Usually the southwest side of the tree bark heats up on a sunny day causing some cell activity under the bark. Often the heat is strong enough to cause some sap to flow. Freezing temperatures at night cause the watery sap to expand, thereby damaging the cells directly under the bark. These cells cannot expand as the tree grows in the spring and summer. The circumferential expansion of the tree in the spring causes these weak, damaged areas to crack open.
Open bark cracks lead to problems:
* Wood-decay fungi and water enter the core of the tree causing the spread of wood-decay organisms that weaken the trunk.
* Open cracks allow the penetration of severe cold that desiccates the wood. This effect further damages the wood, causing it to weaken and create wider cracks.
With the return of warmer weather in the spring, most bark cracks start to close as the wood becomes more humid. The observant person will notice that the tree tries to grow a new layer of wood along the edges of the crack with the aim of sealing the opening. On a few occasions in small trees, this actually succeeds; however, the seal is rarely complete. As the tree grows, there will normally be a raised ridge of wound-closing wood trying to seal the opening.
Annual bark cracks can lead to the failure of the wood to hold up the tree. Internal decay becomes so widespread that the tree can collapse during a strong wind.
Have the tree examined by a professional, certified arborist for possible repair before a decision is made to remove it. Several bolts inserted through the entire trunk diameter and secured with washers and nuts can significantly extend the life of the tree. Other factors that may play into the decision are the lean of the tree; the presence of many broken branches; the presence of two or more very large, wide-spreading branches; a lopsided tree crown; and decay cavities in the trunk, especially at the base of the trunk. This list is not meant to be inclusive but does give one food for thought when you want to repair a tree. Depending on these issues, wide-spreading upper branches may also require cabling, especially if there is a visible crack in the crotch between two or more very large branches.
I recommend that owners of trees consult with ISA-certified (International Society of Arboriculture) arborists to have the tree assessed before any work is done. In addition, always ask to see the arborist's valid (current year) Manitoba Arborist Licence.
Mike Allen is a consulting urban forester and certified ISA arborist with Viburnum Tree Experts. He makes house and garden visits to assess tree and shrub problems. He can be contacted by calling 831-6503 or by e-mail at viburnumtrees@shaw.ca You can also mail questions to Mike Allen, c/o Newsroom, Winnipeg Free Press, 1355 Mountain Ave., Winnipeg, MB, R2X 3B6. His website is www.treeexperts.ca
