Measuring the Urban Forest

Please feel free to contact the Greenspaces Arboriculture Team for clarification or further information on Merton’s Urban Forest.


The Urban Forest encompasses the trees of a city in their totality – in streets, parks, schools, and gardens; private or publicly owned. The benefits trees bring to the population of a city know no boundaries so the Urban Forest of Merton is part of the Urban Forest of London, merging imperceptibly into the forested parts of the adjacent countryside. The concept unites all the benefits of trees in one idea.


The Urban Forest is an intimate mix and mosaic of ownerships. LBM has some influence over privately owned UF via Planning such as Tree Preservation Orders (TPOs) and Conservation Areas (links).

It has direct control of only a part of the Urban Forest – typically around a third to a quarter – in highways, parks, schools, etc,  

Canopy cover

Canopy cover is the percentage of the land area covered by the foliage of trees.

It is the most usefully convenient measure of the Urban Forest.

The volume of vegetation might be more informative but is far more difficult to calculate.

Tree numbers are not a relevant measure of the Urban Forest for a range of reasons to which will return.

Canopy can be assessed even with the limited resources available to LBM Greenspaces. The many technologies which can provide this data are fairly new, so there are omissions and gaps in our knowledge. However, Merton’s Greenspaces Arboriculture Team has undertaken a number of studies which are useful introductions to meaningful knowledge of our assets.

i-Tree Canopy and OSCCA (Open Source Canopy Cover Assessment) are the sampling tools used to assess tree canopy cover.

OSCCA was developed through the London Tree Officers Association, in which Merton’s Greenspaces Arboriculture Team play a significant role. It has the advantage of having been used in many other London boroughs so that datasets can be compared. It requires the use of GIS systems such as those used by us.

i-Tree Canopy is part of a free-to-use suite of tools for the same purpose but using Google maps and is available on the internet, requiring only a boundary map (shape file) of the area of study.

At present data is limited to the years 2012 onward but these surveys reveal a number of facts. Three surveys provide a range of different categories of information and determine that:

Merton’s Urban Forest Canopy is between 20.5 % and 24% of the land surface of the borough

Green Infrastructure (GI) of all types – trees and other natural surfaces and cover – are nearly 50% of the surface of the borough but less than one third of this is in Local Government control (12-15% of total area) as already noted above.

Close to half of all Merton controlled GI is made up of trees and woodland (c.6.5% of total area ). This consists of:

  • Individually identifiable trees 4%
  • Woodland cover 2.5%

Further studies are needed to confirmation and expand our knowledge base.

For instance we need:

  • Studies with more recent aerial mapping or historical surveys with older aerial sets to indicate our direction of travel – increase or loss of canopy cover
  • Comparative studies to discover the suspected extent of loss of private garden space and trees
  • A study to map the extent of Highway tree cover as being the most promising location for future canopy increase and the most useful from the point of view of public health and wellbeing  (air quality among other measures)
  • General repeat surveys for the same purpose as any other similar ‘safety check’ would be carried out to ensure the health, sustainability and longevity of any public assets


We use CAVAT – (Community Asset Valuation of Amenity Trees). This puts a monetary valuation on both individual tree and the whole urban asset.

Inspection generates the the monetary value via a background formula. This is updated automatically as the tree condition and size changes

The valuation of the Merton sector of the Urban Forest is currently calculated at approximately £1 billion.

Woodland, Groups and “Parcels”

Many trees are measured and surveyed as groups and “Parcels” and, like individuals, are digitised on maps and thus have area, rather than number in calculations, though a number of stems is included for valuation purposes

Fellings are seldom recorded individually as they are irrelevant in woodland management terms, unlike with, say, Highways.

Coppicing an area would count as felling by any criteria which sees cutting at ground level to be felling. Most UK (temperate zone) species are not killed by cutting at ground level.


Trees of a significant size (greater than ‘whips’ and typically 12/14 centimetres girth) are sometimes required to be planted for immediate effect or to avoid the vulnerability of very small specimens in heavily used areas:

  • 2012/13- 190
  • 2013/14- 145
  • 2014/15- 184
  • 2015/16- 120 current estimate.
  • 2016/17 - 115

Private trees

Please see the planning web site

We have no viable estimate of tree numbers for the whole of the borough and have neither resource nor inclination of attempting such an estimate. Canopy studies referred to earlier are certainly needed to determine whether this resource – available to all in many ways – is growing, shrinking or stable – an important consideration in strategic terms.


As we noted earlier numbers of trees are of very little significance. We only record numbers as an incidental towards details of tree health or risk. The measure of the extent or quality of the Urban Forest is unrelated to total numbers and  numbers are misleading by-product  of survey.  We identify species, location , size, condition , form, etc, to help determine our management actions.

We are nevertheless often asked for such numbers. We emphasise the lack of utility of these fluctuating facts but give a snapshot below.

December 2015 Parks totals

  • 14753 trees

Trees removed:

  • 2012/13 - 106
  • 2013/14 - 114
  • 2014/15 - 143


  • 2873 trees

Tree inventory is a constantly changing field as we manage a huge urban forest and tree asset, with pruning, felling and other works taking place constantly and growth always taking place.