TDA 134 Beauty before the Beast
Tree Damage Alert No 134
11th January 2010
Beauty Before the Beast
The longest winter cold spell in Britain for thirty years included locally some very significant falls of snow causing disruption to travel and many abandoned vehicles. Those fortunate enough to be able to avoid travelling enjoyed the beauty of snow laden vegetation and the contrasts of dark tree stems silhouetted against the pristine white of the snow. Unfortunately, such weather conditions have two very significant impacts on the trees we manage!
There are two types of snow (see TDA 131 What’s next?) but on both occasions so far this winter it was large fluffy flakes that fell for at least part of the time. These have the ability to cling together and make good snowballs, and snowmen. Hopefully children with a windfall holiday were allowed to exploit these properties and enjoy what have become uncommon winter conditions in many parts of Britain.
This clingy property of large fluffy snow flakes allows them to accumulate on branches, and, if there is a wind at the same time, the deposit is also along one side of the trunks. This can have two effects. Branches, or even whole trees may brake under the cumulative weight. Particularly vulnerable are evergreens notably Cedars (Cedrus spp.) and there have been reports of trees in this genus suffering – some would say destroyed by the loss of branches.
A more immediate effect this winter has been the blocking roads because trees bowed under the weight of snow. In some instances the tops of trees bent and touched the ground. It appears to have been this that caused most disruption to traffic rather than either parts of trees actually fracturing and shedding material, or trees toppling. This bending of trees and causing transport problems has been described by the media as “Trees being brought down onto the roads.”
Actions and Options
Trees, particularly those in high risk situations, should be inspected to identify branches that have twisted and developed longitudinal splits. These branches, like damaged safety helmets, may fail under a much lesser force than the one that caused the initial weakening. Work should be programmed to reduce the threat such branches pose to people and property.
[NOTE: Branches and stems displaced by a load of snow should be cut with caution to avoid sudden fracture and hazards for the pruner.]
The significance of branches and even whole trees being displaced and bending, will become clear with time. If the snow melts quickly the pressure on the trees may have little or no lasting effect. Prolonged displacement by the weight of snow is likely to leave permanently bent and displaced trees and branches. Decisions will be needed about how to manage these trees – prune or fell.
Lessons must be learned from the occurrences this winter. Why have some trees bent under the weight of snow and others not done so?
It is generally the etiolated fast grown pioneer species that have succumbed to the weight of snow (e.g. Birch and some conifers). In woodland such trees would normally become shaded by other trees growing around them. This does not happen on roadsides because there is light available from the side which encourages one sided and etiolated growth. Once these trees are recognised there is need for silvicultural thinning to make space for retained trees to develop to their fuller potential. Selected trees should then provide a more durable, and less problematic resource along the transport routes.
Derek Patch and Harry Pepper, AAIS, Farnham.
AAIS l Alice Holt Lodge l Wrecclesham l Farnham l Surrey l GU10 4LH
Call the Tree Helpline on: 09065 161147
Calls cost £1.50 per minute