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Do trees make rivers flow?

How does a missed call translate into saving rivers?   | Photo Credit: M. Samraj

Suddenly, the electric blue billboards were everywhere. And printed in white on them the words ‘Rally for Rivers’. A brain child of Sadhguru Jaggi Vasudev, the ‘mystic’ and founder of Isha Foundation, the campaign swept across the country with an undeniably compelling request: ‘Everyone who uses water must Rally for Rivers’.

The means to lend support appeared incredibly simple: give a missed call to a phone number. But it was also somewhat befuddling: how does a missed call translate into saving rivers? The idea was possibly only to illustrate public support for the cause, and the calls did pour in by the millions.

As the campaign gathered steam, a team of volunteers sought consultation from farmers, industry professionals and select scientists to put together a draft policy document. On October 3, Jaggi Vasudev presented this document to Prime Minister Narendra Modi.

Intention does not make science

The draft policy, to ecologists, is a mixed bag — well intentioned perhaps but not all of it based on sound science. The rally’s primary plan of action, for instance, to ‘save’ India’s beleaguered rivers, has been to plant trees along the length of the 20,000 kilometres of river in the country, in 1 km strips on either side. Forest trees on government land and fruit trees on land owned by farmers.

Tree planting, the rally claimed, would achieve a slew of benefits: improve the seepage of water into the soil, regulate river flow, enhance rainfall and improve biodiversity.

Now, planting trees in degraded forest land may have its uses. This, however is contingent on planting the right mix of species, and at densities that the natural environment can support. Local communities that use the area for farming or fishing must, of course, be consulted during the process.

The policy document has recommended 16 tree species, including teak, mahogany, and shisham that are of high timber and industrial value. The team, to its credit, acknowledges that the list is not exhaustive. But it is striking that most of the species on the list are not quite representative of riparian landscapes.

“This is a gross misrepresentation of riparian vegetation across India. Even in dry landscapes with comparatively fewer species, the forest patches along rivers are far more diverse. The species list is just too small,” says Siddarth Machado, botanist and researcher at the National Centre for Biological Sciences (NCBS) in Bengaluru. The campaign has stressed on the need to plant native tree species. But mahogany is from southern U.S. and the Caribbean islands; and casuarina is from Malaysia and Indonesia.

Even the native species listed in the document are not without problems. The shisham tree, for example, is native to northern India but doesn’t naturally occur in the south. Similarly, red sanders sandalwood occurs naturally only in the Eastern Ghats and cannot be randomly planted anywhere.

Trees don’t belong everywhere

Besides, the vegetation along a river’s length is not static. Along its meandering path, the mix of species changes as the river snakes through different landscapes, and is eventually replaced by mangroves where the river meets the sea.

Riverine vegetation may not be uniform and may also not extend as far as a kilometer from the river’s edge. For example, vegetation along banks of rivers flowing through more open, sparsely-treed landscapes can be restricted to strips sometimes as thin as 30 to 40 meters from the river’s edge.

Then, there is the fundamental question: Do trees make rivers flow? Trees indeed help water seep into the soil and eventually to the water table via connecting tunnels fashioned by their roots. Soil can thus absorb more rainwater, potentially mitigating floods. Also, evapotranspiration from tree leaves reduces the excess water in the soil.

But the optimism of the rally relies heavily on the rationale that forests act as sponges, absorbing rainwater and releasing it during dry periods, ensuring that rivers flow year-round.

This is indeed true in some cases. In the Uttara Kannada region of the Western Ghats, forests in the catchment areas have allowed the release of water into the river, even after the rains. But it is perhaps true in only 30% of the studies from around the world. “70% of the time, increase in tree cover can decrease the stream flow, including in the dry season,” says Jagdish Krishnaswamy, a leading ecohydrologist from Bengaluru’s Ashoka Trust for Research in Ecology and the Environment.

Also, trees don’t belong everywhere. Planting a tree in an arid landscape is like bringing a polar bear to the Thar. Trees are unlikely to survive in places better suited for grasses and shrubs. Likewise, places that see heavy monsoon flooding could kill the saplings of several species.

In some cases, single-species plantations have been raised on grassland and open shrubland. These trees, it was later found, increased rainwater infiltration into deeper soil layers, but dramatically reduced outflow into nearby streams and rivers, thus creating greater water shortage in an already parched landscape. Grasslands and scrubs, it must be underlined, are unique and valuable in their own right and harbour considerable biodiversity. Local, often landless, communities use these areas to graze livestock. Thankfully, the rally has recognised that such landscapes should not be forested.

The rally argues that the trees along the river banks will generate rainfall through evapotranspiration. However, most studies that reported this phenomenon of forests enhancing rainfall have done so over a much, much larger area, such as in the Amazon rainforest.

A study published in 2012 in the journal Nature argued that over large scales — such as entire countries and continents — the presence of trees does indeed increase rainfall. However, this requires the passage of clouds over hundreds of kilometres of forests or dense vegetation.

Steering clear

Krishnaswamy is not convinced that the rally’s plan will have such a linear benefit. “It’s unlikely that just a 1 km strip on both sides of a river is going to make a really big difference to rainfall,” he says.

Forest-water relationships are complex. Our knowledge of tropical, riverine ecosystems, especially with respect to soil and hydrology is limited. Given this, the claims made by the Rally for Rivers team are not sufficiently nuanced.

During its month-long stint, the rally steered clear of the larger and more important issues — river-linking, excessive damming, sand-mining and pollution. Befittingly, however, some of these issues are discussed in the policy document and potential plans of action suggested.

Fortunately, there is a three-month feedback window for State governments, NGOs, scientists and concerned citizens to share their knowledge and voice their concerns. If these other social and political issues are addressed, there is greater hope for India’s rivers — and more to be achieved than by randomly planting trees.

Lalitha Krishnan is a research scholar at NCBS and is interested in ecology, behaviour and neuroscience.

Priyanka Runwal is an ecologist affiliated with NCBS with a special interest in arid grassland and savanna ecosystems.

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Printable version | Jun 12, 2021 12:14:13 AM |

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