Michael Allen / Winnipeg Free Press
Hollow decayed ash tree with vertical bark cracks.
Mature ash trees can be beautiful on our properties and public open spaces, however, some do have a tendency for bark to split and may also split at the crotch.
There have been a number of strong wind events in southern Manitoba this summer.
It is not surprising weakened trees have a tendency to fail under strong winds.
Knowledge of susceptible trees in such conditions can prevent a great deal of damage by observing problematic trees and acting on those observations to make the situation safe.
You should be concerned about trees that have long vertical bark cracks and other trees that could potentially split at the crotch.
Ash and many other types of trees, especially poplar and basswoods, are known in the tree business as being potentially damaging trees if — and I stress if — they show significant signs of decay, hollow cavities, dead branches or physical damage.
It is not uncommon to see ash trees with a noticeable vertical split in the bark. The vertical split usually also occurs in the bark on the side directly opposite the visible side.
Over time wood decay fungus gets into the tree and hollows out significant areas. Trees with this feature should be inspected by an experienced licensed arborist.
A larger tree with a significant split in the crotch between two or more large limbs or trunks can cause damage. Initially, the wood of a mature green ash tree at the top of the crotch may be the first area to start splitting.
The vertical cracks on both sides of such a tree may be visible before the tree splits apart. The upper areas of the interior may become dark grey from decay fungi infection. The decay may be made worse by the wood’s exposure to weather conditions such as rain, snow and ice. The greyish decayed area may extend into the core of the tree. Vertical white-coloured wood fibres that are living wood called sapwood may also be present. They occur adjacent to the interior brown wood fibres, called heartwood. Grey regions of wood indicate they were splitting away from the decayed centred area in the not too distant past. Orange cream-coloured callous wood that forms from a vertical crack in the trunk’s bark and underlying wood may also appear. Callous tries to seal up wound damage. Unfortunately this is a very common feature in many trees in addition to ash. The callous strip is created when the living wood tissues (called xylem and phloem) located just under the bark are attempting to seal over the vertical crack. The increased weight from year to year of the branches does not allow the living tissues to seal over the crack, and so the splitting of the wood continues to occur.
The weight load on each of those limbs will have a tendency to increase pressure at the weakest part of the tree, namely the crotch. As the weight of the branches increases on both sides of the crotch, there is a tendency for increased splitting of the crotch to occur. Over the course of time, the newly exposed wood caused by the separation oxidizes and turns grey. The tree is giving us fair notice that its structure is being compromised by gravity. Ultimately the weakened wood will split apart.
A very serious situation can also occur when the crotch develops a major wood decay feature between two significant trunks of the tree.
A very dark, viscous region in the large crack in the crotch of a poplar tree may appear between two major limbs. The crotch is open to the core of the tree where decayed wood is producing bacterial wetwood and slime flux which are popularly called ooze. If left undisturbed in a tree, wetwood is not too serious a problem as long as it does not infect living twigs and leaves. The opening is enhanced during periods of high humidity, and is closed off during periods of dry, low humidity.
Older trees can be amazing features in our landscapes. They have evolved to withstand the environmental stresses caused by nature, and by us.
Learning how to read the condition of trees can produce very important lessons for us all.
Michael Allen M.Sc.F., RPF (retired) is a consulting urban forester, tree diagnostician and certified arborist. He owns Viburnum Tree Experts. He can be reached at 204-831-6503 or 204-223-7709. His website is www.treeexperts.mb.ca.
viburnumtrees@shaw.ca
