Torturing Trees in Car Parks

Learning lessons from the tree plantings of the 1980s and ‘90s in the UK

Introduction

Figure 1: Coming out of its cage, almost, is a specimen of wild cherry (Prunus avium ‘Plena’) in a retail car park. That is has been bashed and maltreated is not that surprising – but that it is still this small 16 years after it was planted is very shocking!

My mother was an avid grower of bonsai trees. If you are unfamiliar with the process of bonsaiing a tree, it is essentially the cruel torturing of tiny baby trees by clipping their tops, clipping their roots, wiring and distorting them whilst giving them a minimal amount of soil to grow in – and continuing that torture regime for decades (Fig. 2). The fortunate trees that did not respond well to my mum’s ‘loving’ treatment of them – especially those that refused to miniaturise their leaves - I got to release into our woodland. This made my role the arboricultural equivalent of Amnesty International, supporting a refugee population to overcome their previous mistreatment. I learnt a lot about nurturing trees through this process of releasing them to ‘the wilds’.

Figure 2: One of my mother’s bonsai trees, a specimen of Pinus parviflora Siebold. & Zucc. which I have inherited and that I continue to “torture” by restricting its top growth and the amount of soil it has access to. Given its age, it should be about 15 metres tall now – but it’s only 30 centimetres tall. It takes considerable care each year to maintain the health of this miniaturised tree, particularly the refreshing of the soil resource.

In my mother’s defence, for the several hundred trees she did bonsai successfully, she did give them the much-needed individual care and attention to keep them healthy. Whole days were given over to re-potting them, I recall. In comparison, you can visit a local retail park and, in the car park – particularly in the centre of the car park – you will find some of the most neglected “bonsai trees” in your town.  

Stunted trees in car parks

Figure 3: A Norway maple ‘unhappily’ growing in a supermarket car park, Lancashire. The soil volume this tree is growing within must be minimalistic, as it was planted 34 years ago and yet it has only grown to 2.8 metres in height. The crown of this tree shows evidence of multiple incidents of dieback and partial recovery – identifying the physiological stresses that this tree must be under, trying to grow in this harsh urban location.

The Norway maple (Acer platanoides L.) shown in Figure 3 was planted in a supermarket’s car park and is currently 2.8 metres tall with a trunk diameter of approximately 8 centimetres. One might mistakenly think it has only recently been planted, but looking in more detail at the image identifies that this tree has suffered several episodes of dieback. In fact, this tree was planted in 1982 – with probably a six year period of production in the nursery prior to planting, which makes this stunted little Norway maple tree a gob-smacking 40 years of age.

How has this extreme growth retardation happened? Well, look at the ‘spec’ of the planting: above-ground we can see the planting specification is typical of 1980’s landscaping standards – in that it is tarmacked right to the base of tree’s trunk, there are no protective bollards to stop the tree being bumped into by parking vehicles and there is no evidence of a prepared planting pit. What has made this potentially large-growing tree into a ‘dwarf’ is that it has been starved of a key resource: SOIL. As a consequence, it lives the life of a tortured bonsai.

In the background of Figure 3 is a shelter belt of Norway maple trees growing at the edge of the car park and planted in the same year (1982). These shelterbelt trees are between 12 and 15 metres in height, with trunk diameters of 30 to 35 centimetres, as they got to grow in enough spoil from the development that was shunted to the side of the site before the process of compacting the layers of crusher run and laying the car park surface condemned the rest of the trees within the car park to their tiny ‘islands’ of soil.

When you survey and assess tree plantings from decades ago which are situated in the middle of car parks, they highlight the very minimal benefits that can be gained from planting trees in poorly designed pits with limited soil resources (Ennos et al., 2014). What has been achieved from 34 years of ‘growth’ in the case of this Norway maple? Very little. It is not providing significant shelter, shade nor many other of the benefits that one would want from an urban tree planting. Many of these stunted urban trees are the tangible symbols of environmental tokenism – they represent the small nod to the environment that was given at that time to facilitate some large retail site developments getting through the planning stage. Yes, trees were planted on these sites to improve amenity, screening and to tackle issues of pollution, surface water runoff and the need for shade, but the stunted trees that have actually grown in these dire conditions have failed to deliver the promised idyllic car park experience that the promotional landscape drawings depicted.  

Researching the extent of the ‘stunting’ effect

Based on the work of two recent arboricultural graduates of Myerscough College, we have collected data from thirteen car park sites, each with multiple trees growing within the car parking areas, but each site also having a set of ‘control trees’ of the same species, planted in the same year within the landscaping strips at the edges of the sites. This allowed comparisons in growth rate and the performance of different species. And for some sites the differences in tree sizes between the control trees and car park trees were truly shocking – heights, trunk diameters and crown spreads of the trees planted within the car parks were often half of what they could have been, sometimes as extreme as the hapless maple above (Fig. 3). The death rates of these trees were also much higher when they were planted within the car parking areas. 

