Trees in lakebeds: Boon or bane?

Farmers and fishermen have the impression that Acacia nilotica and Prosopis juliflora, are high water consumers, which they think affects water levels in the lake. Scattered trees on a lakebed may not pose a significant issue, but the scale and density of plantations can be problematic, say experts.

Bengaluru: Researchers of Paani.Earth, a Bengaluru-based non-governmental organisation focusing on water, were puzzled to see some patches inside the Hesaraghatta lake, when they were observing the satellite images in 2022.

“We later figured out that the patches were nothing but trees inside the lake, in the water,” said Khushbu K Birawat, a researcher at Paani.Earth. Out of curiosity the researchers checked many other lakes, and found the same trend.

Paani.Earth team, including the curators Nirmala Gowda, Madhuri Mandava and Khushbu K Birawat, attributes the origin of the idea to plant inside the lake to Lakshman Rao Report released in 1986. The report advocated the “foreshore planting” of saplings and developing tree parks on dried lakebeds. As a result, the Karnataka Forest Department (KFD) started planting inside lakebeds to prevent lake encroachment, since the 1990s, for next tow decades.

In addition to planting native trees along the lake foreshores, the KFD also planted or sometimes dispersed seeds of the invasive fuelwood species, such as Prosopis juliflora and Acacia nilotica, directly on the lakebeds.

Paani.Earth explored the consequences of this through fieldwork, spatial analysis, online research, and discussions with various stakeholders—including limnologists, professors, fishermen, local activists, gram panchayat members, and forest department officials. The findings have been uploaded on its website.

Water storage affected

Open water in a lake, that can be seen and measured from satellite images, serves as an indirect indicator of changes in lake water storage levels. In October 2022, the lakes in the Hesaraghatta catchment overflowed. However, by April 2023, just five and a half months later, the open water levels had declined by half.

When submerged trees die, they release nutrients that deplete oxygen levels, leading to the asphyxiation of sensitive native aquatic species.

This reduction in storage is largely due to silt accumulation. Informal desilting by sand miners and brickmakers typically removes silt and increases water storage. However, the presence of trees prevents these activities. As a result, areas with tree growth remain largely untouched and become shallower, drying up quickly, found out researchers. Although the lakes overflowed in 2022, creating an illusion of ample water, their actual storage capacity is limited by this shallowness.

A spatial analysis by Paani.Earth revealed that nearly half (47%) of the lake area in the Hesaraghatta Lake catchment is covered by trees and shrubs. Data from European Space Agency World Cover 2021 they analysed showed that in Bengaluru Urban district, 49% of lakebeds are covered with trees and shrubs, with areas like Anekal Taluk experiencing even higher coverage at 57%. In some instances, entire lakes were transitioned into forest-type woodlands or tree parks.

Water guzzlers?

Farmers and fishermen have the impression that Acacia nilotica and Prosopis juliflora, are high water consumers, which they think affects water levels in the lake. Recently, Yamare Gram Panchayat in Anekal wrote to the forest department, requesting the removal of these two species from the lakebed. They cited significant reduction in groundwater levels and surface water storage, leading to drinking water shortages.

The panchayat’s concern is consistent with Paani.Earth’s observations, say the researchers. Though the forest department officials maintain that there is no scientific evidence to support this, researchers believe the traditional knowledge of farmers needs to be recognised, while more scientific studies must be done to find out the truth.

Paani.Earth’s report points out that farmers living near Sidlaghatta Lake in Chikkaballapur also support the removal of trees from lakebeds, citing that large areas of dense trees can reduce water storage capacity. This concern was raised in the Karnataka Legislative Assembly in 2018.

Removal: A solution?

The methods used for planting or removing the plantations can significantly alter aquatic biodiversity. Fishermen from Doddaballapura, who have been fishing in the lakes that form Hesaraghatta lake’s catchment for over four decades, note that fish eggs and other aquatic life forms can remain dormant in the dry lakebed, only to become active when rainwater replenishes the lake. Using heavy machinery, such as excavators, to dig pits for planting saplings or removing plants can disturb the lakebed and destroy the fish eggs.

