Droughts can kill deep-rooted tree species more

The deepest water compartments that dried up during the drought took longer to recharge

Updated - January 20, 2018 06:34 pm IST

Published - January 20, 2018 06:08 pm IST

 A view of the 50-hectare forest plot in Mudumalai National Park, where the team worked.

A view of the 50-hectare forest plot in Mudumalai National Park, where the team worked.

Droughts can kill, but you would imagine that deep-rooted forest trees – whose roots tap into more permanent water resources – would be the least affected. But a study now finds that droughts killed tree species that access deeper water much more.

Tree deaths due to droughts are a major threat in both temperate and tropical ecosystems. This could further aggravate with climate change, with droughts predicted to increase in many parts of the world.

In a deciduous forest, where water is scarce especially in summers, how do different tree species with varying root depths deal with such water stresses? An inter-disciplinary team from institutes including Bengaluru’s Indian Institute of Science (IISc) studied how different species partition underground water resources up to a depth of 30 metres (just above the groundwater table) in Tamil Nadu’s Mudumalai National Park. In a 50-hectare forest plot, the scientists measured how much 7,677 trees belonging to 12 common tree species grew and how many others died between 1992 and 2012, which also saw an intense drought (2000 to 2003). The team collected local hydrological data including daily rainfall and water-holding capacities of local soils to estimate how much water is available across varying soil depths.

Devising a novel eco-hydrological model to quantify the depths from which trees took up water, the scientists find that while species like teak that absorbed water from near the surface may be adapted to droughts by surviving on scanty rainfall across the year, others like axlewood (Anogeissus) and laurel (Terminalia) took water from the deeper depths composed of weathered rocks. Their study, published in the Journal of Ecology, shows that though species that access deep water experience fewer droughts, they are more vulnerable to protracted droughts.

“The deepest water compartments that dried up during the drought took longer to recharge,” says lead author Rutuja Chitra-Tarak, then doctoral researcher at IISc. “We found that co-existing tree species diverged in water uptake depths, species using deep-water experienced drought more intensely, and thus died more – the first study to demonstrate this.”

Climate change—droughts could kill such species, says Chitra-Tarak. “At large scales, this can even lead to lesser water being recycled into the atmosphere, worsening droughts,” she says.

This novel hydrological modelling approach can help scientists model the impacts of increasing droughts on forests and their feedbacks on climate change. Chitra-Tarak is currently testing this across forest types and climates across the world.

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