix attempting to extensively classify and assess tree species with regards to their usability after predicted climate change. It works on the hypothesis that in the near future trees will become more important in the urban environment but will have to cope with increasingly extreme climatic conditions, especially an increase in the frequency and severity of summer drought and heat waves.

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The analysis categorised trees according to their suitability, with regard to drought tolerance and hardiness. The table reproduced below is representative of the total work and is not the work in its entirety. This cannot, because of space, be reproduced in full here but focuses only on trees and shrubs which were considered suitable based on two assessment categories (drought tolerance and hardiness) taller than 10 metres.

Acer zoeschense                  Zoeschen maple

Cladrastis sinensis               Chinese yellowwood

Fraxinus pallisiae                 Pallis’ ash

Ostrya carpinifolia                European hop-hornbeam

Phellodendron sachalinense  Amur corktree

Pinus heldreichii                   Bosnian pine

Quercus bicolor                    Swamp white oak

Quercus macrocarpa             Bur oak

Robinia viscosa                    Clammy locust

There are many aids to species selection and information as to how diversity can be achieved. The range of species which could enhance diversity in our tree populations is plentiful but it is worth referring to CITREE, a web-based species selection programme produced by the Technische Universität in Dresden, Germany, which makes recommendations as to species choice based on constraints criteria and required design characteristics (www.citree.com).

The 2009 publication recommended Urban Trees: Site Assessment and Tree Selection for Stress Tolerance (Bassuk at al), which remains a useful reference.

However, the challenge is to encourage designers, specifiers and tree managers to step outside their comfort zone and move beyond the often narrow range of species and cultivars they are familiar with and perhaps use rather formulaically.

Forestry Journal: Foliage of Celtis occidentalis, a tree underused in the urban environment in the UK.Foliage of Celtis occidentalis, a tree underused in the urban environment in the UK.

NURSERY SUPPLY

Of course, even if this challenge is met, there is the question of availability within the nursery industry. It is obvious from the species list used as examples above that many would not be available.

In an article published in Arboriculture News, August 2018, Dr Gary Watson, a lead researcher from the Morton Arboretum, Chicago, USA, summarised the dilemma as he saw it, drawing attention to substantially increased production necessary, the likelihood that the production costs of previously underused species being considerably higher and the need for growers to develop the confidence that the trees would be purchased if they were prepared to invest in the growing of them.

He concluded: “Arborists and foresters would have to learn how to match each species with appropriate sites, be willing to pay more and to commit to using a wider variety of more challenging species.”

Dr Watson’s points are well made, but the educated matching of species to site should surely be a prerequisite of all urban plantings.

The growers’ reluctance to invest is related to market uncertainty. This uncertainty is related to the lack of long-term strategic planning. With long-term strategic planning comes certainty and the prospect of more contract growing. This method was successfully used in the delivery of New York City’s Million Trees campaign, completed in the autumn of 2016.

Forestry Journal: Sorbus aucuparia: It has to be remembered that many Sorbus selections are clones.Sorbus aucuparia: It has to be remembered that many Sorbus selections are clones.

NATIVE VS EXOTIC

Discussion of species diversity in the urban environment frequently polarises into whether the planting of native species is preferable to the use of so-called exotic species. Some argue that native is best, some argue for a mixture, some prefer the use of exotics. The real question is whether true diversity is achievable using native species alone.

Research has shown that cities are capable of supporting higher species diversity and generally a higher species diversity is to be found in urban areas and can be explained by four factors:

  • High incidence of introduced species already present
  • The socio-economic factors present in city areas
  • Land use and land use heterogeneity
  • Diversity of environmental factors like soil and climate diversity.

The same source (Morgenroth et al, 2016) also states that: “The use of non-native species often refers to fluctuating conditions which are expected to increase under climate change. Non-native species have a better chance to cope with these fluctuations than native species.”

READ MORE: Barcham Trees and Treeconomics produce groundbreaking rating

Urban areas are not only divided by an urban-rural gradient but also consist of areas separated by socio-economic and cultural differences. Socio-economic status and culture are shaping forces for urban bio-diversity, for example higher socio-economic status is correlated with greater species diversity. Landowners in higher socio-economic areas tend to shape their surroundings and plant a more diverse range of species.

It has been argued that urban landscapes represent the most complex mosaic of vegetative land cover and multiple land uses of any landscape and are characterised by a diverse range of site conditions not found in the surrounding countryside. Urban areas can accommodate a surprisingly varied flora.

Forestry Journal: Ostrya virginiana fruits.Ostrya virginiana fruits.

Given that research has demonstrated the urban environment is already a diverse one with regard to tree species, it is perhaps surprising to find that other research indicates that many practice-oriented publications, research papers and governmental websites in the fields of urban planning, urban forestry and urban ecology argue for the use of native species and the avoidance of introduced species.

If Santamour’s widely used 10-20-30 formula is used as a guide where no more than 20 per cent of any urban tree population should be made up of a single genus, it is interesting to note the number of UK tree species considered native. According to the RHS website, 18 genus are native: Acer, alnus, betula, carpinus, crateagus, fagus, fraxinus, ilex, malus, pinus, populus, prunus, pyrus, quercus, salix, sorbus, tilia and ulmus. However, many of these are represented by a limited number of species.

Genus                      Species native to UK

Acer                        Acer campestre

Alnus                      Alnus glutinosa

Betula                     Betula pendula

                              Betula pubescens

Carpinus                 Carpinus betulus

Fagus                     Fagus sylvatica

Fraxinus                 Fraxinus excelsior*

Ilex                       Ilex aquifolium**

Malus                    Malus sylvestris**

Pinus                    Pinus sylvestris**

Populus                Populus nigra subsp                                               

                           Betulifolia**

Prunus                 Prunus avium**

                           Prunus padus**

                           Prunus spinosa**

Pyrus                   Pyrus cordata**

Quercus               Quercus petrea*

                           Quercus robur*

Taxus                   Taxus baccata

Tilia                     Tilia cordata

                          Tilia platyphyllus

Ulmus                 Ulmus glabra*

                          Ulmus minor*

The two genus remaining are Sorbus and Salix. These are represented by several species: Salix** (salix alba, salix caprea, salix cinerea, salix fragilis, salix pentandra, salix triandra, salix viminalis) and Sorbus** (sorbus arranensis, sorbus aucuparia, sorbus bristoliensis, sorbus devoniensis, sorbus domestica, sorbus eminens, sorbus hibernica, sorbus lancastriensis, sorbus porrigentiformis, sorbus psuedofennica, sorbus rupicola, sorbus subcuneata, sorbus torminalis, sorbus vexans and sorbus wilmottiana).

*Genus which are currently affected adversely by a known pest or disease where planting is restricted or banned.

** Genus where native species would not be suitable for extensive use as trees, particularly street trees in the urban environment either because of size, form or availability. It is to be noted that S. aucuparia, S. domestica and S. torminalis are used but normally they are represented by cultivars.

The above is the interpretation of the author and is likely to be contested by some, but it is readily apparent that the choice, which at first seems ample to meet diversity criteria, is limited. The native versus exotic discussion is not going to be resolved here and is going to continue to be, in some instances, controversial, but the palette of trees which can be used in the urban environment is vast (see TDAG Guide to Species Selection authored by Dr Andrew Hirons). The stresses and strains caused by either climate change or imported pests and diseases is only going to grow and it is at the very least questionable as to whether the palette of available native trees can deliver the resilience necessary.

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