We intend to publish our findings soon, the key two findings being a) that the extent of the stunting (for most species surveyed) is significantly related to the tree pit volume supplied (Fig. 4) and that, b) from analysis of two car park sites planted with London plane trees (Platanus x acerifolia (Aiton) Willd.), this species copes far better with restricted pit size and compacted ground. This latter point could be easily anticipated – Alan Mitchell described the capacity of London plane to cope with soil compaction in his field guide to the trees of Britain and Northern Europe: “More important in streets is the ability of [London Plane] roots to function in compacted and covered soil” (Mitchell, 1974). Unfortunately we are not currently in a position to conclude our study with a recommendation to fill new UK car parks with London plane trees: the threat of plane wilt disease (Ceratocystis platani) now looms over this hybrid and it would be unwise to add further to this existing vulnerability of the UK’s urban forests.

Figure 4: Specimens of whitebeam (Sorbus aria cv.s) growing in restricted soil volumes in a retail car park in Preston, Lancashire. The largest tree pictured is in the greatest soil volume and the smallest tree has a highly restricted rooting area, as identified by the white borders drawn around the planting areas.

Why collect this data on stunted car park trees, then? We now have several design techniques and technologies that overcome the issues of inadequate soil volumes for the car park trees, so we have a set of solutions to this malaise. Why look back at these failed plantings? 

First, it is to draw a ‘line in the sand’ – if we mistakenly follow planting specifications that were used in the 1980s and ‘90s for car park plantings, then we now know just what stunted little trees will come of that approach and how little they will then contribute to our townscapes. We should not accept such specifications for future developments, as it fails to provide a sustainable future for our urban forests.

Second, these sorts of specifications haven’t gone away – only the other month was I asked by a landscape architect as to what size of concrete-sided pit should a particular tree be planted into – 1 metre by 1 metre, or perhaps 1.2 metres squared?! This long-term and continuing torture reminds me of an old Milton Jones joke: “Dressed in orange – Asked questions they don’t know the answers to – How would you like to work at HomeBase?” (Jones, 2008). The corollary one-liner for car park trees is “Prettily drawn for the planning application – Asked to cope with too little soil and no aftercare – How would your tree like to grow in a car park at Wickes?” (Fig. 5).

Figure 5: View of four trees (in this case, Prunus avium ‘Plena’) at a DIY store car park, Preston, Lancashire. Note that the two more stunted trees are situated further into the car park, and the two trees nearer the edge have managed to root into the surrounding landscaping strip, allowing them to develop to a significantly larger size. Penetrometer readings confirmed that soil compaction was highest nearer to the building set within this car park, where the most stunted cherry trees were growing and the tree death toll was highest (not shown – too horrific!).

Third, I have had arguments with other arboriculturists who want to give up the fight – who think I’m wasting my time advocating for trees in the centre of car parks. I respectfully disagree. On a warm day an acre of unshaded black tarmac is going to be getting pretty hot, the cars parked within that car park are going to get very hot and the retail therapy seekers hauling their heavy (5p) bags of purchased goods across that acreage will get unhealthily hot and bothered. Besides which, it is the centre of the car park which has heightened air pollution and where controls on surface run-off are needed – not at the edge of the site. It does matter that trees are established in these locations – and that they are not stunted bonsai trees, but make a worthwhile contribution to the thermal comfort of car park users. These token gestures at tree planting that one can see in many of our retain parks are not what we should accept as a society.

Conclusions

Technical solutions to this problem are available – we can insist on minimum soil volumes for the mature size of the tree being planted (e.g. Doncaster MBC, 2015), we can install sub-surface “rootable” volumes of soil (Urban, 2008; Deeproot, 2016; GreenBlue Urban, 2016) and perhaps we can explore the traits of P. x acerifolia and look wider for other species that may grow well on the meagre soil offerings of our local retail parks (Sjoman et al., 2015). We should be sure to learn ALL the lessons from these ineffective planting projects, starting with discovering just how stunted the trees become if they are treated so brutally. Perhaps you might be surprised at just how old your local supermarket’s car park trees really are, if you want to explore this topic further: the fact that the trees are small does not mean that they were recently planted. We hope to report our findings as a contribution to this debate soon.

Citations and related publications:

Deeproot (2016) Company web pages; accessed at https://www.deeproot.com/

Doncaster MBC (2015) Doncaster Council Development Requirements and Guidance: Supplementary Planning Document, section 8.9; available at https://www.doncaster.gov.uk/services/planning/development-guidance-and-requirements-spd

Ennos A R, Armson D & Rahman M (2014) How useful are urban trees?: The lessons of the Manchester research project; Conference paper presented at Trees, People & the Built Environment II, Birmingham, England.

GreenBlue Urban (2016) Company web pages; accessed at https://www.greenblue.com/gb/

Jones M (2008) Hello; comedy performance audio recording; Laughing Stock Productions Ltd.

Mitchell A (1974) A field guide to the trees of Britain and Northern Europe;  1st Edition; Collins: Glasgow.

Sjoman H, Hirons A D & Bassuk N L (2015) Urban forest resilience through tree selection – Variation in drought tolerance in Acer; Urban Forestry & Urban Greening 14, 858-865.

Urban J (2008) Up by roots: healthy soils and trees in the built environment; ISA: Illinois, USA

Acknowledgements:

Many thanks go to Daniel Goulbourne, BSc (Hons) graduate, and Jaimey Richards, MSc graduate, for willingly carrying out the data collection for this research project.

**THIS ARTICLE FIRST APPEARED IN THE ARB MAGAZINE -             DECEMBER 2016 EDITION**