Árbol urbano



Fresno con un desgarro en el tronco, por mala praxis de arboricultura

Un árbol urbano es aquel especímen o colección de éllos creciendo dentro de una localidad urbana o suburbana. En un sentido amplio, incluye cualquier clase de vegetal leñoso creciendo en asentamientos humanos. En un sentido más estricto (encajando en parque forestal) describe áreas donde los Ecosistemas carecen de sobreviventes silvestres o remanentes.

El cuidado y manejo del árbol urbano se llama forestación urbana.

Los árboles juegan un rol importante en la ecología de los hábitats humanos de muchas maneras: filtran aire, agua, luz solar, ruidos; enfrían el ambiente; dan sombra a animales y a otros vegetales, y área recreacional para los habitantes. Moderan el clima urbano, reducen vientos y tormentas, proveen de sombra a viviendas y comercios, ayudando a conservar energía. Son críticos en enfriar la isla de calor urbana, y sombrear reduciendo los potencialmente peligrosos días de reducción del ozono, que azotan las megaciudades en los meses de verano picos.

En muchos países hay un crecimiento del entendimiento de la importancia de la ecología en la forestación urbana. Surgen numerosos proyectos para restaurar y preservar los ecosistemas, desde la simple eliminación de las hojas caídas, a la eliminación de vegetales invasores, y a la reintroducción de especies nativas originales.

Beneficios

Los beneficios del arbolado urbano son muchos: embellecimiiento, reducción de los efectos de la isla de calor, reducción de las escorrentías y escurrimientos, reducción de la contaminación atmosférica (química, física, biológica), reducción de costos de energía por perfeccionar el sombreado de edificios y de calles, mejora del valor de la propiedad, atenúa la amenaza a la fauna y flora silvestres, y en general mitiga todos los impactos urbanos ambientales.

Social, psicológica, recreacional, vida silvestre

La presencia abundante de arbolado reduce estreses, beneficiando la salud psicofísica del habitante urbano. El sombreado agradable de calles y parques atrae a la socialización y al juego. Nuevamente, la planificación y el involucrarse de la comunidad son importantes para obtener resultados positivos.

Los árboles dan sitios de anidación y alimento a aves y otros animales. La gente aprecia el avistaje, alimentación, safaris fotográficos, pintura artística, y vida silvestre. La combinación del arbolado y la vida silvestre ayudan a mantener su conección con la naturaleze.

Beneficios económicos

The economic benefits of trees have been understood for a long time. Recently, more of these benefits are becoming quantified. Quantification of the economic benefits of trees helps justify public and private expenditures to maintain them. One of the most obvious examples of economic utility is the deciduous tree planted on the south and west of a building. The shade shelters and cools the building during the summer, but allows the sun to warm it in the winter after the leaves fall. The USDA 'Guide" notes on page 17 that "Businesses flourish, people linger and shop longer, apartments and office space rent quicker, tenants stay longer, property values increase, new business and industry is attracted" by trees. The physical effects of trees--the shade (solar regulation), humidity control, wind control, erosion control, evaporative cooling, sound and visual screening, traffic control, pollution absorption and precipitation--all have economic benefits

Reducción de la contaminación atmosférica

As cities struggle to comply with air quality standards, the ways that trees can help to clean the air should not be overlooked. The most serious pollutants in the urban atmosphere are ozone, nitrogen oxides (NOx), sulfuric oxides (SOx) and particulate pollution. Ground-level ozone, or smog, is created by chemical reactions between NOx and volatile organic compounds (VOCs) in the presence of sunlight. High temperatures increase the rate of this reaction. Vehicle emissions, emissions from industrial facilities, gasoline vapors, and chemical solvents are the major sources of NOx and VOCs. Particulate pollution, or particulate matter (PM10 and PM25), is made up of microscopic solids or liquid droplets that can be inhaled and retained in lung tissue causing serious health problems. Most particulate pollution begins as smoke or diesel soot and can cause serious health risk to people with heart and lung diseases and irritation to healthy citizens. Trees are an important, cost-effective solution to reducing pollution and improving air quality.

