Contrary to popular belief, afforestation programmes that blindly plant trees in the hope of conserving water only end up consuming more water
Nature-based solutions like planting of trees and restoration of forests are often touted as the panacea for water conservation. This is because forested watersheds — lands covered by forests which drain all the water flowing through them into waterbodies like rivers or lakes — provide a whopping 75 per cent of the world’s accessible freshwater resources. But many organisations implementing this crucial nature-based solution have been unable to differentiate between restoration of forests and planting trees.
For instance, in India, afforestation was one of the interventions of the Union government’s Jal Shakti Abhiyan, launched in July 2019, to make the country’s most water-stressed districts water secure. Under this programme, district administra tions were encouraged to under- take planting of trees in a big way. The enthusiastic local authorities reported a staggering number of afforestation activities which turned out to be fudged data, as admitted by district officials as well as a senior official in the Jal Shakti ministry. Even if these numbers were real, simply planting trees will not conserve water. In fact, trees can suck up water and release it through evapotranspiration — water lost by trees to the atmosphere through tiny openings on the underside of their leaves known as stomata.
“Studies conducted in various parts of the globe, especially in semi-arid and arid regions have shown that blind afforestation does not increase water supply,” says Gopal Singh Rawat, former dean of the Wildlife Institute of India, Dehradun. “When sparsely vegetated land is converted into forest, there is a reduction in blue water (available for human use) and increase in green water (part of water available for plant use). Trees can consume more water than other shorter vegetation. According to the mass balance principle, if more water is used by trees, less water will flow into rivers and lakes or recharge the groundwater that people can directly use,” he says.
There are three aspects to be considered while planting trees for water conservation. First is that of scale. In general, forest expansion of 2 sq km or more can increase the possibility of rainfall. Trees transport water to the air, and water vapour moves to another location, which can be far from the afforested area. “On a global scale, afforestation can bring benefits to the water cycle,” says Rawat. The second aspect is what kind of tree species must be planted for water conservation. Invariably, fast-growing broad leaved species such as eucalyptus and poplar consume more water as compared to needle-leaved species, such as casuarina and pines. The third aspect is that of site characteristics. Areas with varying geology, soil and patterns of precipitation have different responses to large-scale plantati ons. For instance, a study carried out for 30 years till 2011 in China shows that different regions experienced varying changes in precipitation and soil moisture with increasing number of trees.
In north and southeast China, enhanced precipitation resulting from increased tree cover was able to cancel out water loss due to evapotranspiration leading to no changes in the regions’ soil moisture levels. In southwest China, during the same period, the researchers observed a significant decrease in soil moisture, while there was also a weakening of the summer monsoon season. In northeast China — the only region where a decrease in forest cover was observed — soil moisture went down drastically because of an anomalous anti-cyclone (high pressure area that disrupts the formation of rain bringing low pressure areas) formation during summer. A study published in PLOS One in August 2016 found that soil moisture in the topmost layer of soil decreased after afforestation and this decrease was different for different species of trees and varied with regions.
In India, the problem began a long time ago. “Misplaced tree plantations began when some trees like the eucalyptus were used for draining swamps in the Nilgiri hills of Tamil Nadu, especially near Ooty. This was done in the middle of the 19th century by the British,” says M D Subhash Chandran, a senior scientist with the Centre for Ecological Sciences of the Indian Institute of Science, Bengaluru. The British saw these areas as health risks as these were mosquito habitats which could cause malaria. Moreover, most of the tribes who lived in the area also practiced slash-and-burn cultivation, which changed the character of vegetation, from primary to secondary. In later years, the British harvested teak trees for timber from secondary forests. “The British faced significant problems as sometimes the old evergreen trees would grow back in these secondary forests as well. They shifted to large-scale tree plantations which introduced monocultures to India for the first time,” explains Chandran.
Post-Independence, governments have pursued the same strategy, sometimes even more vigorously. “Eco-restoration of degraded natural terrestrial ecosystems would be better as compared to blind afforestation,” says Rawat.
Model of succession
Natural ecosystems, especially evergreen systems found along the catchments of rivers in the Western Ghats, are much better at conserving water as they have complex root systems, which can hold large amounts of soil together and that can, in turn, hold large quantities of water in place. They also slow down the flow of water streams through them which helps the soil absorb and hold more water.
“Plantations, on the other hand, lead to soil erosion and greater water flow,” says Chandran. Moreover, secondary forests can also be restored back to primary forests scientifically. “This can be done using the model of succession. You cannot plant sensitive species of an ecosystem in the open areas as they will be scorched in sunlight. There are certain transitional species that need to be planted first. Then, the ecosystem needs should be allowed to grow around them, with sensitive species being introduced at a later stage,” says Chandran.
There are myriad problems with tree plantation exercises being carried out in India, even by forest departments. “Forest departments generally use tree species which can give good results, which in this case means survival,” says Chandran. So they have very few choices of tree species that are fire-resistant and consume less water. This criterion for the choice of tree species to be planted needs to change if we have to restore secondary forests and plantations back to primary forests, he says.
“At least in Uttara Kannada district of Karnataka, I have observed that wherever teak plantations have come up, water in the surrounding areas have dried up. If water flows of natural ecosystems and randomly planted systems are compared during the monsoon season, the former are far more consistent than the latter,” adds Chandran. Areas where trees are randomly planted are also prone to landslides, forest fires and weed infestation. This is what happened in the case of bushfires in Australia and forest fires in California. “Fire hazard will be far less in the restoration model. The plantation policy of the country needs to be redesigned,” adds Chandran.
This was first published in the 16-31 May, 2020 print edition of Down To Earth
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