India’s open ecosystems face an unusual threat: trees Premium

Study reveals increasing tree cover negatively affects grassland birds, highlighting the threat of woody encroachment on open ecosystems.

Increasing tree cover is often seen as a positive outcome of biodiversity conservation, and a much-needed effort to combat climate change. What happens, then, if tree cover increases in areas that historically hosted a different habitat?

In a study published on June 5 in the journal Global Change Biology, scientists of the Universities of Witwatersrand, Cape Town, and Oxford reported that more trees in open ecosystems like savannahs and grasslands have substantially reduced the number of native grassland birds. In the African Savannah in particular, the population of grassland birds has declined by more than 20%.

Grasslands and savannahs are biodiverse habitats in tropical and temperate regions throughout the world. They cover nearly 40% of the earth’s total landmass, and are home to many endemic and at-risk species of plants and animals. From megaherbivores like elephants, rhinoceroses, and buffaloes in Africa and Asia to grassland birds like the bustards, floricans, and grouse of the Himalayan grasslands and American prairies, open ecosystems have it all. However, we are rapidly losing them.

Activities threatening them include the conversion of grasslands, intensive agriculture, loss due to erosion, large-scale development projects, overgrazing. But lurking among these usual suspects is also a highly unusual one: trees. 

The increase of tree and shrub cover is called woody encroachment — and it is widespread across most ecosystems. Woody encroachment entails the conversion of open habitats to habitats with greater tree cover and/or shrub density. The end result is the homogenisation of an ecosystem, meaning a diverse, multi-layered ecosystem turns into a uniform layer of woody plants.

This is a dire prospect because open ecosystems are characterised by a grassy understory and a scattering of native tree species. They are generally maintained by certain natural as well as human activities like grazing and fire, which are called disturbance regimes because they work in tandem to limit the growth of tree species. But once these regimes are disrupted, trees have the calm they need to establish themselves and start woody encroachment.

A higher concentration of carbon dioxide in the air due to ongoing climate change also encourages deep-rooted woody plants in grasslands to proliferate.