An Expert Explains: In alarm over Conocarpus trees, echo of faulty policies on invasive exotics

Problem with exotic plant species 

It has been shown repeatedly that many species that were introduced to India previously for greening or ornamental projects eventually turned invasive. Native species that failed to compete for space and soil resources were driven to extinction, and significant costs were incurred to ensure the management of the invasive species. The list of these species is long — among the notable stories are those of Eucalyptus, Prosopis juliflora (vilayati kikar), Acacia mangium, and Lantana camara — and they continue to be used until considerable focus is brought on to their invasive characteristics.

Flowers and leaves of the Lantana camara. (Via Wikimedia Commons)

There are two species of Conocarpus (buttonwood) trees, with several varieties and hybrids — Conocarpus erectus, which is more widely used in India, is native to South America, and C. lancifolius is native to East Africa. They are easily propagated and multiplied in nurseries through stem cuttings.

Serious pollen allergies and respiratory problems have been reported in the vicinity of their plantations, which has led to civic agencies taking action to ban them. The problems had already been noted in Kuwait and other parts of the Middle East, as well as in Karachi and Faizabad in Pakistan, when they started being used in greening projects in India.

India’s losing battle

So while banning Conocarpus plantations seems to be the right thing to do, what is being done to address the root cause of this kind of problem?

It is Conocarpus today, it could be Leucaena tomorrow, and Mangium the day after. As is the case with exponential growth, by the time adverse impacts are visible to the public, it’s already too late.

Conocarpus can hopefully still be managed, but we have certainly lost the chance to manage Prosopis juliflora and Lantana camara on public lands and commons. We can also no longer manage Leucaena leucocephala, which is battling it out with vilayati kikar for dominance in the urban woodlands of Delhi while nearly extinct native species gradually perish.

Vilaiti Keekar/ Babool or Southwest thorn Prosopis juliflora in Krishna Wildlife Sanctuary, Andhra Pradesh, India. (Via Wikimedia Commons)

Short-sighted policies

Why do urban greening initiatives end up deploying these exotics in the first place?

The non-native species, in a biogeographic sense, often face very few or no pests or pathogens in new habitats, which makes their proliferation easy. Often, these species may require very little aftercare. Some species like Conocarpus are not browsed by livestock, and are thus favoured for horticultural or landscaping projects.

The problems surface only slowly and often much later.

The human population gets familiar with the local pollen calendar and, to a large extent, their immune systems coevolve. People in cities are exposed to a spectrum of allergens in any case, and respiratory allergies are common. Into this already precarious landscape comes new and newer plantations of exotics, which are ironically brought in often to improve urban air quality and aesthetics.

What can be done

Almost all horticultural species are well studied, and propensities, including phytochemical volatiles and allergies, are well known. It is certainly possible for landscape managers to choose from among native species pools, species that are ecologically appropriate and also meet aesthetic needs. Tragically, urban landscaping and agroforestry projects often do not even refer to existing information on these species before taking up their large-scale use.

Based on available research, it is almost unanimously recommended by scientists that urban greening projects should use native species for long-term sustainability. The avenue trees in Lutyens’ Delhi are of species from Central India and the Gangetic plain, and they have served the city well both in terms of aesthetics, and by harbouring a large number of native bird species and other dependents. Doing so may marginally increase the costs and effort, but will add immense value in terms of ecosystem services in the medium to long term.

Cassia Fistula or Amaltas trees seen near PAU in Ludhiana, Punjab. They are also native to Delhi and seen in Lutyens’ Delhi in particular. (Express Photo by Gurmeet Singh)

The import and use of horticultural plants need to be monitored by maintaining a watch list of plants that are known to harm native species and ecosystems, and raise public health concerns. The numbers of mega greening projects are likely to increase in the coming years in step with India’s climate commitments, and landscape planners and land managers need clear guidelines outlining these principle.

Furthermore, the value of Carbon sequestration by urban greens has been highlighted by recent research to contribute significantly to mitigating Greenhouse gas emissions, thereby contributing significantly to the green GDP of a country.

The EU for instance, has adopted comprehensive policy on Urban Green Infrastructure – that includes all the natural, semi-natural and artificial networks of ecological systems in urban and peri-urban areas, such as forests, parks, community gardens, backyards and avenue trees. The policy is aimed at sustaining biodiversity and ecosystem services in the urban areas as part of the Nature Based Solutions (NBS) package for climate mitigation.

India aspires to meet its global emission reduction commitments and NBS is a critical component of our climate strategy. It is in this context, with the expanding market and scope of NBS, that greening projects need to be mindful about the consortium of species that are used.

Suresh Babu is a Professor at the School of Human Ecology and Director of the Centre for Urban Ecology and Sustainability at Dr B R Ambedkar University Delhi. His research interests include conservation biology, ecological restoration, and invasive species management.