Woodland Planting
Trees are planted in an area with a given spacing between trees. Total canopy area is limited to the area planted.
Individual trees
Each tree can develop a full canopy, so the total area is the sum of the individual canopies.
Tree loss
Trees lost in one year or slowly over a number or years.
The same plan can be undertaken for a number of years in succession.
Examples
2026 BCC plan to plant 2300 trees over 3 years
Bristol City Council announced a plan to plant 650 trees in 2025/6 and 840 trees in each of the following two years to replace trees lost to ash dieback and as part of the effort to reach 20% tree canopy by 2040. The claim is made that this will increase tree canopy by 10 hectares when trees have reached full maturity.
It is assumed that these trees will be planted as individual mature trees. The modelling period relevant to the canopy goal is not full maturity, which could be 100 years but 2040, in 14 years time. This model predicts that the programme will add around 2.5 hectares by 2040 and 23 hectares in 100 years time. This programme will make a significant contribution to trees in streets and parks, provided planting spaces can be found, but the estimated cost of the programme is around £700,000, so it shows just how difficult it is to achieve the 2040 goal.
Modeling approach
Tree growth rate
The first step in modelling the growth of a tree requires the prediction of the Girth (circumference) or equivalently the Diameter at Breast height (DBH) over time.
There have been a number of studies of the related problem, that of estimating the age of a tree given its girth.
A commonly quoted rule is that trees put on an inch of girth per annum i.e. 4mm radius per annum.
In practice, growth rates depend on species and the site. One source of data is the paper by John White from the Forestry Commission
Information Note FCIN12 "Estimating the Age of Large and Veteran Trees in Britain"
In this model, the tree puts on rings of constant width up to the age of maturity, and then rings of constant area until senescence.
This model is also used in our Tree Age Calculator
Another source which we have not yet integrated is this paper by a Veteran Tree Association