Trees reduce temperatures and smog

With an extensive and healthy urban forest air quality can be drastically improved. Trees help to lower air temperatures and the urban heat island affect in urban areas (see: 'Trees are energy savers' for more information on this process). This reduction of temperature not only lowers energy use, it also improves air quality, as the formation of ozone is dependent on temperature.

• As temperatures climb, the formation of ozone increases.
• Healthy urban forests decrease temperatures, and reduce the formation of ozone.
• Large shade trees can reduce local ambient temperatures by 3 to 5 °C
• Maximum mid-day temperature reductions due to trees range from 0.04 °C to 0.2 °C per 1% canopy cover increase.
• In Sacramento County, California, it was estimated that doubling the canopy cover to five million trees would reduce summer temperatures by 3 ºC. This reduction in temperature would reduce peak ozone levels by as much as 7%and smoggy days by 50%.

Lower temperatures reduce emissions in parking lots

Temperature reduction from shade trees in parking lots lowers the amount of evaporative emissions from parked cars. Unshaded parking lots can be viewed as miniature heat islands, where temperatures can be even higher than surrounding areas. Tree canopies will reduce air temperatures significantly. Although the bulk of hydrocarbon emissions come from tailpipe exhaust, 16% of hydrocarbon emissions are from evaporative emissions that occur when the fuel delivery systems of parked vehicles are heated. These evaporative emissions and the exhaust emissions of the first few minutes of engine operation are sensitive to local microclimate. If cars are shaded in parking lots, evaporative emissions from fuel and volatilized plastics will be greatly reduced.

• Cars parked in parking lots with 50% canopy cover emit 8% less through evaporative emissions than cars parked in parking lots with only 8% canopy cover.
• Due to the positive effects trees have on reducing temperatures and evaporative emissions in parking lots, cities like Davis, California, have established parking lot ordinances that mandate 50% canopy cover over paved areas.
• "Cold Start" emissions

The volatile components of asphalt pavement evaporate more slowly in shaded parking lots and streets. The shade not only reduces emissions, but reduces shrinking and cracking so that maintenance intervals can be lengthened. Less maintenance means less hot asphalt (fumes) and less heavy equipment (exhaust). The same principle applies to asphalt-based roofing.

Active pollutant removal

Trees also reduce pollution by actively removing it from the atmosphere. Leaf stomata, the pores on the leaf surface, take in polluting gasses which are then absorbed by water inside the leaf. Some species of trees are more susceptible to the uptake of pollution, which can negatively affect plant growth. Ideally, trees should be selected that take in higher quantities of polluting gasses and are resistant to the negative affects they can cause.

A study across the Chicago region determined that trees removed approximately 17 tonnes of carbon monoxide (CO), 93 tonnes of sulfur dioxide (SO2), 98 tonnes of nitrogen dioxide (NO₂), and 210 tonnes of ozone (O₃) in 1991.

Carbon sequestration

Urban forest managers are sometimes interested in the amount of carbon removed from the air and stored in their forest as wood in relation to the amount of carbon dioxide released into the atmosphere while running tree maintenance equipment powered by fossil fuels.

Interception of particulate matter

In addition to the uptake of harmful gasses, trees also act as filters intercepting airborne particles and reducing the amount of harmful particulate matter. The particles are captured by the surface area of the tree and its foliage. These particles temporarily rest on the surface of the tree, as they can be washed off by rainwater, blown off by high winds, or fall to the ground with a dropped leaf. Although trees are only a temporary host to particulate matter, if they did not exist, the temporarily-housed particulate matter would remain airborne and harmful to humans. Increased tree cover will increase the amount of particulate matter intercepted from the air.

• Large evergreen broad-leafed trees with dense foliage collect the most particulate matter.
• The Chicago study determined that trees removed approximately 234 tonnes of particulate matter less than 10 micrometres (PM10) in 1991.
• Large healthy trees greater than 75 cm in trunk diameter remove approximately 70 times more air pollution annually (1.4 kg/yr) than small healthy trees less than 10 cm in diameter (0.02 kg/yr).

