Black wattle, invader of the Nilgiris

They [the Nilgiri hills] are as smooth as the lawns in an English park, and there is hardly one of them which has not a mass of dark wood terminating suddenly as if it had been planted…” The former governor-general of Madras Presidency Thomas Munroe’s description in 1826 of these ancient mosaics of forests and grasslands no longer holds. Extensive plantations of various exotic species by both the colonial and Indian governments have changed the landscape.

Across the Nilgiris, and indeed other grasslands in India, invasive species are taking hold. And there is ample reason to worry, judging by the results of a recent study on these mountain ranges.

The study area is part of the Nilgiris, a biodiversity hotspot in the Western Ghats, where several of the natural grasslands were converted into plantations of black wattle or Acacia mearnsii by the forest department. These trees have now invaded several of the montane grassland systems in the Nilgiris.

A study by a team from the Indian Institute of Human Settlements, Bengaluru, Foundation for Ecological Research, Advocacy and Learning (FERAL), Tamil Nadu, and the University of Lancaster, reports in the Journal of Hydrology that wattles increase flood risks during heavy and extreme rainfall events, have a significant water footprint and might be detrimental to the hydrologic services of catchments in the Nilgiris, in both dry and wet seasons.

“Very few studies have been conducted in the Nilgiris that have looked at the impact of invasive species on hydrological services,” Rajat Nayak, a researcher at FERAL who co-authored the study, told DH. A few studies that have been conducted have mainly looked at the impact of invasive on plant diversity.

The researchers have set up a high-resolution network of rain gauges and stream-level monitoring units in the Nilgiris, which can help provide data that can be used to improve flood forecasting, develop climate change mitigation strategies in the Nilgiris and provide baseline and reference data to identify sites for restoration and monitor the success of future restoration activities in the Nilgiris.

“Anecdotal evidence suggested that wattle reduces stream and groundwater through excess evapotranspiration losses, thereby increasing drought risk,” says Nayak. “Our study, suggests that in addition to the reduction in water availability during summer, wattle invasion can also increase flood risks in mountainous regions.”

“This is a unique study,” says Kamal Bawa, president of Ashoka Trust for Research in Ecology and the Environment (ATREE), Bengaluru. Land cover impacts the flow of water, especially after heavy rains. In India, vast areas of natural ecosystems have undergone land cover change due to the degradation or conversion of land for agriculture, plantations and other purposes.

“Although land cover change is widespread in India, this is one of the few or perhaps the only study of its kind that examines the impact of such changes on the flow of water after rainfall,” says Bawa. “The study is extremely important for meaningful action and policy changes related to mitigation of flooding, especially in the context of climate change.”

It is widely assumed that Acacias were planted in natural grasslands for two reasons, says Bawa. First, the bark of black wattle is a source of tannins used for tanning leather, a vital enterprise in the Nilgiris. “The forest department must have viewed plantings as a source of revenue.”

“The second is the misguided policies.” For quite some time, the forest department has believed that all lands that are devoid of tree cover need to be restored by planting trees, not recognising that some ecosystems, particularly grasslands, are naturally without trees. Policymakers have classified grasslands that occupy vast areas in the country as wastelands.

The invasive Black Wattle introduced in India from Australia has spread rapidly on its own in the evergreen Shola forests in the middle hills of the Nilgiris.

“The Tamil Nadu forest department has now realised the mistakes of the past and has a programme for the replacement of wattle with native grassland species,” says Bawa.

“Our interventions in intact natural ecosystems must be minimal,” he adds. “Tens of millions of hectares of degraded land in India need to be restored. We must rely on science to guide what to plant where and when.”

Nayak says that the wattle removal activities prevalent now have mainly targeted clear-felling of large matured wattle stands, but this may prove counter-productive too. Removing large stands may result in increased soil erosion and increased sediment load in the water bodies in these mountainous landscapes.

“Our study findings could help in designing management plans which could protect the ecological functions of a mountainous watershed system. The study also emphasises the need for the management and restoration of high-altitude grasslands.”

Also, the coppicing stems and regenerating plants from seed banks could amplify water losses as young plants have far higher evapotranspiration rates compared to mature stands. “Hence, a rigorous study on possible soil erosion and the gain in ecosystem conditions by wattle removal should be conducted to identify and prioritise areas for wattle removal,” says Nayak.