The Lifespan of Trees
By Jim Hole
When it comes to understanding lifespan, most people have a pretty good grasp of the concept--at least when it comes to animals. But we’re not quite as clear on the lifespans of many plants. Everyone knows that bedding plants are only supposed to last one season, but when it comes to perennials, trees, shrubs and hardy roses, sometimes gardeners forget that these organisms, like any other, are mortal. This especially applies to trees; people seem to expect them to live forever, and often can’t figure out what they’re doing wrong when venerable trees begin to die. The truth is, you may have done nothing wrong at all; the bell may simply be tolling for your tree, so to speak.
Old Soldiers, Fading Away
While it’s true that a few trees can and have lived for thousands of years, most typical backyard trees will experience considerably shorter life spans. Older trees have one obvious feature that separates them from their younger cousins: they are a lot bigger. And although this isn’t a startling revelation, being taller and broader does have some serious implications on tree health. Water and nutrients travelling through an older tree have a greater distance to cover than those travelling through a younger tree of the same species, for example. As a result, there’s more opportunity for that journey to go awry.
Cavitation, or the breaking of the continuous, cohesive chain of water columns can be a problem in large trees. When a solid column of water breaks apart, an air bubble is formed that is comparable to an embolism in a human body. The air bubble can force water to take an alternative route. Sometimes the rerouting causes certain branches to lose their water and nutrient supply, leading to their eventual death. If enough branches are affected this way, the tree can be seriously weakened.
Drought conditions frequently put a greater strain on older trees than younger ones because older trees require more water due to their size – the greater the number of leaves, the larger the water loss due to evaporation.Therefore, one key component of managing older trees is to provide them with lots of water, particularly during prolonged periods of drought.
Another question that many homeowners ask about older trees is whether or not to fertilize. The answer is yes, but sparingly. With younger trees, the goal of applying fertilizer is to promote growth; with older trees, the priority should be to maintain health. A large dose of fertilizer won’t help an older tree. It shifts the tree’s biochemical balance in favour of producing new branches and buds while sacrificing the production of a tree’s defensive chemicals, used to ward off pests. So while an older tree will likely produce more leaves when it gets a surge of nutrients, it will be less able to fend off insect and disease attacks.
Finally, pruning is often the last thing that one thinks about with old trees, unless a branch is about to tumble from the tree and damage the house or car. Pruning large old trees requires specialized equipment that homeowners simply don’t have, and therefore pruning can be expensive if a professional arborist is hired.
Nevertheless, pruning is beneficial to old trees. Perhaps the most important reason for pruning older trees is to reduce the risk of limbs falling off. But pruning has other benefits: it reduces the distances nutrients and water must cross, and therefore also reduces the risk of cavitation. It also eliminates dead branches, removing them as a harbour for insect pests and as an entry point for disease. Keep in mind that pruning a little every year is far better than pruning in one big session once a decade. Just as with human beings, steady care over the years is far better than emergency surgery.
To Everything a Season
Although they are mortal, with proper care most trees will have a long and rewarding life. They key is addressing their changing needs as they grow older, a practical approach that, when you think about it, applies whether you are a tree or a human.