Urban Trees - Part 1

Much has been said lately about the value of trees for the environment. Among other things, trees filter the air and water, provide oxygen, shade and cool their environment, prevent soil erosion, and manage storm water. I envision this entry as the first in a series about trees in the urban and suburban environments.

American Forests, an organization that “works to protect, restore and enhance the natural capital of trees and forests”, recommends urban areas set tree canopy goals. For my hometown of El Paso, Texas, the organization makes the following recommendations:

Average tree cover counting all zones	25%
Suburban residential zones	35%
Urban residential zones	18%
Central business districts	9%

I wonder if 25% is a reasonable figure in what naturally is a desert shrub environment. I often fear that environmentalist underestimate the value of shrubs in their push to encourage people to plant more trees. Perhaps it is because the strongest environmental movements began in or focused on forested areas, but even forests have understory plants that serve important functions. Perhaps this discounting of shrubs is due to disparities in what is defined as a tree. In a previous post on this blog, I shared pictures of a few shrubs, including Honey Mesquite, Palo Verde, and Desert Willow, growing in a natural area near my house. All three of these are typically limbed up as trees in cultivation but in reality are large shrubs. Outside of oases, there are no true trees in the natural environment, so if large shrubs are excluded from the 25% tree canopy goal, it is quite unrealistic. If at least these three large shrubs are included, the 25% figure seems reasonable to me.

It is also common to under appreciate the value grassland. In a recent entry on his blog concerning invasive species, author and Master Gardener George Brookbank shared this tidbit about the natural history of the desert environment he calls home:

“The Santa Cruz valley was, within living memory, a broad valley where grass grew as tall as a man’s knees on his horse. Annual fires prevented the growth of trees and shrubs--it was only grass that thrived--until cattle were driven up from Mexico to the train station in Tucson. They ate the grass on their way north, of course, but they ate it so thoroughly that there ceased to be controlling grass fires every year and the mesquite trees moved in.

“The mesquite trees didn’t invade the area and kill out the grass. Man did it. Who is going to uproot the mesquite trees in the Santa Cruz Valley? Perhaps no-one because the mesquite trees are, of course, native plants and should be tolerated.”

Even though the area in which I live was previously arid grassland, I have never heard any environmental or governmental organization encourage me to plant native grasses. On the other hand, I hear and see encouragement to plant trees on an almost daily basis from several organizations through several different media. While they do not provide shade (to humans or our structures anyways), native grasses provide many of the same benefits that trees do, and they do so on unimproved soil with minimal irrigation.

In order to achieve the goal of a 25% tree canopy, El Paso, with an area of roughly 250 square miles, needs a total of 2.5 million trees with an average width of 30 feet. According to this document (pdf), hosted on the city’s website, El Paso’s actual tree canopy is only 0.1%. There is great disparity between the goal and reality, and we are not making up the difference with shrubs and grasses. In economic terms, the unemployment rate among native plants in our area is quite high; perhaps you have a place where you can exploit their cheap labor rates for a high return on investment.