Aarey, smog, wildfires — forests dominate headlines. But what exactly is a forest?

Forests can be defined as areas with a closed canopy of trees without grasses in the understorey. But there’s more to it.

Aerial view of the Amazon rainforest | Commons

Bengaluru: The north Indian pollution crisis, wildfires in the US and Australia, and the protests over deforestation in Mumbai’s Aarey Colony — forests have dominated headlines of late on account of many reasons.

Apart from being vacation spots and providing great photo-ops, they also sustain diverse animal populations, provide cleaner air and control warming, help regulate water quality and flow, and are a matchless ally as the world grapples with the worsening climate crisis. 

But what exactly is a forest? India’s green cover is not easy to understand. There are different types of greenery, lands where trees shouldn’t crop up even if they can, and the definition of forests, savannas, and other green ecosystems vary across the country. 

What is a forest?

Forests can be defined as areas with a closed canopy of trees without grasses in the understorey. In 2001, the United Nations (UN) defined a forest as “an area of >0.05–1 hectares with >10–30 per cent cover of plants >2–5 m tall at maturity”. 

Ecologically, forests are created by a diverse group of tree species, each with their own role. And these trees provide a habitat to countless inhabitants like insects, reptiles and birds, worms, amphibians, and mammals.

For example, it is primarily because of the Delhi Ridge that the city is the world’s second-most bird-rich capital, after Kenya’s Nairobi.

Forests are found in tropical and temperate regions with varying levels of seasonality. Tropical forests experience dry seasons but can still be sources of water. Some of the world’s most famous forests are found along the Western Ghats, eastern Himalayas and wet regions in the western Himalayas.

Borneo, the Congo basin in Africa and the Amazon in South America are examples of tropical forests. Siberia, high-altitude regions of the Himalayas and the Andes, and the Pacific coast of North America are examples of temperate forests.

According to a 2010 study, 7 per cent of India’s land-area is a forest, i.e. approximately 22 million hectares. 

However, these forests are scattered across regions with diverse evolutionary histories. Over half, 53 per cent, of India’s forests are found in just five places — Andaman and Nicobar Islands, Mizoram, Nagaland, Manipur and Arunachal Pradesh.

Is Aarey a forest?

The Aarey Colony in Mumbai became a flashpoint this year as the Mumbai Metro Rail Corporation (MMRC) felled hundreds of trees to clear land for a car shed. Angry residents took to the streets in protest against what they saw as an assault on a forest that houses deep biodiversity and provides clean air to a city often described as a concrete jungle.

A big part of the row centres on whether Aarey is a forest at all.

A look at the list of 1,300 plant species recorded from the Sanjay Gandhi National Park (SGNP), which adjoins Mumbai’s Aarey Colony, indicates a mix of dry-adapted (eg. Dillenia pentagyna, Polyalthia cerasoides, species from the Capparaceae family, Bombax ceiba, Sterculia urens, species of the Rutaceae family, some species of the Anacardiaceae family) and wet-adapted (eg. Michelia champaka, Flacourtia montana, Kokam and the Clusiaceae family, Melia azedarach, Toona hexandra, trees from the Leeaceae family, some trees from the Anacardiaceae family) species.

These plants are specifically suited to survive in dry or wet ecosystems, just like succulents evolved to survive dry ecosystems and willow trees grace Himalayan lakes.

In 1962, ecologist Robert Whittaker classified the world’s biomes by grouping regions based on their average annual temperature and total annual rainfall.

It was a good start to a rather complex problem as transitions between biomes are often gradual, rather than abrupt. The region of Mumbai (from Colaba to Sanjay Gandhi National Park and Gorai to Mulund) has an average annual temperature of 24°-26°C and an average total rainfall of 220-370 cm.

This data means Mumbai can host a forest ecosystem. The biodiversity of the 100+ square kilometres of SGNP and Aarey colony is typical of a forest.

It is a thriving ecosystem with trap-door spiders, endemic ground dwelling geckos, native plants, and leopards, besides countless other species.

