City trees combat temperature rise and pollution

With 41 percent of India’s population expected to be concentrated in urban centres by the year 2030, according to a UNDP report, it makes sense to rethink ‘ecology’ and understand its urban manifestation, wherein the role that trees, green or unconcretised spaces, water bodies and urban wildlife play in defining the city’s climate, pollution, cultural values and socio-economic opportunities can be evaluated. ATREE’s urban ecology team carried out an assessment of the environmental impacts of street trees in reducing pollution and mitigating microclimate in Bangalore. The results are compellingly in favour of using trees as shields against rising temperatures and pollution.

Bangalore’s large canopied trees are islands of shade for all classes of citizens, providing aesthetic relief and a break in concrete landscapes. Their functional value however exceeds their aesthetic appeal. ATREE’s studies show a dramatic 5ºC maximum difference in temperature and a maximum of 20 percent increase in humidity at mid-day between tree-lined and bare stretches of the same street. Suspended particulate matter on stretches of road without trees was as high as four times the permissible limits of 180µg/m3, but reduced to near permissible limits when trees were present; obviously serving as a partial sink for airborne pollution.

So would it help if we resort to planned planting of trees in order to cool the city and relieve its citizens of air and dust pollution? The answer seems to depend upon the planning. ATREE’s study on tree distribution and changing planting practices in Bangalore shows a disturbing trend of big canopy trees being replaced with narrow canopied species. The same study shows a trend of wider and older roads supporting larger and older trees with substantial canopy umbrellas, and narrow, residential roads supporting narrow canopied species, which do not provide the same ecosystem services as the larger canopies. Since Bangalore has witnessed large scale expansion of its wide roads for creating signal free corridors, six lane highways, and the Metro project, this means that the city is being selectively denuded of its largest and oldest trees. Bangalore’s stately giants, the rain tree, African tulip, copper pod and big leaf mahogany are slowly giving way to an entirely different landscape that is more brown than green.

Fortunately Bangalore also has a citizenship and media far more vocal about felling of trees than other cities in India. The urban ecology team has reached out (see press links below) to wider circles to disseminate information about the role of trees and garner support against their indiscriminate felling. This is a work in progress and has lessons for other cities in India too.

Press
Trees help in temperature reduction, pollutants Deccan Herald, 30 January 2010
Drive in Bangalore to choke on poisonous air DNA, 17 February 2010
City swelters: 10º hotter without trees Deccan Chronicle, 18 February 2010
Harini Nagendra, Whither giant trees? Deccan Herald, 6 June 2010
Tree is company Deccan Herald, 27 July 2010