Forbidden forest: Mridula Ramesh, on the alien invaders in our midst - Hindustan Times

Forbidden forest: Mridula Ramesh, on the alien invaders in our midst

May 04, 2024 04:16 PM IST

Some foreign tree species, like the African baobab, are bountiful. Others, like the Prosopis juliflora, were brought here for byproducts, have grown to dominate landscapes. And, as tussles over water intensify, have morphed into villains.

The giant piqued my interest. It stood nearly 24 metres tall — about the height of a five-storey building — within a well-kept school campus, in a small town in Tamil Nadu, Rajapalayam.

The baobab within the school campus in Rajapalayam, Tamil Nadu. This tree swells up and stores water in the monsoon, almost like a well. (Image courtesy Mridula Ramesh)
The baobab within the school campus in Rajapalayam, Tamil Nadu. This tree swells up and stores water in the monsoon, almost like a well. (Image courtesy Mridula Ramesh)

My first feeling when I saw it was awe, followed by a sense of peace engendered by the tree’s serene majesty. It had shed its leaves to conserve water in the summer, and I could see why people in this state called it “Pei Maram” or “Ghost Tree”. Its bare branches made it seem as though it was brandishing its roots in the air.

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Indeed, an old African folk tale goes that the gods, angered by this tree’s arrogance in coping with drought, ripped it from the Earth and flung it back, upside-down. The tree had the last laugh; it continued to survive. This species, the African baobab (Adansonia digitata), is one of the world’s strangest-looking and oldest trees. Radiocarbon analysis of interior tissue from separate branches is the only reliable way to date a baobab. Using this method, scientists have found that some of the oldest baobabs have been around 2,000 years, making them the oldest living angiosperm trees on the planet.

Their age is a nod to their brilliance: every part of the baobab supports life around it, and helps it thrive in harsh climates. Its stem swells as it gathers up water in the monsoon, for use in later dry months. It has multiple stems that fuse over time, leaving hollows in the centre. These cavities within the trunk store water that others can tap into. The 14th century traveller Ibn Battuta wrote of how Africans used the baobab as a well.

Tribes across that continent, such as the !Kung of the western Kalahari, use reed pipes to sip water directly from the tree. It provides lifegiving shade too, and is a rich source of nutrition. Its leaves are rich in calcium and antioxidants, while its fruit, a nutrition powerhouse, is easy to dry and carry. Maybe that’s why Africans took the fruits and the seeds with them, as they were brutally kidnapped and shipped off into slavery.

We know they did this, from the baobabs that have sprouted in the Americas and the Caribbean — natural monuments to historical wrongs. Indeed, no one quite knows how (or when) the baobab I visited came to be. Since Rajapalayam lies close to an ancient trade route, perhaps the seed was dropped by a trader, or more likely his slave.


Botanical guests come in all shapes. Some come unbeckoned, in secret. Others have a red carpet rolled out for their arrival. Some are bountiful, like the baobab, while others, like the Prosopis juliflora (or mesquite; henceforth referred to as Prosopis), are grabby.

In the late-19th century, after a vicious famine, the colonial British government in India wanted a drought-resistant plant that could supply firewood for the masses without impinging on reserve forests. So, in 1877, Prosopis was imported and planted in India, for the first time. People called it “the exotic lady from South America”. By 1940, it had expanded its reach and grew so well that the Maharaja of Jodhpur dubbed it a “Royal Tree”. Soon, it had invaded and thrived in the arid parts of former colonies, including in South Asia, Eastern Africa, Australia and north-eastern Brazil.

By the 1960s, as the Rann of Kutch encroached upon the Banni grasslands, the then-Gujarat government turned to Prosopis to hold back the desert. It played its role all too well. It beat back the sands, but took over the grassland. Today, it has colonised more than half the grassland, and burrowed its tap root into the deepest reserves of water.

The tap root is this plant’s secret weapon. A 1960 excavation in North America found a Prosopis root at a depth of 53 metres. The Prosopis is Nature’s borewell and so, doesn’t need (like the baobab, for instance) to try and save water. Indeed, a recent study conducted by the Indian Institute of Technology (IIT)-Bombay suggested that Prosopis’s water use may be upsetting the Banni ecosystem’s water balance.

Studies show that P. juliflora is an aggressive invader: from impacting elephant corridors to reducing nesting success in bird sanctuaries, it hits native species in every landscape where it has been introduced.