However, not all kinds of green cover hold such healthy ecosystems, and not all ecosystems driven by plants look like forests. For example, savannas.

What are grasslands and savannas?

Television shows have made us familiar with expansive vistas of African grasslands teeming with lions, hyenas, elephants and giraffes. 

These grassy plains scattered with trees are called savannas and are present on all continents bar Antarctica.

In 2011, in a study titled “When is a ‘forest’ a savanna, and why does it matter?”, Indian scientists concluded that “savannas are mixed tree-grass systems characterised by a discontinuous tree canopy in a continuous grass layer”. 

According to them, the actual tree cover (total area of tree canopy in a given area) in the world’s savannas varies, ranging from grasslands with sparse trees to woodlands with many trees. This variation is driven by rainfall.

The study also shed some light on the ecological processes and the architectural differences between forests and savannas. If the canopy is closed, it is a forest. If it is not closed and the grass layer is of the ‘C4 type’, it is a savanna, the researchers said.

C4 is a relatively recent evolutionary group among grasses, which date back 100 million years. They are called thus because the first product of photosynthesis is a four-carbon compound. 

The unique anatomy of the blades allows for higher productivity when atmospheric CO2 concentration is low and temperatures are high.

Savannas are ancient ecosystems too, and C4 plants are believed to have begun dominating the landscape 22 million years ago.

The evolution of herbivore-adapted savannas (arid, nutrient-rich savannas with thorny trees), development of seasonal and monsoonal climates, and intensified fire cycles are the factors that have likely driven the global transition from forests to savannas.

Forest species don’t survive in savanna conditions (high light, low rainfall, frequent fires). Adult forest trees are relatively longer, with wider canopies, thin fire-sensitive bark, and large light leaves.

In savannas, adult trees are the exact opposite: Short, with thick fire-resistant bark, open crowns, and small thick leaves. Peanuts, rice and most trees are C3 plants, while maize, wheat, millets are C4.

Agricultural greening versus forest cover

Earlier this year, a team of scientists from Boston University, NASA and the Indian Institute of Science (IISc) published a paper titled “China and India lead in greening of the world through land-use management”.

The study has been misunderstood and was frequently misinterpreted to argue that deforestation and loss of savannas are compensated by this increase in green cover.

The paper defines greening as “a statistically significant increase in the annual average green leaf area at a location over a period of several years”.

Around 82 per cent of the increased green cover was contributed by cropland greening, while forests contributed only 4 per cent. The greening of croplands has been facilitated by heavy fertiliser use and increasing irrigation.

Crop greening is nowhere close to natural ecosystems, especially when it comes to the benefits of biological diversity. Savannas and forests provide ecosystem services to humans such as food, medicines, water, wood, grazing land for livestock, carbon sequestration, resilience during droughts and floods, and biodiversity. 

The only ecosystem service provided by croplands is food.

The increase in green cover in India only shows that the country is moving towards food self-sufficiency. Deforestation rates in India point to the grim status of its forests. 

Scientists, for example, have criticised the UN’s simplistic definition of forests for not taking into account the ecological complexity and biodiversity found in natural ecosystems.

By this definition, whether a mangrove swamp is replaced with trees or a rainforest completely felled and a few saplings planted, they will be considered forests.

Comparable loss rates for savannas are not available, and are the need of the hour to scientifically assess the health and extent of our natural ecosystems.

The planting of trees in a savanna for it to be redefined as a forest is the equivalent of environmental destruction.

It is time for our lawmakers to enable the protection of our natural ecosystems rather than plant trees where they don’t belong or fell forests on the promise of restoration.

Indian scientists recently showed us the way ahead for forest restoration. We must learn from them if we are to adapt to the ongoing climate crisis.

Chintan Sheth is a research scholar at Bengaluru-based National Centre for Biological Sciences, an affiliate of the Tata Institute of Fundamental Research. He studies landscapes, climate change, rocks and plants, and can be found on Twitter at @blueczkfox. The views expressed are personal.