Why achieving diversity in arb is not as simple as it sounds

IT is now widely accepted that one of the ways of achieving resilience in the urban forest to the challenges of climate change and the increased likelihood of threats by imported pest and disease is by increasing the diversity of species within tree populations. This sounds easy when written in one sentence, but there are many complexities and nuances which must be addressed if such increased diversity is to be achieved.

Firstly, it is worthwhile to try to outline what is meant by diversity in tree populations. There have been several theoretical models which have found their way into common discourse. The most often used is that of Santamour, published in 1990, which suggests that no urban tree population should comprise more than 10 per cent of any given species, no more than 20 per cent of any genus and no more than 30 per cent of any given family. Miller and Miller in 1991 suggested that proven species should not exceed more than 10 per cent of the population, while Moll in 1989 stated that no species should exceed five per cent of a city’s tree population and that no genus should exceed 10 per cent. Even earlier, Grey and Denhe put forward the idea that no one species should amount to more than 10–15 per cent of the total population, while as early as 1975 Barker suggested that communities should establish maximum population densities for each species as a percentage of the entire street tree population and no more than five per cent of any one tree species is used. More recently, the 3-30-300 rule published by Konijnendijk in 2021 has gained some traction suggesting there should be at least three trees visible from your window, there should be 30 per cent tree cover and no more than 300 metres to the nearest park. 

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There are inconsistencies in the above and many nuances which are open to debate. Certainly they are useful as guidelines and should not be ignored, but it has to be remembered that there is no scientific basis to any of them. They are all interpretations – well made, yes, but interpretations just the same. If diversity is the desirable outcome, then it must be remembered that diversity in urban tree populations is multifaceted. True diversity is not just as simple as the number of different species present. The true health of an urban tree population must surely include not only the diversity of species but the diversity of age classes, the diversity of size, the diversity of genetics and the diversity of public good achieved. It seems to me that all the guidelines focus on the planting of trees as opposed to the management and maintenance of the trees already present in the population. It is from these trees that diversity of age and size will be achieved and therefore their retention and health is critical if diversity is to be achieved.

At present, from central government through local government down to the many valuable and enthusiastic NGOs, the focus for new planting is on numbers. The number of trees planted is seen as being meritorious, but within what is undoubtedly a well-meant series of programmes and initiatives there is very little sign of any strategy. Numbers of trees are planted, but there seems to be no real consideration of which species are planted where and to what long-term purpose. There is enough knowledge in the public domain for tree planting to be strategically managed with planting planned for the long term, with the inclusion of new species and cultivars included in planting programmes. There are examples of this type of strategic planning in the public domain exemplified by the work carried out by Treeconomics working in partnership with Belfast City Council and Birmingham Tree People, where comprehensive urban forest master plans have been produced. Sadly, these remain beacons to highlight what is possible rather than being the norm and no national strategy appears to be on the horizon.

This lack of a strategic approach to tree maintenance, management and planting has implications for everyone involved in the creation of diverse and sustainable urban forests into the future, not least tree nurseries. While the focus is on numbers alone, there appears to be no focus on where all the trees to be planted are going to come from. Obviously, this is the tree nursery, arguably the most critical element in the whole process. It must be remembered that a 12–14 cm girth street tree, when delivered from the nursery, is between 7–10 years old. The nursery must plan for the long term.

While numbers remain the principal indicator of success, tree nurseries can sell all trees irrespective of the species. Where is the incentive for tree nurseries to experiment with new species and cultivars when they can safely continue to grow in numbers the species they are familiar and comfortable with? Yes, the numbers will be planted, but where does the diversity agreed to be necessary come from? It is a chicken and egg situation exacerbated by the lack of strategic direction. Given notice and time to grow, nurseries are capable and willing to extend the species range produced but are, not surprisingly, unwilling to gamble and produce different species which might not sell, especially as the numbers game ensures that they can sell what they already produce.

The introduction of new species will also present the nursery with challenges in production techniques. The introduction of new tree species involves challenges in propagation and cultivation. These challenges are apparent when considering marginal trees already in production but not used as extensively as they might be. Examples are easy to find. The pruning and development of trees such as Styphnolobium japnicum (Japanese pagoda tree) or Koelreuteria paniculata (golden rain tree) often produces specimens which are ill-equipped structurally to thrive in the urban landscape. 

Other species present challenges regarding propagation either from seed or cuttings which are not addressed. It is easy to talk about diversity, but the question of where such tree stock is to come from is rarely addressed.

There are many other challenges to achieving diversity in our urban forests which will be the subject of future articles in this magazine, but cannot be discussed fully here because of limitations of space. There are many new and different species and cultivars growing quite happily in the botanical garden which could make an incredible contribution to diversity, but these are locked up in a system of limitations which frustrate commercial production. 

Koelrueteria paniculata, one of the species which present challenges. (Image: Stock)

The perennial discussion about natives versus exotics continues to rattle on, with nonsensical insistence that only native trees should be planted in the urban environment when it is quite clear from anyone who chooses to look that there are not enough native trees which are not already impacted on by pest and/or disease, or are wholly unsuitable for the urban environment because of their inherent characteristics, to achieve diversity in an urban environment.

However, the key to achieving diversity is a strategic approach to urban forest management with long-term planning for population management. This must include investment in the trees we already have as well as investment in tree planting. Tree planting must be divorced from what is purely a numbers game at the moment with new planting and the introduction of new species with purpose.