ENVIRO NOTES: Don’t just plant any trees

City of Surrey's tree canopy strategy requires critical thinking

A recent report to city hall has put quite specific numbers to Surrey’s loss of tree canopy, something that has been apparent qualitatively to even the casual observer.

We know now that there has been a reduction of almost 18 per cent in tree-canopy area during the past 13 years.

That number raises alarm bells because of the value of trees, which is well recognized.

Not only are trees a renewable source of wood. They provide habitat for birds and small mammals; they can grow edible fruits; they serve as wind-breaks and provide shade; they store carbon and release oxygen; some of them fix nitrogen and are thus soil improvers; their leaf fall promotes water infiltration into the soil and diminishes run-off; the roots of some species penetrate deep into the soil bringing up and recycling stored nutrients; they have aesthetic and spiritual values that cannot be replicated anywhere else.

It’s not hard to understand why trees are important, but it’s too simplistic just to say we should have canopy cover of 40 per cent, an arbitrary and unsubstantiated number.

It’s much harder to decide what to do about the removal of trees.

If buildings are to be constructed, the site must be cleared, and that means taking out trees. If some are left standing close to the building excavation, it’s likely their root systems will be damaged thus compromising their stability and future health.

It’s much preferable to retain groups of trees that will be mutually supporting rather than isolated individuals, and their differing management needs must be addressed.

There’s more to the conundrum. In our climate, trees grow quite rapidly, and so attractive views can become obscured in just a few years.

Is that a good enough reason to fell the offending trees?

And what about the risks of wind-throw?

Several difficult questions arise when there is a legal requirement to replace trees that have, of necessity, been removed from a building site.

Should we favour native species over introduced ones? Are conifers preferable to deciduous or hardwood species?

Apart from the Pacific yew, all our native conifers will grow to 70 metres or more in height; in its early years Douglas-fir will put on a metre or more in height annually. How will trees of that size fit into the planned construction over time?

Though they are called ‘evergreen,’ conifers constantly shed needles – actually small leaves – but not all at once, so there’s a continuing cleanup issue.

Deciduous species, on the other hand, shed their leaves over a few days or weeks in fall, so that they can provide shade in summer and let sunlight in during winter.

But not all our native hardwoods are suitable in built-up areas.

The big-leaf maple can grow to 50 metres or more with a spreading crown; poplars, also called cottonwoods, are shallow-rooted, tall and subject to sudden breakage; neither nitrogen-fixing alders nor birch are very long-lived and they can begin to break up at about 70 years.

Fortunately, there are at least five smaller native deciduous species which can be utilized – cherry, crab apple, dogwood, Garry oak and vine maple, each with its particular site requirements.

If it’s decided to use introduced species, the number of possibilities increases greatly but also potential additional problems, such as disease introduction and incompatibility.

Please do not read into this account any defence of tree removal or uncritical wringing of hands over loss of trees. Rather, it’s a call to think long and hard about how our desirable tree canopy may be preserved, enhanced or, if need be, replaced.

Dr. Roy Strang writes monthly on the environment for the Peace Arch News. rmstrang@shaw.ca