What do we need to consider in order to achieve a truly diverse urban forest?

THERE is an increasing appreciation of the benefits trees provide, especially in the urban environment. It is generally accepted that more trees need to be planted. Ambitious numeric targets for tree planting, coupled with equally ambitious targets to increase canopy cover by a certain number of percentage points, emerge on an almost daily basis, initiated by government, both central and local, along with the many initiatives of NGOs and community groups.

Such initiatives at both local and national level are to be welcomed and, some would argue, long overdue. Yet, more often than not, there is a lack of any evidence base to support the achievability of such targets. The question ‘what have we got?’ remains largely unanswered and the question ‘where do we want to be?’ is rarely asked or defined in any sort of long-term strategic management plan for the urban forests of the UK. There is rarely a baseline which indicates the current position accurately, making it difficult if not impossible to strategically plan for urban forest development into the future.

Forestry Journal: A mixed species avenue in the USA.A mixed species avenue in the USA.

The work of Treeconomics and others has produced baseline data for many local authorities and landowners using i-tree and other tools to provide extensive detail, including ecosystem service benefits, population mix and economic value.

It is true that urban tree populations, at a time when more planting is being encouraged, face increasing pressures, the most notable being the accelerated introduction of imported pests and/or diseases and the effects of climate change on urban environments both now and into the future. They also deliver many social benefits such as improved health and well-being which, at present, cannot be quantified accurately using the tools available. It is fair to say that the estimation of the public goods delivered by trees is undervalued.

Increasingly, there is a focus on diversity within urban tree populations and how the strategic pursuance of such diversity offers, through inbuilt resilience, the potential to, at the very worst, ameliorate some of these pressures and at the very best prevent wholesale damage to highly vulnerable tree populations, as well as adding to the benefits and public goods delivered.

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The subject of strategic urban forest management begins with the question, ‘what have we got?’. Without this knowledge, it is difficult to see how resilience can be achieved through diversity. A knowledge of the composition of existing tree populations, with evidence of existing species diversity, age and size class diversity and – if possible – genetic diversity is essential if resilience is to be achieved. How is it possible to plan what to plant where and in what quantity, without a basic understanding of what is already there?

Forestry Journal: A one-species avenue, folly or not in terms of diversity.A one-species avenue, folly or not in terms of diversity.

In the UK, at the time of writing, the evidence base is fragmented and partial, but what information exists suggests that urban tree populations, while diverse at an individual species level, are highly dependent on relatively few species.

Trees in Towns II, the last comprehensive review of trees in urban areas in England, reported that six species accounted for 37 per cent of all trees and shrubs planted in England’s cities. Evidence from i-tree studies from around Europe suggest this has not changed a great deal and that our urban spaces are heavily reliant on a few species. Recent work carried out by Forest Research examining 12 i-tree studies in the UK concluded that locations were typically represented by a small number of species. Over the years, several theoretical models have emerged as to the metrics which can be used to achieve sensible and resilient species diversity within the urban environment. Of these, the most frequently quoted is Frank Santamour’s rule. He states the ideal would be that no urban tree population should comprise more than 10 per cent of any given species, no more than 20 per cent of any particular genus and no more than 30 per cent of any given family.

It has been suggested elsewhere that communities establish maximum population densities for each species as a percentage of the entire street tree population and that no more than five per cent of any one species is used.

More liberal recommendations suggest that proven species do not exceed 10 per cent of the population.

It is recognised and accepted that the larger the tree, the greater the range of benefits it provides. However, tree populations are dynamic. Trees are lost and planted, and succession has to be provided, which poses the question as to whether there is an ideal age/size matrix which can be applied to any given tree population to ensure a resilient and sustainable future for that population.

Forestry Journal: Young planting in a grass verge.Young planting in a grass verge.

The matrix most often quoted and used is one which has suggested that tree and park populations should be comprised of trees of approximately 40 per cent with a dbh of 0–20 cm, 30 per cent with a dbh of 20–40cm, 20 per cent with a dbh of 40–60 cm and 10 per cent with a dbh greater than 60 dbh.

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?’. The well-worn phrase ‘the right tree for the right place’ is often shown up to be meaningless unless supported by worthwhile and well-researched evidence.

