'Reading the Leaves: Planting for the future' was the title of the talk given by the guest speaker, Clive Stephens at the recent Bishopston Society AGM.

Clive is the Chair of the Redland, Bishopston and Cotham Neighbourhood Partnership and our local Tree Champion. In his interesting and entertaining illustrated talk he looked at the topic of the importance of planting urban trees.

With predictions of hotter and wetter weather for a future Britain as a result of climate change, trees can play a significant role in helping to ameliorate these conditions. Towns and cities create their own urban heat islands, where the large areas of concrete, brick and tarmac absorb heat in the day and then, like old fashioned storage radiators, release this heat at night. Trees, by creating shade from their canopy of leaves, reduce the heating effect of sunlight falling on hard surfaces, and at the same time, through the effects of the continual transpiration stream of water that passes from their roots, up the trunks and finally evaporating out of the leaves, act as a cooling system, reducing the temperature of the surrounding air space. Infra red surveys of wooded urban parks have shown these areas are 4-5% cooler at night than similar areas without trees.

Particulates and waste gases in car exhaust fumes in conjunction with the effects of UV sunlight on these gases can produce unpleasant and even dangerous levels of pollutants  both in urban street air and in the rain water washed off these streets. Trees can help reduce these effects by filtering some of the pollutants and by slowing the rate at which the runoff is carried away as storm-water, making more surface water available to the soil and thus to the trees themselves as well as other plants.

The planting and management of urban trees, if not carried out carefully, can of course result in problems. Thus the often necessary requirement to regularly pollard some street trees can seriously shorten their life. Sprouting of numerous and unsightly short side branches low down on the trunk (epicormic growth) can result from serious pollarding. Badly chosen species planted in close proximity to housing can, where there is shrinkable clay, cause subsidence to foundations. Pavements can be lifted by roots and in autumn, leaves can block drains. The covering of gardens with impermeable surfaces will exacerbate tree root problems because rain fails to soak into the ground and instead flows directly into drains, depriving tree roots of the water they need to absorb for the healthy growth of the tree. The reaction of a tree in this situation is to extend root growth outwards in an attempt to find water, often undermining any house foundations nearby.

Bishopston is one of the areas of Bristol that has a rather low number of trees. The measure of the trees planted in any given area is expressed, internationally, as a % tree canopy cover figure. The overall % canopy cover for Bristol is 14%, while for Bishopston it is around 10% to 11%. Because of the high density of housing and the layout of the streets in Bishopston, it is unlikely that this figure could be realistically increased to more than around 15%, but thanks to a street by street survey carried out by Clive, many of the neglected and ‘forgotten’ tree stumps that existed around Bishopston have been or are now being replaced with new trees and our % canopy cover is slowly improving.  

Bristol City Council has recently come to adopt a much more positive view of the value of planting street trees and has set up the ‘Bristol Tree Forum’ http://www.bristol.gov.uk/page/bristol-tree-forum to help develop a policy that places tree planting as a central part of any building construction and development in the city. BCC now believes the city as a whole has the potential to increase its % tree cover from its present 14% to 30% over the next few years. Significantly, this figure of 30% is believed by experts to represent the point at which the cumulative effect of these trees would be to reduce the absorbed heat in a city and thereby lower the temperature to around the same level as that of the surrounding countryside.

In The BCC’s new Planning document, as part of its ‘Core Strategy’, Policy BCS9 specifically spells out for any developer that they must take into account the ‘green infrastructure’ of the site, replacing and increasing in number any trees that are destroyed in construction work. The two relevant paragraphs are given below:

Individual green assets should be retained wherever possible and integrated into new development. Loss of green infrastructure will only be acceptable where it is allowed for as part of an adopted Development Plan Document or is necessary, on balance, to achieve the policy aims of the Core Strategy. Appropriate mitigation of the lost green infrastructure assets will be required.

Development should incorporate new and/or enhanced green infrastructure of an appropriate type, standard and size. Where on-site provision of green infrastructure is not possible, contributions will be sought to make appropriate provision for green infrastructure off site.


For those interested in reading more about the Core Strategy Policies see:

http://www.bristol.gov.uk/sites/default/files/documents/planning_and_building_regulations/planning_policy/local_development_framework/Adopted%20Bristol%20Core%20Strategy.pdf

Locally, there are several examples of the way in which this BCS9 policy can be seen to be operating in practice. There is  the street tree planting as part of the recent developments along Ashley Down Road where 17 new trees have now been put in. The small Sainsburys on the Gloucester Road near the prison should soon have 2 new trees planted outside. An avenue of Norway Maples will be planted on the new development at the Gloucester County Cricket Ground site. Trees, additional to those already on the site will also have to be planted by Sainsburys if their planned development at the Memorial Ground goes ahead though Clive is having to fight hard for trees to be included in the car park area.  Finally, even though St Andrews Park is very well endowed with trees, a ‘Tree Planting Plan’ is at present being formulated for the park. The local community is to be invited to take part in helping plant new trees there this autumn; an important first step in planning for a future well ‘wooded’ park, as its existing mature and ageing trees slowly succumb to disease or old age.