This is not so on its home turf. One fascinating University of Delhi study found that in its homeland, Venezuela, the soil was richer and the biodiversity higher under the shrub’s canopy. In foreign climes like Hawaii and India, soil richness and biodiversity under the same Prosopis canopy was much lower. The study also found that, while P. juliflora’s leaf litter and leachate hurt native Indian plants, India’s own, native Prosopis species, the khejri, helped native biodiversity. Sadly, while the slow-growing khejri can cope with drought and heat, it could not meet the rapacious demand for firewood, and struggles to hold out against the invader brought in to meet that need.


Water is the axis on which the Prosopis story turns. After a drought in the late 1950s, the then-chief minister of Madras State, K Kamaraj, had seeds of this plant distributed widely (my father tells me they were even sprayed from the air), especially in the dry Ramnad district. Prosopis took to tank beds better than a fish to water, and soon there were few tanks in the region that were not overrun.

One study published in the International Water Management Institute -Tata Water Policy Research Highlight publication found that more than a third of the tank water spread area in present-day Tamil Nadu was infested with the shrub. This lowered the water available for downstream irrigation, which was a problem for some, not all.

Tank irrigation benefits mainly large-landholding farmers. Most rural people gain more from Prosopis-on-tank-beds made into charcoal or firewood, or from their goats and sheep feeding on the plant’s seed pods. So, as long as free electricity ran borewells that spewed groundwater, the large landholders did not bother about the shrub’s invasion. Everybody won, until groundwater began to run out. Then Prosopis was hauled into the courts.

In 2015, a public interest litigation called for its removal. In response, the court ordered the removal of P. juliflora from riverbeds, tanks, and common lands in several districts in the state. But in 2017, there was pushback, including claims that there was little scientific evidence that Prosopis was a water-guzzler.

This isn’t strictly true. Consider the evidence from the Afar region of Ethiopia. This dry (annual rainfall is about 560 mm) and hot (average annual temperature is 31 degrees Celsius) region is made for this plant, which has gone on to infest 1.18 million hectares, or about a quarter of the region. Researchers from Addis Ababa University placed probes in Prosopis trees in four study sites (wet and dry) to measure the actual water use of the plant. They collected this data along with local weather data for about a year between 2016 and 2018. They found that, on average, a tree of this species transpired seven litres of water a day. Prosopis trees in dry soils used more water than trees in wetter soils. They used more water in dry seasons than in wet seasons. So different from the baobab that sheds its leaves in summer to conserve water, and swells in the monsoon to store rain.

When the team carefully upscaled the results for the Afar region, they found that P. juliflora consumed the equivalent of about half the rainfall of this dry region.


Inderjit Singh of the Centre for Environmental Management of Degraded Ecosystems at the University of Delhi says Prosopis is ubiquitous in South Delhi, which, incidentally is a groundwater hotspot. And while we focus on the borewell controls, we ignore the seemingly harmless green plant whose tap root is sucking out precious groundwater. Unwise.

About half a century ago, a forest department official penned a paean to this plant: “Prosopis Juliflora has crossed the continents and oceans, and has come to India at the invitation of the Foresters. She has served the poor and the Forest Department faithfully for years… She completes a century of colonization, stabilization, and expansion in 1977, and let us celebrate the centenary of Prosopis juliflora, and sing her saga of achievements and glory.”

By 2015, she had morphed into a villain. This transformation is a marker of the intensifying tussle over water. As long as groundwater held out, this plant wasn’t seen as a problem. But as groundwater reserves began to sputter, the piper of Prosopis had finally to be paid.

There are other invaders among us, including the lantana, water hyacinth and tomato. Some consider paddy an invader in Punjab and Haryana. After all, in the 1960s, paddy grew on less than 7% of the crop area in Punjab. Today, it is ubiquitous, sucking the groundwater out of the state. It’s been a case of so far, so good.

But oh! The changing climate! By upsetting the water balance, it is forcing us to confront the trade-offs involved in keeping the invaders versus uprooting them. The decision is easy with the baobab, harder with Prosopis, as it is so hard to eradicate. But the trade-off with paddy? Brave is the one who tries to make it. Until, of course, the climate makes it for us.

(Mridula Ramesh is a climate-tech investor and author of The Climate Solution and Watershed)

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