This was initially going to be my last post on widely unknown functions that trees can provide, but it got long so there are going to be more of these. Previously I wrote about their ability to increase cloud creation and precipitation, as well as some trees’ ability to manage their own hydrology by actively pumping water from wet areas to dry ones. Bear in mind that i’m not covering all the functions trees can provide–just those that i think are relatively unknown, and highly relevant to the work of afforesting deserts. This post is about how trees affect air quality in urban settings, and how that affect could be used to dramatic effect in an arid setting.
There have been various studies documenting the value trees create in urban environments by reducing dust and pollutants. This value is typically calculated by summing up the costs to society these pollutants and dust would create if the trees were not there to reduce their impact. These values accrue specifically in the fields of air quality & public health, energy, and flood and drain management, and are valued in the hundreds of millions of dollars per year in large metropoli.
Trees, Dust, and Pollution
Trees remove a host of pollutants including nitrogen dioxide, (NO2), sulfur dioxide (SO2), ozone (O3), carbon monoxide (CO), and particulate matter of ten microns and less (PM10), including fine particulate matter (fpm) of less than 2.5 microns. Particularly with the fpm, a study in the UK found that urban trees reduced fine particulate matter by 50%.
This is not simply an issue of breathing easier. Regularly breathing fine particulate matter of 2.5 microns or less is proved to be a direct cause of ischemic heart disease, dysrhythmias, heart failure, and cardiac arrest, increasing the risk of these diseases by 8 to 18%. The other pollutants are known irritants that cause emphysema, bronchitis, asthma, cancer, or even death (like with carbon monoxide poisoning). High concentrations of the sulfur oxides are associated with increased visits to emergency rooms, and increases of death in at-risk populations such as the very young, very old, and those with already existing cardio-pulmonary diseases. Notice in the chart above the sources of fine particulate matter–these are very common in industrial desert cities.
David Nowak, Ph.D., of the USDA Forest Service calculated that the trees in New York City save the city approximately $60,100,000 per year in health costs alone. On small scales, strategically planted trees have been found to reduce pollution by 50% on urban and suburban lots.
What About Dust Storms?
Of course urban trees work primarily on pollution that is created within the city but what about small particulate matter and pollution, such as that carried by huge dust storms, brought from outside?
First, it should be noted that many dust storms that hit urban areas are the direct result of deforestation and agricultural dust bowls, such as some that have occurred in Australia, Iran, Pakistan, Northern China, and the American Southwest. What can trees do about these storms brought from outside?
Second, these storms are created by strong winds impacting bare soil, which cause fine dust particles to be swept up into the atmosphere. The winds are driven over and over toward the ground, thus picking up more and more dust, and unless they face major barriers will sweep into cities, causing the health risks I mentioned above as well as shutting down traffic and businesses.
This cycle could be eliminated by using waste water from cities to plant belts of forest on their windward sides, which would intercept the wind, add humidity to the air (which would weigh it down, slowing it further), and intercept huge amounts of dust. The density, size, and frequency of the belts would have to be designed commensurate with the severity of the dust storms that seasonally sweep in–in places like Riyadh (see the picture below), you would probably need multiple belts probably 300-400 meters wide. The design would have to intercept all dust at 100 meters or lower, and cause the rest of the dust to sweep higher up into the atmosphere.
It also bears mentioning that in cities where the dust storms are the direct result of deforested areas, particularly those areas where forest was replaced by monocropped agriculture, it would be better simply to manage those forests as a productive landscape than to eliminate them.
Imagine putting wasted waste water to use in cities such as Jeddah, Riyadh, Baghdad, Cairo, Phoenix, and others, by planting dense food forests to block out these storms! Imagine the improvement in the quality of life, if the skies in these cities were blue instead of hazy yellow from pollution and dust! Imagine children with asthma and grandparents with heart diseases feeling like it is okay to go outside! Aside from the ecological services, the forests could provide a host of other resources–jobs, food, timber, fuel, forage, medicines, ambers, incenses, gums, nuts, oils, tourism, hunting, and more, and it could all be done without negatively impacting the ground water situation. Imagine, because once we have imagined it, then we can start to do it.