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It is worth reflecting on how trees contribute to our lives—beyond their role as a source of wood and paper. Should we preserve trees? Are some species better than others? Are large trees better than small trees? These and a host of other questions must be carefully considered if landscape architects and town planners are to design the best spaces for human habitation.
Plant growth derives from photosynthesis, in which plants take up water from the soil and carbon dioxide from the air to produce carbohydrates for their growth. Oxygen is released as a byproduct. About half of all photosynthesis on Earth occurs on land and about half of that occurs in forests.
Trees and Carbon Dioxide:
Tree growth is a means of slowing the rise of carbon dioxide in Earth’s atmosphere, to stem global warming. Indeed, forest destruction is responsible for about 20% of the human-caused emissions of carbon dioxide to Earth’s atmosphere, so the preservation of old forests is an important part of climate-change solutions. Urban areas cover less than 5% of the land surface, so the contribution of trees in urban landscapes to total carbon uptake by plants on land is likely to be rather small. Generally, trees in urban and suburban areas are less productive than the natural landscapes that they replace, but trees are always more productive than open grasslands and farmland. Young, planted trees grow rapidly, so major programs to plant lots of them in cities result in carbon uptake of some significance.
One might argue that if young trees grow faster than old trees, then why not cut old trees and plant young trees in all areas. Reality is not that simple: the benefit depends strongly on what happens to the old, cut trees, the species and density of the new, young trees, and what other environmental attributes are important to us. Let’s look at each of these briefly.
A variety of studies show that when a large, old tree is cut, nearly all of its carbon is returned to the atmosphere within a few years—at a rate which is often much greater than the carbon uptake by young trees that may have replaced it. If a tree is burned or allowed to decay, the carbon in its biomass is returned to the atmosphere as carbon dioxide. Unless, the wood is used in long-lived products—housing, furniture, coffins—cutting an old tree to replace it with young trees has little benefit to the climate.
The ornamental trees planted in many suburban settings seldom achieve the same final biomass as native species. Ornamental trees are usually planted at lower densities, surrounded by lawn. The carbon uptake by such a landscape will inevitably be lower than a native forest or a suburban forest of large trees. The maintenance of such a landscape also uses considerable fossil fuels in lawn mowing, fertilizer, and pesticides. Natural landscapes are more likely to result in net carbon uptake from the atmosphere.
If young trees are planted in a denser array than old trees, one might expect that the collective uptake of carbon by a suite of young trees might exceed the carbon uptake of a single, large, old tree. Indeed, this is the basis of much plantation forestry. However, when the canopies of the trees begin to overlap, leading to shading and competition, the weaker trees will die, so that the final density and total biomass of the surviving trees will be similar regardless of initial density. Although trees may grow at different rates, there is little opportunity to design forests that will have a significantly greater final total biomass by planting decisions alone.
Trees and Water:
It is worth considering the other attributes that trees cast upon our environment. Trees take up water from the soil. A small portion of the water is consumed in photosynthesis, but a much larger portion (1000X or more) is simply evaporated from the leaves, in a process called transpiration. Transpiration keeps the leaves cool and inadvertently provides regional cooling, because each gallon of water lost as vapor to the atmosphere carries away heat—known as latent heat—that would otherwise remain to warm the environment. One doesn’t need much experience to recognize that suburban parking lots are warmer than shady lawns on a summer day—that is transpiration in action. This cooling effect of trees overwhelms the greater reflectivity of bare surfaces. One recent scientific paper suggests that the loss of water by urban trees has a cooling effect that is 4X more effective in managing climate change than their uptake and storage of carbon dioxide.
Large trees take up and transpire more water than small trees, largely due to their greater leaf area. Large trees are typically more deeply rooted, so they remove water from the soil to greater depths and are also more resistant to short-term summer drought. Depending on size and species, urban trees can use between 1 and 60 gallons of water each day. Total water loss by trees will depend on how big they are and how many of them are on the landscape. All these statements are also conditional on what species we are considering. Some species, like sycamore, are profligate users of water in all conditions; others, like hemlock are miserly in their transpiration. Overall, a complete canopy of deciduous trees is likely to remove the equivalent of 4 mm/day of rainfall from the soil. We can expect the cooling associated with a canopy of large, old trees will be greater than that associated with a few smaller trees planted in an open lawn.
Trees and Soil Erosion:
By slowing the movement of water off the land surface, binding soil particles with their roots, and enhancing the water uptake-capacity of the soil, trees also slow the erosion of soils. Floodplains or riparian areas with a natural growth of trees are an important part of flood control. These areas add capacitance to the system—storing water during periods of high flow and releasing it to streams during periods of low flow. Trees in these habitats also remove plant nutrients from floodwaters, improving the quality of water downstream.
Trees and Air Pollutants:
Trees also remove pollutants, such as ozone and nitric oxide (NOx), from the atmosphere. Again, the process is dependent upon leaf area, rather than size, so one must consider whether an array of small trees has greater or lesser leaf area than a single large tree. The concentration of NOx controls the formation of ozone, and trees are important in removing NOx from the atmosphere. Thus, trees reduce the regional concentration of ozone, which is a toxic air pollution that exacerbates asthma, emphysema, and other respiratory diseases.
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