We need to stop working with tree-species limitations

IT has become, over the years, almost a standing joke that when asked how large the palette of tree species used by landscape architects is, the answer is: ‘Oh, 10 or fewer.’

This is of course unfair on landscape architects, not because it is untrue, but because landscape architects are singled out for ridicule. Experience suggests that in some way we are all culpable. 

The phrase ‘the right tree in the right place’ has almost become a mantra for tree selection. It implies that for every situation there is the perfect tree and that choice is just a question of turning to the right page and ticking the appropriate box. If the boxes are limited to 10, then the ‘right tree in the right place’ has obvious limitations.

There is, accompanying this, much talk of the resilience of tree populations. Obviously, tree health and condition are important, but perhaps the most important factor is the number of species growing in any tree population. Numerous species diversity and, by default, resilience guidelines exist, with the most well-known being that published by Frank Santamour in 1990, resulting in what is known as the 10-20-30 rule. It suggests that no tree population has more than 10 per cent of a single species, not more than 20 per cent of a single genus and not more than 30 per cent of a family. Numerous i-tree studies, both in the USA and more recently in the UK, have demonstrated that most if not all (particularly urban) tree populations are heavily dependent on very few species and are therefore highly vulnerable to imported pest and disease. 

It is worth looking briefly at two well-known cases where an imported pest has encountered a tree population heavily dependent on few tree species. The arrival of Dutch elm disease in the UK in the late 1960s led to the eventual death of an estimated 25 million elms in the country and the virtual disappearance of elm as a significant tree of stature. The USDA Forest Service estimated some 38 million ash trees are vulnerable to emerald ash borer in a 25-state area centred on Detroit and that simulations have estimated some 17 million trees will probably have to be felled and replaced.

The London i-tree report estimated that around 370,000 ash trees are vulnerable in London with a replacement cost of £450 million and that London plane, which provides nearly nine per cent of the canopy cover in inner London, would cost some £350 million to replace. Plane is already affected by Massaria and there is the threat of plane wilt hovering in the background. The same report suggests that Asian longhorn beetle could impact on some 3.8 million trees, representing 31 per cent of the total population with a replacement cost of a staggering £23 billion. 

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It is therefore reasonable to question the palette of 10–12 trees. When next out and about, have a look and see how many Liriodendron, Celtis, Gymnocladus, Pterocarya, Carya, Ostrya, Koelreuteria and Styphnolobium you can see. There will be odd specimens, but I will be surprised if there are large numbers, yet all have characteristics which make them suitable for different aspects of urban planting and this is just the tip of the iceberg. If resilience through population diversity is the objective, then the ‘blinkers’ have to come off, with the limited palette discarded. It has become so easy to adopt a fallback position and select from the well-known list, which includes Pyrus chanticleer, Liquidambar styraciflua, Betula utilis Jacquemontii, Betula pendula, Amelanchier Robin Hill, Tilia cordata Greenspire and so on and so forth until we reach the magic 10 or 12 species, a self-imposed ceiling.

This implies that those who select trees need to develop a greater understanding of the trees available and the intricacies of selecting the somewhat nebulous ‘right’ one for the right place. Tree nurseries also must become more adventurous in the species they grow, although it is difficult to justify nurseries speculatively growing trees which specifiers will not use (but this is potentially the subject for another article). 

The mantra of ‘right tree in the right place’ as used currently becomes a lazy excuse for just looking for another off-the-shelf solution. Tree selection is the outcome of a thought process which involves the consideration of many factors, all of which will influence the likely success or failure of the tree post-planting. More often than not, tree selection is guided by aesthetics and design briefs rather than any real consideration of the longevity or health of the tree in the landscape.

All planting sites have constraints. These need to be considered and assessed prior to tree species selection taking place. Many of them can be ameliorated, modified or controlled through engineered solutions, design variations, horticultural or arboricultural techniques and modified maintenance practices. There will remain, after such interventions, variable environmental and other conditions on site. These will often be localised. 

Pterocarya fraxinifolia. (Image: EA)

Individual tree species have different genetic characteristics and strategies for coping with different environmental conditions. These characteristics and strategies manifest themselves in different tolerances, growth habits, attributes and adaptations suitable for successful growth in the natural range of the species involved. Species selection is about matching the tree and its characteristics with the prevalent environmental conditions and remaining constraints of the planting site.

It is here that a knowledge of individual tree species is critical and the usual palette of 10 should be discarded. Individual species have different tolerances and strategies which can make them more, or less, suitable for any planting site. Drought-tolerant trees will often have small leaves or, in the case of conifers, needles which have a low surface area. Salt-tolerant species such as Tamarix have developed specialised structures which enable them to function in conditions of high salinity. Trees such as Taxodium, Platanus, Populus and Alnus, among others, have characteristics which enable them to cope with varying degrees of waterlogging. Different species can thrive with varying levels of pH and the differing resulting variations in nutrient availability. Different species are more or less vulnerable to pest and disease and have developed different strategies for coping with stress.

The vulnerability of tree species to pests and/or disease is important but equally, if not more so, is the resilience of the tree population. The more species present in any population, the more resilient it is likely to be and the lower the impact any imported pest and/or disease will be. This is of course significant when considering which tree species to select for planting.

In nature, trees grow in environments where they have adapted to survive. These environments offer clues as to the suitability of an individual species for any planting site. The hardiness zone in which the tree grows and the natural range it occupies offer clues and information which can be used to inform tree selection. Trees have developed strategies and characteristics which enable them to survive in certain environments which are often very specific. There is an opportunity to match the conditions found at any planting site with those occurring naturally and then select tree species that have evolved to cope with those conditions. The provenance of the tree species offers specific information, as does its place in natural forest or woodland succession. For example, warm, dry mountain slopes with a limited soil volume may replicate conditions found in a paved urban environment. Here, early succession species such as Pinus nigra, Quercus petraea and Koelreuteria paniculata are likely to be successful. 

It is well recognised that trees deliver many ecosystem service benefits. It is possible to quantify many of these benefits, in both volumetric and monetary terms. The benefits trees provide is related to the leaf area index. Generally, the larger the tree, the greater the benefits delivered. Trees have the potential to positively affect pollution levels at the human level. They sequester and store carbon, intercept rainwater and slow flows into the drainage system and provide shade. Trees also can provide disservices in terms of BVOC emissions and allergenic potential, as well as the well-versed disservices such as fruit drop, root damage and leaf deposits.

It is beyond the scope of this article to explore any of the above in any depth, but hopefully it will reinforce the suggestion that ‘right tree in the right place’ is inadequate and that tree species selection is more than a slogan. It involves many factors and considerations and could be regarded as a specialism. 

Celtis occidentalis. (Image: EA)

At Barcham, we have prepared and published a free-of-charge species-selection guide which explores some of the above in greater detail and another manual on tree diversity.

Dr Andrew Hirons at Myerscough College has prepared a more detailed guide on behalf of the Trees and Design Action Group, which is now available as a free-of-charge download on the TDAG website. He has also written a book along with Dr Peter Thomas from Keele University titled ‘Applied Tree Biology’. While not specifically about tree selection, it offers, in addition to detailed explanations of tree biology, many, many examples of how trees have adapted to different, often extreme, environments, the strategies they have evolved and how these can be applied.

So the challenge is there, the tree species are there, a great deal of knowledge is there. All that needs to happen is for the potentialities to be realised.