Sunday 7 August 2005

Trees a Record of Time

Trees record the passage of time in the texture and size of there yearly growth rings.

Amazing research is being done by Dr Tardiff at The University of Winnipeg. Dr Tardiffs specialty is measuring the age of trees and extracting climactic data based on the size and characteristics of the tree growth rings. This process is known as dendrochronology. The word means literally tree-time, and gives historical and prehistoric climate data for locations in Manitoba. This research goes way beyond counting the rings of trees and estimating their age. So how old is my tree? This question is a frequent one and not always easy to answer. It’s much easier to answer in the past tense, how old was my tree. This is best done after the tree is cut down. There is no easy way to count growth rings in a living tree. It can be done, but not without injuring the tree. A skilled arborist with good local tree growth knowledge can often approximate the age of a tree by comparing it to similar trees that have been taken down in the area.

A tree growing in the forest may live to be hundreds of years old. This is the case for the Old Oak Tree in Souris, Dr Tardiffs studies have located stands of black spruce in the Duck Mountains that are over 300 years old. These are examples of trees that have lived in natural undisturbed environments. As the level of disturbance increases, the life span of the tree decreases. One thing that is known is that site conditions affect the average survival rates of trees.

Research by the International Society of Arboriculture has yielded interesting results on the lifespan of urban trees. All trees need three basic things to survive, soil, air and water. In our increasingly urban environments any one of these could be lacking or present in a form that is not best suited to a tree’s needs. Extreme climactic events do play a role in weeding out the weak and aged trees but are relatively rare. A tree planted in an urban park will last approximately ½ the time it would in nature. Given this fact and the trees we have in our parks this works out to approximately 75 years. In a suburban lot tree life is again halved to 35 years. The trees planted in cut outs in sidewalks can be expected to live 7 years. By comparison how long would you expect a goldfish to live in a tea cup? The single most limiting factor in tree survival is soil compaction. Soil compaction is the breakdown of soil structure through vibration or weight being applied to the soil. Compacted soils are so dense that they exclude air and water that is essential to root growth. Trees without roots don’t survive very long. Soils that were once compacted around Roman villages in England can still be identified from the air by differences in the colour and vigor of the trees and vegetation growing on them. These trees and grasses are less able to survive drought and other stress factors. Romans traveled on foot and built there roads and settlements with out heavy equipment. Two thousand years later we can still see evidence of soil compaction. If you consider the impact of vibratory packers, skid steer loaders and other modern building equipment, the long term impacts of construction must be even greater than we expect. The only effective solution is to avoid soil compaction in the first place and to mulch properly trees to increase the organic portion of the soil. So why plant a tree at all? If you ask this question you may have missed the point. Even if a tree lasts 7 years it still contributes dollars to our local community and increases property values by as much as 10%. Urban trees increase in value through out their short lives. Trees are the only urban public asset that increases in value with time.

It’s worth it to keep planting trees even if they may not last forever. Check with your local nursery fall is an excellent time to plant a tree!