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).

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Inevitably whenever the question of species diversity in the urban environment is raised, the discussion is polarised into whether the planting of native species is preferable to the use of so-called exotic species. Some argue that native is best, while some argue for a mixture and others 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 a generally higher species diversity is to be found in urban areas due to the complex nature of urban space. 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 of coping with these fluctuations than native species. Urban areas are not only divided by an urban-rural gradient, but consist of areas separated by socio-economic and cultural differences. Socio-economic status and culture are shaping forces for urban biodiversity. 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. 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 many practice-orientated 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 formulae 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 genera are native. These are acer, alnus, betula, carpinus, crataegus, 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 and the question has to be asked: how many of these are successful as significant urban trees, particularly in roads and streets?

The native vs exotic discussion is still ongoing and will, it seems, continue long into the future, but all planting opportunities have to be considered in terms of the benefits they bring and how those planted opportunities are converted into long-term planting success and the delivery of essential public good.

Forestry Journal: Trees planted on the Navy Pier in Chicago, USA.Trees planted on the Navy Pier in Chicago, USA.

It has also to be remembered that diversity is not just a question of species diversity. There are many forms of diversity within the population which need to be considered if a truly diverse urban forest is to emerge. These can be summarised as follows:

SPECIES DIVERSITY: The proportions of any given family, genus, species, and cultivar which make up the total tree population.

AGE CLASS AND SIZE DIVERSITY: Size and age diversity are generally inextricably linked. The older the tree (of any species) the larger it is, but the size of that tree relative to other trees of other species is obviously related to the genetic potential of individual species. Diversity within the population is the mixture of ages and sizes present in that population.

GENETIC DIVERSITY: There is very little in the literature which deals adequately with the question of genetic diversity within tree populations. Certainly, in the urban environment, many of the more successful tree species are in fact clonal selections which have been propagated vegetatively either through cuttings or budding. The consequence is that all these selections are genetically identical. Only trees raised directly from seed can display genetic variation. The consequence of clonal selection is that being genetically identical they are equally, universally, vulnerable to stresses and strains whether from climate change or pests and diseases.

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Before COVID-19 so unceremoniously interrupted our lives so dramatically, I was working on the fourth of the Barcham Manual series. The others, entitled Specification Manual, Species Selection Manual and Planting Manual, have all been in circulation for some time with the Specification Manual being on its fourth print run. The new manual is to be called Resilience through Diversity and has reached a first draft stage, with specialists asked to offer their comments and observations. Work has now stopped and it is unclear when publication will take place, but several conclusions were arrived at which may be modified and built upon as time goes on and we all return to something like normality:

  • The impacts of climate change and the increased risks to our urban tree populations by invasive pests and diseases are real and growing.
  • It is generally accepted that the resilience of urban tree populations can be increased by the diversification of those populations. This diversity can be achieved through species diversity, age and size diversity and genetic diversity.
  • Metrics exist and are widely accepted to evaluate diversity within urban tree populations.
  • Evidence suggests diversity in the urban tree populations of towns and cities, not only in the UK but across the world.
  • The foundation in planning to increase diversity is understanding fully the situation as it is currently, with a supporting evidence base, and answering the critical question, ‘what have we got?’
  • To succeed, diversification needs to be part of an overall long-term visionary strategic management plan.
  • Research has indicated that there are many species which are not currently used or infrequently used which have the capabilities to thrive in the urban environment.
  • Specifiers need to be more adventurous in their species choice and relate that species choice to the overall diversity objectives.
  • Diversity in the urban environment cannot be achieved through the use of native species alone.
  • Tree nurseries have to be part of the dialogue and be involved in discussions about species choice and be given the confidence to invest in producing an increased range of material with contract growing, made possible by long-term planning of a possible solution.

It has been impossible to condense the full contents of the manual or present a full discussion about tree population and diversity in this article, but I hope a summary of some of the main points might prove useful and encourage further thought on the subject.

For further information contact keith@barchamtrees.co.uk. For information about i-tree and its use in the UK, contact Kenton Rogers at Treeconomics kenton@treeconomics.co.uk.

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