“If a forest isn’t replacing itself, it’s a garden. Just standing back and letting trees grow is not going to work.” – Dr. Robert J. Warren
Delhi, the capital of India, enjoys a special place in history. Situated on the alluvial plain of Yamuna, it is a unique landscape. To the south-west lies the desert, in the north lies the Himalayas and from north-east to south-west runs the oldest hill range – the Aravalli. It serves as a gateway between the Thar Desert, Aravalli Range and the freezing cold Himalayas.
This geographical location makes Delhi an ecologically important place which provides a unique microhabitat for different species. It has riverine species as well as species from desert ecosystems. It is this unique combination of river and hills that may have attracted inhabitants since the 6th century BC. It has seen the rise and fall of several cities. Delhi remains the capital of India since 1911 when the colonial government shifted their base from Kolkatta.
This political shift had a huge impact on the ecology of Delhi’s Ridge forest. When British planners first visited Delhi, they termed it barren with little or no vegetation cover.
Inspired by the European idea of Garden city, Sir Edward Lutyens wanted a greener city in which Delhi Ridge was supposed to play a prominent role by providing a green backdrop to the new city. Their plan for the city included a greenbelt, lush green forest on hills and trees along all avenues, and a spacious compound. A massive afforestation program was prepared for the ridge to stop surface water runoff, and to protect the new city from dust blows. This ridge forest was also supposed to be the recreation space for the residents of the new city.
For this task, Sir Edward Lutyens collaborated with celebrated horticulturist William Robert Mustoe in 1919. He and his assistant architect Walter George, along with Mustoe jointly decided on which species to plant. During this period, many species were introduced to Delhi’s environment, most of which were alien to Delhi.
Afforesting the Delhi Ridge was most challenging. It was difficult to choose suitable species for a place like the Delhi Ridge which is located in semi-arid climate that does not support the growth of luxuriant vegetation. The Delhi Ridge belongs to Tropical thorn forest – a forest type which has been described as “an open park like forest in which trees grow in groups separated by grassy glades” (Champion & Seth 1963).
The vegetation of this forest can be divided into two groups i.e. temporary and permanent. Temporary vegetation comes up after the monsoon, has a short lifespan, and is resistant to seasonal change. It dies quickly, adding humus, and a pool of nutrients to the soil, thereby creating ideal conditions for the growth of permanent vegetation. Permanent vegetation is xerophytic in nature and is highly drought resistant.
Mustoe had to find the best species that could tolerate drought, remain green throughout the year, and grow quickly. After trying and testing several species, Mustoe was finally satisfied with the Mexican Morn or Prosopis juliflora (henceforth P. juliflora) . Although he had hand-planted seeds of several species in the ridge, yet the Delhi Ridge was eventually afforested by P. juliflora. Today, it has taken over not just the entire ridge area, but also the riverine tracks of the Yamuna. Excepting the Aravalli Biodiversity Park, if you take a walk in any other part of the ridge, you will have a completely different view of these thorn forests. You will find a dense forest devoid of grasslands, and which is dominated by P. juliflora.
The negative impacts of P. juliflora are now widely known. P. juliflora is known to create monoculture patches spreading into pastureland, agriculture fields, as well as forests; altering the soil nutrient profile, and in many cases also proving fatal for livestock if eaten in large quantities. It absorbs nutrients from the soil much faster than old resident species such as Prosopis cineraria.
Invasive species like P. juliflora are considered notorious because they negatively affect native species by competing for resources and altering ecosystem conditions, in turn affecting native fauna as well as the people dependent on traditional species for livelihood and cultural practices. It can also change the physicochemical properties of the soil, and its extensive lateral spread makes it a dangerous competitor.
A quantitative study on woody biomass of the Asola Bhatti Wildlife Sanctuary in Delhi revealed that P. juliflora was the dominant vegetation, covering 23.43 km2 of the total area – much more than any native species found in that area (Kushwaha et. al. 2014).
Similar results were found, when in 2013 we conducted an independent study in six fragmented forest patches of the southern ridge which were not part of the Asola Wildlife Sanctuary but notified as reserve forests. Our results revealed that P. juliflora was the most abundant species covering the largest proportion of area in each of the six forest patches. We also recorded the presence of many other invasive species including Lantana camara. 80% of the species that were recorded in the ridge were foreign species that must have been introduced to the landscape at some of point in the past.
This is just one story of the many challenges faced by urban forests. This is a story of the introduction of an alien species to a new place where it has not only succeeded in establishing itself, but also managed to modify the landscape to suit its needs, replacing old native residents of these forests.
Alien species are sometimes introduced intentionally, the Delhi Ridge being the case in point. At other times they are introduced unintentionally, such as when the invasive species Parthenium hysterophorus is believed to have entered India in the 1950’s along with imported wheat from USA.
Urban areas are the centres of mobility where both man and material constantly move. This, along with increased connectivity between places increases the vulnerability of natural landscapes, making them prone to biological invasions.
In such a scenario, several questions need to be addressed. What is it that we are looking to conserve in the forests of Delhi? Are present management practices successful in limiting the expansion of P. juliflora? And the most important, how do we save our forests from such alien invasions?
Bowe, P. (2009). The genius of an artist’: William r. Mustoe and the Planting of the City of New Delhi and its Gardens. Garden History, 68-79.
Maheshwari, J. K. (1963). The flora of Delhi. New Delhi: Council of Scientific & Industrial Research.
Kushwaha, S. P. S., Nandy, S., & Gupta, M. (2014). Growing stock and woody biomass assessment in Asola-Bhatti Wildlife Sanctuary, Delhi, India. Environmental monitoring and assessment, 186(9), 5911-5920.
(The article originally appeared in the blog of Centre for Urban Ecology and Sustainability, Ambedkar University https://cuesataud.wordpress.com/2017/03/03/alien-in-the-forest/)