Thinning green is thick with exotica

Environmentalists all agree that more and more trees need to be planted in urban spaces. But there is little discussion about which variety of trees might be ideal.

A recent report prepared by Divya Gopal and Harini Nagendra of the Ashoka Trust for Research in Ecology and Environment (ATREE) indicates that exotic species are being introduced in city parks on a scale that is worrisome: nearly 77% of the trees in city parks are now exotic species. These are often not varieties that native birds and other species have adapted to.

The report, Tree Diversity, Distribution, History and Change in Urban Parks: Studies in Bangalore, India, was written after research undertaken in early 2008. The report will soon be placed before the Bruhat Bangalore Mahanagara Palike (BBMP). The authors said that the findings could serve as inputs while planning the green cover of the city. Many exotic species of trees like the royal palm, the authors said, had no larger ecological significance as they provided neither shade nor fruit.

Sankara Rao, distinguished fellow at the Centre for Ecological Sciences, IISc, agreed with the findings of the report and said that Bangalore did indeed have an excessive number of exotic varieties of trees. The diversity in the naturally-occurring species in the city is thus adversely affected, Rao said. Many small mammals and birds are affected by the growing number of exotic trees in the city, he said, adding that the BBMP and the horticulture department have remained insensitive to this problem.

Rao also pointed out that fast-growing trees have found preference when it comes to planting activity; however, such varieties of trees are also more prone to breaking. Over the last three months, nearly 80% of trees that fell in the heavy rains and strong winds were copper pod trees which typically have weak roots. It would be ideal if these trees were not planted any more, Rao said.

A total of 1,423 trees were sampled in 127 plots in the city for the purpose of the study. Of the 80 different species (distributed across 27 families) that were found in the sample, 53 were exotic; only 27 were native. A few commonly-encountered species dominated; Polyalthia longifolia (commonly called Indian mast), was the most dominant species of tree in the city. An evergreen tree, this species was introduced from Sri Lanka.

Nearly a fifth of the trees that comprised the study were of this variety. Five other common varieties, that together comprised half the trees studied, were: honge, royal palm, Nile tulip, orchid tree, copper pod, gulmohar, golden bell, Indian cork and African tulip.