Wednesday, November 2, 2011

The Life Expectancy of a Tree

I can go out into my backyard and put my hands on a big post oak tree that's been growing in that same spot since before I was born. Then I can step over about 40 feet, and lean against a pine that is perhaps half that old, though the two trees are the same height. In the best possible world, my oak tree will be around for a hundred years, and can be expected to outlive its neighbor by a quarter of a century.

Long-lived eastern red cedar.
But just across the fence stands an eastern red cedar—a wildly invasive pest that has taken over millions of acres of  grasslands in the Southern Plains, and is choking out vast areas of native deciduous woods. Hypothetically, that cedar could outlive both the post oak and the pine by a good 700 years.

Obviously, when it comes to trees, there doesn't always seem to be a lot of justice in the age distribution of different species.

So how do the trees in your own landscape measure up? Have you ever even thought about how long you can expect your valuable anchor plants to continue to fulfill their important roles? Perhaps you should. Because a  desirable long-lived specimen might be well worth the cost of restoration if it is damaged by an extreme-weather event, while a short-lived tree might warrant removal and replacement (preferably, with a long-lived species).

The ubiquitous Bradford pear is a classic example of a remove-and-replace tree, should it become seriously damaged or simply begin to "age out." The life span of a Bradford pear is 15 to 25 years—and the last five years are apt to look sad indeed as its lollipop shape tends to open out and spread. So...the first five years are devoted to establishing itself and taking shape, followed by a maximum of 15 years in good-looking prime time (if a severe-weather event doesn't substantially shorten that period—a distinct possibility), ending by at least five years of decline.

Short-lived Bradford pears.
If it seems that I spend a lot of time dissing Bradford pears, well, I certainly do. To that, I could add the river birch, despite the fact that the species' "Heritage" cultivar was named 2002 Tree of the Year by the Society of Municipal Arborists. In 2007, I saw many, many river birches destroyed by an ice storm in my community. Five years later, many more remain out there in landscapes, their severely damaged crowns still sporting visible signs of damage.

In this age of increasingly extreme weather, planting a tree that can't "take the heat" (or ice or wind or flood or whatever your region dishes out) makes about as much sense as packing a bikini on an expedition to the South Pole. A little forethought can save you from a lot of grief.

Every part of the country has its rogue's gallery of tree species that are vulnerable to the specific forms of extreme weather common to those areas. Some trees just don't have the structural habit or strength to stand up to the assaults that landscapes across the country are being subjected to these days.

So what does a tree's life expectancy mean to you and your landscape?

First of all, keep in mind that a long-lived species won't necessarily live out its full life span if  it is planted outside its desired growing conditions. And that means more than just its hardiness zone. You also need to take into consideration a species' soil and moisture preferences, as well as what extreme-weather conditions it can tolerate.

Over the ages, a tree species that is native to your area has evolved an ability to withstand most of the types of extreme weather likely to strike you landscape. (Tornadoes and powerful hurricanes are an exception— they have the capacity to uproot even long-lived native trees.) If these native species hadn't developed their weather-specific strengths, they could never had continued to survive and evolve the capacity to live very long lives.

So when I plant trees, I shoot for species that have a "normal" life expectancy of at least 75 to 100 years. These include oaks, maples and ginkgoes. Yes, these trees will outlive me. But I also know they will grace the places where I've lived for generations after I'm gone—a concept that I rather fancy.
Long-lived oak.

The benefits can be striking.

When the ice storm of 2007 struck, my heavily-treed neighborhood was hit particularly hard. Debris-removal trucks trundled up and down the streets week after week, hauling away shattered tree branches and trunks. The roar of stump grinders made pesky leaf blowers sound like wind chimes. But over on the next block, the sturdy oak shown at the right stood unscathed. The tree is probably about 40 years old now, and Nature-built for our heavy-duty weather extremes. With proper care, this tree should live on for generations longer.

You can learn the life-expectancy of your own trees—or that of a tree you think you might like to plant—by googling the species and "lifespan." For an overview of several dozen species, check out this site.

Then, before you buy another tree, pick up the phone and talk with your local cooperative extension agent or municipal forester (if you community has the latter) to find out if your chosen species is suitable for the growing conditions and extreme-weather events common to your location. (Besides longevity, you'll be wanting to know if your soil, drainage, hardiness zone, and other factors are compatible with your choice.

And don't be shy. It's perfectly okay to go out and hug your trees.

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