Trees in London, by Rebecca Lardeur
Please find below the updates on Rebecca’ research into urban trees.
Putting a monetary value on trees has been one way to fight damages inflicted to urban trees. In the UK, the system which is often used is called CAVAT (Capital Asset Value for Amenity Trees)
Cavat was finalised by Christopher Neilan in collaboration with the London Tree Officers Association in 2008. The system was created to put a financial value on the loss and replacement of individual trees and has since been used in legal and planning cases in boroughs and local authorities. The photo above is a 300 years old London Plane estimated at £1.6m in Islington. Southwark has estimated its trees at £143m, Sheffield at above £11m, and Tooting estimated 56 Chestnuts tree at £2,639,562. However, the system does not take into account environmental services the trees bring such as flood control, water retention, cooling and greenhouse gases intake.
Cavat has two methods: the Full and the Quick one.
The first looks at 1. the basic value of the tree by its height and multiplied by a unit value factor, 2. the surrounding population size living around the tree, 3. the location of the tree, 4. tree’s health, 5. calculating its amenity value, and 6. its life expectancy.
The latter strips most of the above and focuses on step 1, 2, 4, and 6.
Although it is exciting to see such systems, there are some ways CAVAT could be enhanced to calculate and value climate mitigation benefits.
Currently, the ‘amenity’ value of the trees is left to be calculated by the officer on the case. The amenity covers wildlife, habitat, setting, and heritage but is left to the subjectivity of the officer and wouldn’t affect the end value of more than 60%.
Also, the Unit Value Factor is focused on the nursery gate price and the planting cost of an averaged 10 trees for the whole of the UK. Currently, the UVF is set at £16.26. By averaging the value, you miss on the species’ diverse attributes and the importance of local weather conditions.
Photo by Islington Council on Twitter.
You can find trees in the urban environment on three different types of land: private, park, and street.
To experiment with these categories amid lockdown, I used Google Maps and Street View to classify trees in a space of 200 square meters on the west side of Clissold Park. I live close by and often walk there.
I calculated that the highest number of trees was on private land. This is not to be taken as a London average but is a good insight regarding urban trees proportions. In my garden alone, there are five trees: one ash, two different types of palm trees, and two bay leaf trees. The council very likely do not know about them and those trees appear in no official reports.
In comparison, street trees and park trees are monitored by Tree Officers. They trace them and decide which species are planted. Those trees are usually cared for during their few first years before being left to grow on their own.
Each category of trees encounters different benefits and challenges. They face different levels of pollution, invasion from humans and animals, and receive different levels of care.
Urban trees have one massive common aspect: they are seldom ‘born’ on the land itself: they come from tree nurseries. More on this soon!
One way trees in cities differ from their forest counterparts is where they grow. In forests, trees often spring directly from seeds which fell on the ground or were placed there by animals. In cities, most trees come from Tree Nurseries to the exception of some trees in private gardens and old parks.
Nursery trees are organised, monitored, pruned, and also have passports (< EU law requirement). They are cared for until being sent to be planted in their new environments.
Each tree nursery develops its own way to grow trees. There are three main categories, with many different schools within each: Bare Roots, Containerised Trees, Root Balls.
The common aspect between nurseries is that they cut roots early on a tree's life to ease transportation. Keith Sacre, who works at @barcham_trees, is very much aware of roots defects in nursery trees leading to death within their first few years after being planted. To ensure a good survival rate, Barcham focuses on containerised trees.
Little is known about the roots system, especially in the urban environment. I will speak more about this next!
The roots are one of the most important parts of a tree, and yet there is still a lot to learn from them, especially in the urban environment.
Roots create an underground network supporting trees' abilities to take and give nutrients as well as oxygen (with the help of fungi). In plus, they help communication between trees and stabilise their bodies. They might grow shallow or deep, wide or narrow. It depends on species.
In the context of London, roots grow in a soil more condensed, mixed with rubbles from centuries ago, and which is limited in space. Roots grow where opportunities are, most often than not aiming for air.
Image 1 is a re-work of scientific diagrams made by Thomas O. Perry for his article 'Tree Roots: Facts and Fallacies' published at Harvard Arboretum in 2016. The drawing illustrates 9 different tree species' roots, species which are common in North America.
Image 2 is a re-work of scientific diagrams made by Claire Atger, a French researcher and doctor in botany. She regroups roots under two subcategories: the perennial roots supporting the anchoring of the tree, and the deciduous roots helping the tree to colonise and exploit the soil.