The latest issue of the India State of Forest Report (ISFR) — a biennial publication by the Forest Survey of India (FSI), which falls under the Union ministry of environment forest and climate change — released on December 21, 2024, stated that subabul has, in a first, overtaken other all other local species of fauna.

This is a problem because like vilayati kikar, Subabul is also a hardy tree which propagates quickly and is allelopathic in nature, meaning it secretes chemicals that do not allow any other species to grow around it. Indirectly, it prevents growth of Delhi’s native trees, thus taking over a landscape fairly quickly.

Till 2021, the most commonly found tree in Delhi’s urban spaces was the neem (azadirachta indica), with the mulberry, the ashok and the peepal close behind. Renowned environmentalist and author Pradip Krishen said experts have long been warning Delhi’s forest department and other agencies of the dangers of subabul, but no serious effort has been made to stem the tree’s unchecked growth.

“Till a few years ago, we were still at a point where we could warn the agencies, but now it is just everywhere. All across the southern and central Ridges, there are large patches of only the subabul tree. When you drive along the central Ridge, there is a large area of thick subabul forest,” said Krishen.

To be sure, the vilayati kikar remains the most dominant species in Delhi’s forest and rural areas, but it is fast losing ground to its compatriot.

Subabul — also known as river tamarind — was first introduced to Delhi in the 1970s, with the administration choosing the tree for its utility in farm forestry.

“It was introduced as a fast-growing, nitrogen-fixing tree species for agroforestry, fodder, and fuelwood production. The Indian Council of Agricultural Research (ICAR) and other agencies promoted it extensively to combat deforestation and soil degradation,” said Sumit Dookia, assistant professor (environment), Guru Gobind Singh Indraprastha University.

However, experts soon realised the tree’s invasive nature: it tends to grow quickly, and can form dense canopies that crowd out other vegetation. The tree’s high propagation rate and its hardy nature also make the subabul highly adaptable, and its leaves can be toxic for cattle if consumed in large quantities.

With its growing predominance in the Ridge ecosystem, the forest department in 2022 classified subabul as one of the three main invasive alien species, along with vilayati kikar and lantana.

Krishen said subabul can now be seen everywhere — from Sanjay Van to Jahapanah forest to the JNU forest. During a walk inside Sanjay Van, Krishen also demonstrated why it was a species of concern. “One only needs to look under the canopy of the subabul tree — you will find hundreds of small saplings. The seeds spread fast, grow quickly, and take over space and resources, which in turn does not let any other species grow. So, the propagation of native, more ecologically beneficial species is hampered,” he said.

Other experts pointed to the subabul’s hardy nature.

“One subabul tree produces hundreds of thousands of seeds, which germinate very fast and are hardy in nature. That is why it is easier for this tree to quickly propagate even in areas that are already quite vegetated. Additionally, the tree grows fast and produces flowers and fruits within two-three years,” said ecologist Vijay Dhasmana, the curator of Aravalli Biodiversity Park, Gurugram. Dhasmana said the hardy nature of the tree also makes difficult to eradicate.

“It is very similar to the vilayti kikar in terms of propagation, and merely chopping it off will not stop it from reappearing. So, it has to be removed from the root, and active departmental efforts are required,” he said.

The forest department, meanwhile, has formed a standard operating procedure (SOP) to deal with the spread of the subabul. These plans are aimed at the eco-restoration of the Ridge area with a microhabitat of Mangar Bani near the Gurugram-Faridabad border.

“In the case of leucanea leucocephala (subabul), the young sapling has to be removed manually on a regular basis and pruning of trees must be done before seed pods start to emerge on the tree. Elimination of subabul is one extremely labour intensive and high maintenance process involving heavy costs but is probably the only effective method to control spread of this species,” the SOP states.

However, the efforts don’t seem to have translated on the ground. Experts note that while action plans exist for the vilayati kikar, the subabul and its spread has largely been ignored.

Faiyaz Khudsar, the scientist-in-charge at DDA’s biodiversity parks programme, said the species is fast growing, drought resistant and allelopathic — a process by which a plan secretes chemicals to suppress the growth of other plants. “All these factors combine to make this invasive species one that has the capacity to transform ecosystem processes in an area. It can directly impact the growth of other plants and the community structure,” Khudsar said, stating focussed action was required against the species, which can fare well in almost all soil-types.

Dookia, who is also part of Delhi’s Biodiversity Council, says targeted action is needed against the species to remove it gradually. “For all invasive species, we require planning and action plans,” he said.