Graze or blaze: Study looks at ways to manage grass dominance in savannahs

A study from the Eastern Ghats highlights the dominance of native Cymbopogon grass (lemongrass) in a mesic savannah ecosystem and probes the causes and control measures to take it back to the desired or historical state to preserve biodiversity and ecosystem services.

Discussions around plant invasions and dominance are often in the context of foreign or exotic species. The study, however, illustrates that it is not uncommon for native grasses like Cymbopogon to dominate a landscape. The grass is unpalatable, its dominance gives little scope for other, palatable grasses to grow and support livestock and herbivory.

Cymbopogon is a C4 grass or a warm season grass that needs plenty of sunlight and is heat and fire tolerant. The grass contains oil and the dominance of such inflammable grass can result in more frequent, large fires that do not favour the biodiversity and the ecosystem.

The study, by scientists from India and Germany, explored better land management strategies to bring the grass under control.

Unique ecosystems affected by land use change

Open natural ecosystems such as savannahs are unique ecosystems that harbour distinct biodiversity but are neglected as they are often viewed as degraded forests or seasonally dry tropical forests. Savannah ecosystems have existed in India for over a million years, as indicated by fossil and molecular evidence from the diverse and endemic C4 grasses, predating any human modification of the landscape, notes the paper. They are highly vulnerable to various anthropogenic pressures due to lack of protection.

The study was conducted in mesic savannah landscapes in Chittoor and Madanappalle in Andhra Pradesh where the Cymbopogon grass species are found to dominate. The current high dominance of Cymbopogon grass is likely explained by recent changes in land management. The study notes that prior to 1987, Cymbopogon grass in the Eastern Ghats were harvested by local communities to thatch the roofs of their huts. This periodic harvesting coupled with a fire regimen controlled its growth to a great extent and ensured palatable grasses grew for the livestock to feed on. However, this grass harvesting declined in the late 80s with bricks and mortar houses replacing thatched huts. The paper reports that the pastoralists recollect Cymbopogon grass being sparse on the landscape and palatable fodder for livestock was more plentiful.

The study also points to an increase in local population and a corresponding growth in livestock but largely sheep and goats that do not consume this grass. “These species are usually only eaten by large-bodied grazers like gaur and cattle, that too when they (the grass) are young. Once they are adults, their leaves become unpalatable even for large, roughage feeders,” said Jayashree Ratnam, director of the Wildlife Biology and Conservation Program at the National Centre for Biological Sciences, who has studied savannah ecosystems extensively.

The study explored three different management strategies — control of both Cymbopogon grasses and fire; fire exclusion; and manual removal of grass and fire exclusion — and found that both grass removal and fire exclusion were necessary to reduce the dominant grass for a significant growth of palatable herbaceous plant biomass and species diversity.

Controlled fire as a land management strategy

Fire is an integral part of the management of savannahs. Unlike wet ecosystems, where moisture degrades the biomass, dry ecosystems like savannahs depend on fire to recycle the biomass. “The most annual rainfall these regions get is about 500 mm. The rainfall is erratic, too. There is never enough rain to decay the biomass. So, fire is an integral part of the system,” said the author of the paper, C. S. Saneesh. Fire management is critical in these ecosystems as optimal fire regimes promote healthy grass-dominated communities and species that depend on them.

The paper argues for fire exclusion for ideal intervals which is not the case now. “We found the fire return interval to be every 1.7 years. In some of my study areas, there were two occurrences of fire within a year, which is not ideal,” he said. The main finding, according to Saneesh is, if the dominant grass is removed along with fire exclusion for a year and a half or two years, the palatable grass increases to about 90%. However, in the long term, fire needs to be a part of savannah management. It is possible that the best management moving forward would be to bring back the historical fire regime, notes the paper.

The study also underlines the role local communities can play in keeping a check on the grass dominance. Saneesh says he believes in the landscape management by local communities through government initiatives such as the Mahatma Gandhi National Rural Employment Guarantee Act (MNREGA).

Ratnam notes that the management by local communities in these ecosystems is important and can be done through some combination of harvesting, controlled burning and grazing by livestock, depending on the status of the landscape.

Banner image: An aerial view of the mesic savannah landscape with an overgrowth of Cymbopogon grass. Photo by C.S. Saneesh.