Tree planting on grasslands can reduce specialist bird populations

India’s fast disappearing grasslands have long been targeted for afforestation – a practice that encourages tree planting on “degraded” parcels of land. With their vast and open landscapes, savannahs and grasslands are often considered barren land in need of more dense tree cover. But how does converting grassy, arid land to something woody and canopied transform an ecosystem, and the life within it?

New research from Maharashtra studies how old-growth savannahs, when planted with trees, changes bird populations and distributions. Grasslands and savannahs are estimated to cover 15%-20% of India’s land mass, and yet remain under-researched.

The lack of recognition of grassland ecosystems has meant their role in regulating microclimates and hosting endemic biodiversity has largely gone unnoticed. “There are very few studies on dry savannahs in general, and even fewer on the impacts of afforestation on them,” said Abi T Vanak, Director of the Centre for Policy Design at ATREE and a scholar of India’s grasslands.

The study from Maharashtra compared bird species and abundance across undisturbed savannahs and well-established plantations, creating new evidence for how the impacts of afforestation go beyond just changes to tree cover. It found that afforestation rendered the area uninhabitable for grassland specialist species like the Indian courser – a long-legged bird which scans the grass to forage – and the tawny pipit, which nests on the ground – among a host of others.

Instead, bird populations were maintained by an influx of other species better adapted to woody and forested habitats. The shift in species composition is a matter of concern because grassland specialists are declining at alarming rates across the country. The 2023 State of India’s Birds report found that across a 30 year period, from 1992 to 2022, populations of birds occupying open and natural ecosystems declined by half.

The recent study renews focus on the importance of landscape-level conservation, which is gradually finding recognition in state policies. On May 7, the Maharashtra government published a resolution mandating that afforestation drives be avoided on grasslands and wetlands, making it the first state in the country to do so.

“The intent of afforestation is to create rainforest density plantations everywhere, but water is a key limiting factor. Even if tree planting has to be done, it can be done in a way that suits the local climate and rainfall regimes,” said Vanak, adding, “You’d create a more open area that not only looks more natural, but consumes less water and improves the survivability of those trees.”

Relic of colonial rule

The impulse to plant trees in open natural ecosystems is a legacy of British colonial rule, when grasslands were labelled “wastelands” because – unlike forests, which could be harvested for timber – they did not generate revenue for the crown.

British policies in the 1860s thus focussed on making “improvements” to these lands by encouraging cultivation or afforestation on them, a 2019 study tracing the historical roots of India’s grassland governance, notes. “The colonial misconception of grasslands as degraded forests continued post-independence, creating an ostensible need to ‘reforest’ those landscapes that remained,” the paper added.

In Maharashtra, this legacy took shape in the form of Gliricidia sepium plantations. This exotic tree with pink flowers and a wide canopy was first brought to India for its ornamental value, and later planted widely in the state in the 1950s to improve fodder availability for livestock and biomass.

These plantations also provided the ideal setting to study how afforestation affected local bird populations, said Prabhav Benara, a student of wildlife biology and conservation at National Center for Biological Sciences, and a co-author of the Maharashtra-based study. “Birds are excellent habitat indicators, helping us understand how native biodiversity responds to land-use change,” he said. Benara worked with a team of five other students and researchers on the study.

Vishwatej Pawar, co-founder of the Grasslands Trust, a Maharashtra-based NGO dedicated to grassland conservation, who was not part of the study, said research into the state’s grasslands were needed to counter misconceptions about their utility. “There is no clear understanding of what a healthy grassland is, and what a degraded grassland is,” he said, adding, “There’s a basic need to document the biodiversity that exists on these lands, either through government departments or through gram sabha systems and people’s participation, to understand how rich these ecosystems are.”

The Grasslands Trust’s submissions and trainings on grassland management played a significant role in the state government’s adoption of its new resolution.

Winners and losers

Afforested patches of G sepium are located close to native, old-growth savannahs in the Pune and Satara districts, making comparison across both habitats viable.

