A flowering Senna plant looks almost similar to the Golden Shower. (Supplied)
The plant that is sometimes known as the “weeping cassia” is — in a somewhat convoluted way — living up to its name in Kerala: It is making forest officials weep over nurturing it for years.
The reason: The species has proven to be disastrous for Kerala’s biodiversity, and having introduced it to the state some four decades ago, the Forest Department is ruing its decision.
Known botanically as the Senna spectabilis, the plant species is invasive, and has been preventing the germination of local vegetation. So much so, no grass grows below it, nor does it attract insects or many herbivores.
It has also been damaging soil quality, upsetting underground water availability, and incipiently intruding into the wildlife food chain.
Senna plants thrive in the Muthanga range of forests in Wayanad. (Supplied)
It is the last factor that is most alarming for many conservationists; they fear it could worsen human-tiger conflict, a burning issue in Wayanad district.
“See the irony,” said N Badusha, president of Wayanad Prakruthi Samrakshana Samithi, an NGO dedicated to the protection of forests and the environment.
“There was large-scale planting of this plant species in and outside the forests of Kerala in the 1980s, and now it is the biggest threat to biodiversity,” Badusha told South First.
“And the Forest Department is struggling to contain this threat.”
As Badusha indicated, the Senna spectabilis is not an indigenous plant species; it was introduced in Kerala in the early 1980s.
Native to South and Central America, the plant is often grown as ornamental vegetation in front yards, parks and gardens because of its bright yellow flowers that bloom in summer.
In fact, it was its striking yellow flower that misled Forest Department officials into thinking it was the cassia fistula, also called golden shower or Kanikkonna — popular for its yellow bloom and also, incidentally, the Kerala state flower — the species that they actually wanted to bring over to Kerala to beautify its countryside, especially its forests.
Other countries have imported the Senna spectabilis too, but for a different reason. African countries such as Kenya, Malawi, Tanzania and Uganda saw it as a source of cheap firewood.
But as in Kerala, reports say, the authorities in these countries are today also struggling with the disastrous effect that their decision has had on the local ecosystem. Even Australia and Cuba look at the plant as “invasive”.
Apparently, they are finding out what the Global Compendium of Weeds has said of the species: It is an “environmental weed”, while the UK-based nonprofit CABI — Centre for Agriculture and Bioscience International — calls it the “garden thug”.
Unaware of this, Kerala’s Forest Department planted numerous saplings in all vacant and poramboke land, Badusha said, referring to government land outside revenue assessed areas.
The Forest Department even planted them inside wildlife sanctuaries.
“They did it innocently,” Badusha said. “They did not know it was invasive.”
Today, Forest Department officials admit it was a blunder on their part to introduce the weeping cassia, as according to them, the plant has caused biodiversity loss in over 800 hectares of forest land across the state.
The invasive Senna plants inside Wayanad Wildlife Sanctuary. (Supplied)
“Unlike other plants, the senna grows fast,” said Thiruvananthapuram-based environmental activist Veena Maruthoor.
“Flowering happens throughout the year, and the seeds get disseminated profusely.” Cutting the plant doesn’t help, she said: “It re-sprouts soon after you cut it.”
A single tree can produce around 6,000 seeds during every season, while the survival rate of these seeds is almost a 100 percent, as they can survive for eight-nine years even in difficult soil.
A sapling needs hardly two years to become a full-grown tree. Then it starts blooming.
A measure of the tenacity of the Senna spectabilis to survive can be gauged from its spread in the Wayanad Wildlife Sanctuary over the years.
A 2017 report in The Deccan Chronicle said the plant was introduced in the sanctuary as avenue trees, as it grew up to 20 metres in height. That year, the paper found it to have “invaded 2 percent area” of the sanctuary.
By 2021, said a research paper for the UK-based charity Rufford Foundation, it had “covered over 23 percent of the sanctuary area in 40 years” since being introduced in the 1980s.
But official estimates of the forest department put the area under the senna in the Wayanad sanctuary at a much higher 42 percent.
Interestingly, most herbivores — other than the elephant, chital and the porcupine — seem to avoid eating its leaves.
According to the Rufford paper, elephants are the “major disperser of senna”, with the dung facilitating germination of the species.
With the plant making it difficult for other vegetation species to grow, the food chain in the Kerala forests and the Bandipur-Nagarhole-Mudumalai belt linking Karnataka and Tamil Nadu.
This is because, as Badusha said, animals tend to avoid the patches where the Senna spectabilis grows.
This is especially worrisome, say conservationists, as they believe it is because of this that predatory wildlife like the tiger is getting steadily pushed into human habitations in search of food.
“The species with thick foliage arrests the growth of other indigenous tree and grass species. So, it creates food shortages for the wildlife, especially the herbivores,” said Dr TV Sajeev, a scientist with the Kerala Forest Research Institute.
According to him, the species accentuates the existing human-animal conflict in the Wayanad region, where wildlife — impacted by the food chain collapse within the forest — foray into human settlements in search of prey.
With grass and small edible plants fast disappearing, so are the animals that feed on them. This means predators like tigers, which prey on these animals, have to look elsewhere for food.
Consequently, Wayanad has now emerged as the hotspot of human-animal conflicts, with tigers moving around in human settlements targeting cows and other livestock.
A comprehensive mission to eradicate the Senna spectabilis is in the pipeline in Kerala, but officials confirm the target takes work.
This is because the plant is also extremely hardy. As Dr Sajeev noted, “it is categorised as a plant of least concern” in the Red List of the International Union for Conservation of Nature (IUCN) for endangered plant life.
Plucking the plant with its roots is difficult, while cutting and taking away timber would yield no results. The remaining roots and tree portion will replenish and grow.
Seed germination is also high in the case of this species as they grow even in dry, barren lands without any support system.
The plan is for a phased eradication by allowing the plants to dry and wilt by removing their bark. The Forest Department is also exploring chemical options to eradicate the threat.
“Sample testing is now going on,” sources in the department said.
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