Planting trees on lakes alters the habitat, disrupting fish feeding patterns and changing the composition of aquatic species.

Limnologist Ravichandra Reddy who worked with the researchers of Paani.Earth told DH that planting trees on lakes alters the habitat, disrupting fish feeding patterns and changing the composition of aquatic species. “Trees reduce penetration of light, leading to less oxygen production and reduction in amount of fish. The leaf litter produces higher organic matter resulting in plankton bloom and oxygen deficiency” he added.

When submerged trees die, they release nutrients that deplete oxygen levels, leading to the asphyxiation of sensitive native aquatic species. He stressed that much research is needed in this aspect.

When can trees be planted on lakebeds? “If the water stays for more than two months at a level, most plants cannot survive. When planting inside the lakebed, 50 years of rainfall data must be taken into consideration. Saplings should be planted only if the area is under water for less than two months in a year,” explained Brij Kishore Singh, former principal chief conservator of forest, who retired in 2013.

‘No threat from plants’

Regarding plants inside Hesaraghatta lake, which DH captured on drone, Ravindra Kumar, Bengaluru Urban district Deputy Conservator of Forests, told DH the plants exist on the periphery of the lake, not inside the lake. Most of them are dead, which happens when they are submerged in water. There is no direct threat on water storage for the lake because of the trees, he added.

He told DH that the Minor Irrigation Department is planning to pump treated water from KC Valley to tanks in Anekal Taluk, and has requested the forest department to cut the trees inside the lakebed to increase the storage capacity of the lakes. There is no other proposal to remove plants from the lakebed, he added.

In 2021 as well, Minor Irrigation proposed removing 6,316 trees on Singanayakanahalli Lakebed, near Doddaballapura to fill the lake with treated wastewater from Hebbal-Nagawara Valley Project. Around 5,000 of the 6,316 trees to be cut were Acacia and Prosopis juliflora.

Citizens and activists argued that the trees could coexist with the lake water. The Minor Irrigation Department said that tree debris is rich in nutrients, which when accumulated in water bodies, can lead to excessive growth of algae and aquatic plants through a process known as eutrophication.

The forest department may not have planted Prosopis juliflora; they might have spread through bird droppings, said Singh. He felt that total removal of Prosopis juliflora is impossible, and it can only be contained.

Encroachment not prevented

There are no reports from the KFD to show whether the encroachment was prevented by such plantation. The spatial analysis of the lakes in the Hesaraghatta lake catchment area reveals that despite the presence of trees in the lakebed, encroachment, primarily from agricultural activities, brick-making units and built-up areas, has only continued.

The Karnataka Tank Conservation and Development Authority (KTCDA) Act was implemented to manage and conserve tanks and lakes. Since the plantation activities were undertaken before this Act came into effect in 2014, they are not legally considered encroachments. But today similar tree planting would be considered an encroachment under the KTCDA Act.

Several government reports that came out have largely overlooked the issue of trees on lakebeds. The Koliwad Committee, formed in 2017 to assess the nature and extent of encroachment on waterbodies, identified only one instance of tree encroachment by the Karnataka Forest Department (KFD). In the case of Hesaraghatta Lake, the Committee reported that the Forest Department had encroached upon 21 acres and 21 guntas of the lake.

Another report by EMPRI, titled ‘Comprehensive Assessment of Thippagondanahalli Reservoir Catchment Area and its Preservation Zone,’ said that the trees on the lakebed at Kakolu Lake pose a threat to the lake’s ecosystem. It overlooked the forest-type woodlands on all other lakes.

Scattered trees on a lakebed may not pose a significant issue, but the scale and density of plantations can be problematic, said Nirmala Gowda. “The focus should shift to planting only on the foreshore, and species should be selected carefully, to avoid invasive species. Trees should be confined to the catchment areas and the periphery of the lake,” she added.

(Inputs from Adithyan P C, Shree D N and Paani.Earth team)

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