In the concrete jungles that many of us call home, you will find trees planted on sidewalks that may have only a few centimetres of dirt (the actual catchment area for rainfall for the trees) before the asphalt and bitumen take over.
And I’m assuming that under these city streets are contained sewer and stormwater pipes that will not allow the penetration of tree roots to find water.
Where the hell do city trees get enough water to survive?
Although there’s only a small area of exposed soil, there’s plenty of earth under the various surfaces - the concrete/asphalt/paving is fairly thin (compared to the depth of a large tree’s root system. Water and nutrients will soak down through many surfaces and moisten/nourish the soil below the hard surface.
There are pipes and such under there and the growing roots can often actually rupture or distort them.
There are also channels that were dug out to allow pipes to enter buildings, and the roots migrate into those channels. In my neighborhood, built in the 1920s, we are all having our sewer lines dug up and replaced because the roots have found their way into the sewer pipes and are causing blockages and backups. It may be sewage to us, but it’s a banquet to the trees!
Lots of trees on city streets round these parts have a short length of plastic piping protruding vertically from the patch of dirt (or slatted paving) around them. I don’t know this for a fact, but I assume that the pipe is used for watering, to get the water down near the roots.
Can anybody verify this?
I can sort of verify this, although the information comes from my mother, whose sole qualification is that she has lots of trees. Lots and lots of trees. The pipe isn’t exactly for watering the tree, precisely. Trees planted next to sidewalks will send roots out just below the surface, pushing that pesky concrete out of the way in a search for water. The plastic pipe allows water to be introduced under the tree; the tree then sends its roots down, rather than out in the quest for water. Eventually it begins to hit the moisture-rich soil that never quite dries out a few feet (or meters, depending on the type of soil) below that.
The pipes are for watering, but only for long enough to encourage the tree to seek water down, rather than out. The trees quickly become self-sustaining and no further water is introduced via those pipes. By the way, this assumes you are talking about low pipes, projecting a few inches above the ground, rather than the taller ones that trees are tied to to get them to grow up straight.
Using the pipes and watering through those while the tree gets established does seem to work, particularly in hot urban environments (such as Los Angeles) where much of the water would be lost to evaporation if one surface watered.
I apologize for the lack of cites, and hope this isn’t an old wives tale. It really did seem to work for me.
Most cities in the U.S. - and elsewhere, though I know less about them - have City Foresters, or to give them their fancy name, Municipal Arborists. Take a look at the web site of the Society of Municipal Arborists or just google on “urban forestry.”
Learning what trees will thrive in an urban environment is a discipline involving a lot of knowledge of local conditions and microclimes. Different species will do better in different places even within a city. Foresters have also learned to mix various species of trees along a block so that blights don’t destroy whole streets of trees, as Dutch Elm disease did to so many cities.
Maintenance is also a major issue, as proper care and pruning will keep trees alive and the lack of it, seen in many cities because of budget crises, will lead to their death.
And I agree that tree roots can be a real problem for underground pipes. Every year I had to call in my plumber and have him grind the outlet pipe to the sewer free of roots from a tree in my yard until we finally cut the blasted thing down. Although the tree was on my property and not on the “tree lawn” alongside the street, the same principle applies for those. Knowing what trees with what kinds of root systems to plant where is another aspect of the art of urban forestry.
Not that this was part of the OP, but many street trees are chosen because they do particularly well in high- Carbon Monoxide environments. Ginko trees and Flowering Pear are two examples of this. the more CO you throw at them, the better they look. Flowering Pear is a very popular street tree in NYC. Another popular street tree is the Norway Maple. Its pretty much the cockroach of trees – in fact if they get into a large park they are considered an invasive species (as are pin oak, another popular street tree) as they do such a good job of taking over. There’s a joke that the “generic maple-leaf” logo of the NYC Parks Department actually depicts a Norway Maple leaf.
BTW, Ailanthus aka The Tree of Heaven is never planted intentionally but is considered the most urban-tolerant tree of all. As evidenced by this link:
http://www.nps.gov/plants/alien/fact/aial1.htm
people are more concerned with getting rid of it than anything else.
–Once took a class in Urban Forestry–
Adding to Hello Again’s point, nearly as ubiqitous and “urban-friendly” as Ailanthus, at least in the Northeast and Great Lakes states, is Acer negundo, formally known as Ash-leaf Maple, more familiar to most people as Manitoba Maple or Box Elder (not Alder). Not only is it extremely tolerant of urban stress conditions, but it actually seems to thrive on it, probably because it has much less competition owing to other trees’ intolerance.
My hometown (Durham, NC) received an award 20 or so years ago for being a “City of Trees.” Back in the 1940s, a civic campaign raised money to plant mostly pin oaks along many of the streets bordering the downtown area. The trees were never pruned, or at least the city stopped pruning them and taking care of them.
Over the years, layers and layers of asphalt and new sidewalk have covered the ground around the trees, and at this point, half or more are dead or dying. The trees have caused innumerable problems with water and sewer lines, cracked sidewalks and streets, and when they lose their leaves all at once in the fall, they make it impossible to see lines on the road or stop if the streets are wet. In storms with high winds, lots of ice, or sustained rain in already soaked ground, trees fall over and pull up sidewalk with their roots.
The neighbors on my tree-lined street told me that the original owner of my house didn’t want a shady yard, and when the city came through and planted trees along both sides of the street, he pulled them all up. He apparently did this a couple of times. When I lived there, mine was the only house with a sunny yard and nice grass, and a decent sidewalk!
M.
As long as we are naming trees that survive in the polluted atmosphere of the cityscape, don’t forget the locust trees. In fact, IIRC, outside the Daily Center in Chicago are several honey locusts (or black locusts, I don’t remember which).