|Lost Ecosystem Services and Vanishing Ecological Roles. Forest ecosystems in the tropics and subtropics are being quickly replaced by industrial crops and plantations. This provides large amounts of goods for national and international markets, but results in the loss of crucial ecosystem services mediated by ecological processes. In Argentina and Bolivia, the Chaco thorn forest (A) is being felled at a rate considered among the highest in the world (B), to give way to soybean cultivation (C). (Photo credit: Wikipedia)|
Today we review an extensive literature review (with 156 references) of research concerned with the role of trees in an urban ecosystem services (ESS) framework and how that affects the environment, health and climate change mitigation for cities. Past studies have focused not only on the health benefits of trees, but also the conditions where trees can lead to lower air quality.
The paper describes the physical role of trees in allowing for more moisture to be released from the soil to the atmosphere as well as the effectiveness of some trees (with large leaves) to capture air pollutants while at the same time reducing the ventilation and dilution of pollution along tree-lined streets. Pollen from trees causes asthma and allergies in as much as half the population of some cities. The authors suggest that a systems dynamics approach might help to consider the many dynamic processes involved in order to improve urban planning into the use of trees.
“current scientific understanding of the impact of street trees on human health and the urban environment has been limited by predominantly regional-scale reductionist approaches which consider vegetation generally and/or single out individual services or impacts without considering the wider synergistic impacts of street trees on urban ecosystems. “
- “the diversity in ESS for urban street trees, as well as
- the importance of tree species, density and location in service provision for any given location, and
- the implications and potential health and societal effects of optimising for a singular service
“The potential impact of street trees on air quality remains one of the most poorly understood aspects of the studied ecosystem services and benefits …Most studies at regional or city scales show a modest modelled reduction in pollution concentration of less than 5 % resulting from urban vegetation”
“Trees absorb CO2 and gaseous pollutants such as O3, NO2, SO2 primarily by uptake via leaf stomata or surface, and accumulate airborne particulates (by interception, impaction or sedimentation) more effectively than other urban surfaces“
“Trees emit biogenic volatile organic compounds (bVOCs) as a reaction to stress in their environment, such as high light intensities and/or temperatures or low water availability .. Isoprene is the most abundantly emitted bVOC . In the presence of NOx and sunlight, isoprene contributes to ozone formation“
“Exposure to allergenic pollen from trees is associated with a range of health effects, including allergic rhinitis, exacerbation of asthma in susceptible individuals, and eczema.. Estimates of the levels of tree pollen allergies in the population range from around 5 % to over 50 % in Europe”
“systems dynamics approaches could also be used to capture the complexity and dynamic interactions occurring within urban systems, and has been used previously to integrate information from different disciplines and sectors whilst maintaining a health focus.”