Biogenic volatile organic compounds

One important thing to consider when assessing the urban forest's effect on air quality is that trees emit some biogenic volatile organic compounds (BVOCs). These are the chemicals (primarily isoprene and monoterpenes) that make up the essential oils, resins, and other organic compounds that plants use to attract pollinators and repel predators. As mentioned above, VOCs react with nitrogen oxides (NOx) to form ozone. BVOCs account for less than 10% of the total amount of VOCs and BVOCs emitted in urban areas. This means that BVOC emissions from trees can contribute to the formation of ozone. Although their contribution may be small compared with other sources, BVOC emissions could exacerbate a smog problem.

Not all species of trees, however, emit high quantities of BVOCs. The tree species with the highest isoprene emission rates should be planted with caution:

• Casuarina
• Eucalyptus
• Liquidambar
• Nyssa (tupelo)
• Platanus
• Populus (álamo)
• Quercus (roble)
• Robinia
• Salix (sauce)

Trees that are well adapted to and thrive in certain environments should not be replaced just because they may be high BVOC emitters. The amount of emissions spent on maintaining a tree that may emit low amounts of BVOCs, but is not well suited to an area, could be considerable and outweigh any possible benefits of low BVOC emission rates.

Trees should not be labeled as polluters because their total benefits on air quality and emissions reduction far outweigh the possible consequences of BVOC emissions on ozone concentrations. Emission of BVOCs increase exponentially with temperature. Therefore, higher emissions will occur at higher temperatures. In desert climates, locally native trees adapted to drought conditions emit significantly less BVOCs than plants native to wet regions. As discussed above, the formation of ozone is also temperature dependent. Thus, the best way to slow the production of ozone and emission of BVOCs is to reduce urban temperatures and the effect of the urban heat island. As suggested earlier, the most effective way to lower temperatures is with an increased canopy cover.

These effects of the urban forest on ozone production have only recently been discovered by the scientific community, so extensive and conclusive research has not yet been conducted. There have been some studies quantifying the effect of BVOC emissions on the formation of ozone, but none have conclusively measured the affect of the urban forest. Important questions remain unanswered. For instance, it is unknown if there are enough chemical reactions between BVOC emissions and NOx to produce harmful amounts of ozone in urban environments. It is therefore, important for cities to be aware that this research is still continuing and conclusions should not be drawn before proper evidence has been collected. New research may resolve these issues.

Referencias

• Nowak, D. 2000. Selección de Especies de Árboles, Diseño, y Manejo par Mejora de la Calidad del Aire. Annual meeting proceedings of the American Society of Landscape Architects (disponible online, en pdf
• Nowak, D. Efectos de los Árboles Urbanos en la Calidad del Aire USDA Forest Service (disponible online, pdf file
• Nowak, D. 1995. ¿Los Árboles Contamina? A "Tree Explains It All". Proceedings of the 7th National Urban Forest Conference (disponible online
• Nowak, D. 1993. Emisiones Químicas de las Plantas. Miniature Roseworld 10 (1) (disponible online pdf
• Nowak, D. & Wheeler, J. Program Assistant, ICLEI. Febrero 2006.
• McPherson, E. G. & Simpson, J. R. 2000. Reducing Air Pollution Through Urban Forestry. Proceedings of the 48th meeting of California Pest Council (available online, pdf file).
• McPherson, E. G., Simpson, J. R. & Scott, K. 2002. Actualizing Microclimate and Air Quality Benefits with Parking Lot Shade Ordinances. Wetter und Leben 4: 98 (available online, pdf file).
• Hanson, Michael L. 1990. Urban & Community Forestry, a Guide for the Interior Western United States, USDA Forest Service, Intermountain Region, Ogden, Utah.

Enlaces externos

• Urban Forestry South
• Center for Urban Forest Research
• Urban Forest Ecosystems Institute
• Urban Forestry
• USDA Forest Service Northeastern Area

[[Categoría:Arboricultura

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