Importance of Urban Trees.

by Dr. Tim Ball on September 23, 2012

in Atmosphere,Data,Land,Theory

Many are concerned about forests, but urban trees are relatively more important. Urban environmentalists constantly complain about forestry, but extent of forests has increased as much as 30 percent on most continents. We are close to a renewable cycle. Replanting forests (silviculture) that has been going on for upward of 60 years are now reaching harvestable size. Forestry is close to becoming forest agriculture. Similarly, there is less land under cultivation in Canada and many other countries now than there was in the 1930s.

I had the privilege of giving the keynote address at the first conference on urban forests in Canada. Ironically, it was shortly after I’d spoken to a conference at the University of Toronto that was closing its forestry faculty because authorities decided forestry was not necessary in an urban university. Why they chose that faculty is hard to determine because little that goes on in universities is of much relevance to the real world – it’s what they mean when they say, its purely academic. As someone said,

“A professor is a man whose job is to tell students how to solve problems of life which he himself has tried to avoid by becoming a professor.”

A major issue in climatology is the difference in temperature between urban and rural areas. Known as the Urban Heat Island Effect (UHIE) it was first measured by Tony Chandler in London England and described in his 1965 book, The Climate of London. A big reason for the difference in climate between urban and rural areas is the change of the surface from vegetation to impervious surfaces like asphalt or concrete. The centre of the city, defined as the Central Business District (CBD), is 100 percent impervious surface, while the average suburban property is approximately 40 – 50 percent. (Figure 1).

This reduced grass and tree cover means quicker runoff of precipitation and reduced evaporative cooling among other factors. The color of the surface is darkened and this changes the albedo. This is a measure of the difference between solar energy absorbed or reflected – it is why solar panels are black. Treed and grassed areas in cities are measurably cooler than the surrounding urban area. In studies of Winnipeg, Manitoba we found a one-city block park measurably warmer in winter and cooler in summer. Other’s had similar results, such as Tim Oke’s measurement of the impact of Stanley Park in Vancouver.

Figure 1. General surburban lot with impervious surfaces.

Trees also serve another function because they cleanse the air. They transpire oxygen, water vapor and other gases, but absorb CO2 and other gases. Sometimes this is “exhale” isvisible, which is why there are so many “blue” mountains around the world, such as the Blue Mountains of Kentucky. Some species survive rigors of urban air and often become symbols of the city. The Plane tree that Americans call the Sycamore, lined the streets of smog bound London and survived because the bark sheds thus shedding the pollution. Interestingly, this changing color of bark was used to illustrate evolution because pepperd moth came in light and dark. As tree bark became blackened by industrial soot camouflage changed. The black blended and thrived, the light became apparent and numbers decreased.

The second line of Joyce Kilmer’s famous poem,

“I think that I shall never see a poem lovely as a tree, says, “A tree whose hungry mouth is prest against the earth’s sweet flowing breast.”

It speaks to the dependence of the tree on sustenance from the ground. A tree structure includes the trunk that supports the canopy with both supported by the root system. It usually divides into two parts the taproot that anchors the tree and the spreading roots that add stability are the major source of water (Figure 2). When rain falls on the tree, a portion is intercepted and evaporates back to the air, the rest is shed to the outer rim of the canopy. You can stand under a tree for some time before water penetrates. The water is shed to the outside to feed the spreading roots.

Figure 2: Tree root system exposed by erosion.

For many urban trees this is a problem because the impervious surface goes right up to the trunk. Figure 3 shows a tree in downtown Victoria, British Columbia, but you can see this in most cities. It clearly limits the ability of the tree to get water, but most vegetation will put down roots a long way.

Figure 3: Tree on Fort Street, Victoria BC.

Notice the water in the gutter unavailable to the tree.

For example, one species near the Sahara had roots down 100 m. During the Prairie drought of 1988/89 University of Manitoba researchers found wheat roots down 3 m. Urban trees do the same thing, but it takes a delightful form of revenge. City engineers tell me roots work down into sewer and water pipes causing many problems. It’s only fair trees get the water engineers denied them in the first place.