The researchers recorded the presence of birds across 15 sites – each with one old growth savannah and one tree plantation – and divided them into three categories: savannah specialists (adapted to open grasslands), generalists (found in a wide variety of habitats) and woodland specialists (birds that thrive in more densely forested habitats).

Between December 2023 and February 2024, a total of 1,079 individual birds belonging to 69 species were recorded across all study sites. Out of these, 22 species were found only in the old-growth savannas, 21 species were found only in the tree plantations, and 26 species were found across both land uses.

Old growth savannahs included in the study were qualified by those which showed evidence of abundant native grasses, cattle grazing, and fire use – methods that humans have used to maintain grasslands for centuries – but were undisturbed by non-traditional interventions like mining and agriculture. Afforested patches that predominantly contained G sepium, as well as a smaller proportion of Eucalyptus, were also included for comparison.

All sites fell across an annual rainfall gradient ranging from 459 mm to 1,007 mm, indicating wide variation in precipitation levels. “We wanted to see whether rainfall also had an effect on which species were occupying these habitats. Both land use change and rainfall can affect biomes and species pools,” Benara explained.

The results show that while rainfall levels didn’t significantly impact the presence of savannah-specialist species across sites, those with tree plantations saw the biggest losses of these specialists in terms of both richness (diversity) and abundance (population). Woodland specialist species emerged winners, benefiting the most from the afforestation.

“Grassland specialist species like the harriers actually play an important role in stabilising the populations of insects, pests, and even rodents, which helps farmers,” said Pawar, adding, “When their habitats change, either due to afforestation or due to trenches, they compete for space or are unable to nest, roost, or properly forage, which causes population decline.”

Savannah specialist species like the rufous-tailed lark, tawny pipit, rock bush quail and long-tailed shrike saw the steep declines, according to the study.

How rainfall changes the picture

Factoring rainfall into the impacts of afforestation brings out an even more nuanced picture of how they change bird species composition.

For example, in areas that received intermediate rainfall (between 560 and 920mm), old-growth savannahs were found to host 11 more species compared to tree plantations. However, in areas with extreme rainfall (920 mm of rain and higher) or extreme dryness (560 mm of rain and less), afforested patches were found to host more diversity – up to 22 and 14 more species compared to savannahs.

According to the paper, “In the driest rainfall end, tree plantations have sparser canopy and relatively intact endogenous disturbance regimes (fire and cattle grazing), this leads to a mosaic of habitat patches that increases bird diversity in drier plantations… in comparison to the homogeneous, tree-less, drier old-growth savannas.” In the case of wetter regions, savannah specialists were relatively unaffected because these areas were less likely to host a diversity of them anyway.

The worst affected regions by plantations were in the intermediate rainfall zones, where plantations became dense enough to replace grassy understorey. “Tree plantations in this rainfall range are thus structurally homogeneous, and consequently lead to greatest diversity losses — with savanna specialists being the worst affected,” the paper reads.

“It’s not surprising that rainfall plays an important role in species diversity, but an important takeaway is that consistently, they found that savannah species tend to not do well where plantation density increases,” said Vanak from ATREE, adding, “Such insights are often missed in most restoration practices because savannah species aren’t treated with as much importance in India.”

Benera said the paper’s insights could also offer clues into how changing temperature and precipitation patterns due to climate change can affect plantations and biodiversity in the future. “Our findings suggest that as rainfall is projected to increase across Western Maharashtra, drier plantations may support fewer bird species as these habitats transition towards higher-rainfall conditions,” he said.

Apart from rainfall, irrigation from agriculture also contributes to making dry savannahs wetter, added Pawar of the Grasslands Trust.

The foremost recommendation is to stop planting over old-growth savannahs and focus on their conservation. “But a dual focus on preserving old-growth savannas and simultaneously acknowledging the supplementary role of some plantations and production landscapes in general might be pragmatic, given how rapidly old-growth savannas are disappearing despite our best efforts,” the paper stated.

This article was first published on Mongabay.