Users of The Common will notice some tree felling and other tree surgery taking place over the coming weeks. These trees help to form the historic landscape of this wonderful piece of public open space, especially the impressive Horse Chestnut trees. However, there are some measures that we are having to take both to protect the safety of the public and the overall health of the trees.
All trees on Town Council-owned land are inspected every three years - we are responsible for about 1,500 in all. During this inspection they are examined by a qualified aboriculturalist and the overall health of the tree is assessed for what pest or disease might be present, and how vigorous the tree is when in leaf.
This can be assessed by simple visual indicators (leaf size and shape) or by more thorough investigation of suspected rot. A tree might look healthy to the untrained eye but might be rotting from the inside out or suffering from a root fungal infection which will cause sudden death. It's worth knowing too that there are tree pathogens/diseases that are ever present in the soil, waiting for an opportunity to attack a weaker tree.
On The Common there is Dutch Elm Disease and Horse Chestnut Leaf Miner. Here's some information about them:
Dutch Elm Disease is spread by a beetle which flies at a certain height above ground and when it encounters an Elm at this height it bores into the wood and lays eggs. The beetle and its eggs are covered with Dutch Elm Disease (a fungus) which flourishes once introduced under the bark. This fungus then grows in the water carrying tissues of the Elm Tree, effectively killing it by blocking off its water supply. The fungus only grows in the parts of the tree above ground and does not affect the roots so, given time, the roots send up shoots again which grow to be trees. The whole cycle from infection to death, to re-infection of new trees, is about 17 years. So Elms never grow as tall as they once did and need frequent monitoring. New disease-resistant varieties are now available for fresh planting.
Horse Chestnut Leaf Miner is an insect that buries itself in the layer of cells in a leaf and then eats the inner tissues - a 'leaf miner'. It's not the only insect that does this but, it is one of the most voracious. The inner layers it eats are the photosynthesising area of the tree. A tree affected by this insect can see quite a drop in vigour, losing nearly 75% of what it should get from photosynthesis. The Horse Chestnut typically responds by flowering much more than normal. The insect does not kill the tree but, after many years of affecting it, weakens it significantly so that diseases already in the soil can attack and kill it. Effectively, the tree is starved until it is too weak to fight the fungal attack. Unfortunately, this is what is happening on The Common and why those trees weakened by years of leaf miner attack and in close proximity need to be felled - they have finally succumbed to fungal attack.
With both Dutch Elm Disease and Horse Chestnut Leaf Miner, we are faced with severely weakened trees that are now susceptible to sudden death and at risk of falling within a year.
The Town Council will be looking to replace some of the felled trees.
More information about trees is available at http://www.woodlandtrust.org.uk/