Increased tree cover is often seen as a positive outcome of biodiversity conservation, and a much-needed effort to combat climate change. What happens, if tree cover increases in areas that were historically different habitats?
The study was published on June 5 in the journal Biology of Global Change, scientists from the University of the Witwatersrand, Cape Town, and Oxford reported that other trees in open ecosystems like savannahs and grasslands have reduced the number of native grassland birds. In the African Savannah in particular, grassland bird populations have declined by more than 20%.
Many become one
Grasslands and savannas are biodiverse habitats in tropical and temperate regions around the world. They cover almost 40% of the total land area of the earth, and are home to many endemic and endangered species of plants and animals. From megaherbivores like elephants, rhinoceroses, and water buffalo in Africa and Asia to grassland birds like bustards, floricans, and grouse from the Himalayan grasslands and American grasslands, open ecosystems have it all. However, we quickly lost it.
Threatening activities include grassland conversion, intensive agriculture, losses due to erosion, large-scale development projects, overgrazing. But hiding among these suspects is also unusual: a tree.
The increase in tree and shrub cover is called timber encroachment – and it is widespread in most ecosystems. Woody encroachment entails the conversion of open habitat to habitat with greater tree cover and/or shrub density. The end result is the homogenization of the ecosystem, meaning that the diverse ecosystem becomes a uniform layer of woody plants.
This is a daunting prospect because the open ecosystem is characterized by a grassy understory and the spread of native tree species. Most are managed by certain natural and human activities such as grazing and fire, which are called disturbance regimes because they work together to limit the growth of tree species. But if the regime is disturbed, the tree has the necessary calm to establish itself and start the encroachment of wood.
When trees have bad effects
Higher concentrations of carbon dioxide in the air due to ongoing climate change are also pushing deep-rooted woody plants in grasslands to thrive.
“An increase in atmospheric CO2 is likely to promote trees over grass because the C3 photosynthetic pathway used by trees is favored in high CO2 conditions,” Jayashree Ratnam, program director, Wildlife Biology and Conservation at the National Center for Biological Sciences (NCBS), Bengaluru, said. “Once the tree becomes dominant in the system, it can reduce the grass through shading and fire suppression.”
Grass in Hanle, Ladakh, July 12, 2024. | Photo Credit: KSL
Dr. Ratnam studies the biophysical and anthropogenic drivers of tropical savanna and rangeland structure and function and the forest-savanna transition zone.
Woody’s encroachment is spreading worldwide. Many studies have found evidence of various drivers of encroachment on different continents. Fire suppression and fragmentation dominate the grasslands of South America, while more carbon dioxide and rainfall variability prevail in Australia and Africa.
Indoor and outdoor gardens
Closer home, in India, grasslands occur in different climatic regimes: the arid grasslands of the western part of the country; grassy flood plains dot the Himalayan landscape; and the Shola grasslands that crown the Western Ghats, to name a few. In the foothills of the Himalayas, the tall, wet grass is a biodiversity habitat inhabited by beautiful species such as the Indian one-horned rhinoceros, swamp deer, Bengal florican, swamp grass babblers, and several other endemic species.
This grass is critically endangered, not least because previous destruction has broken it into pieces in a sea of forests, agriculture, and other human-derived habitats. That fragmentation makes this patch more vulnerable than ever.
Most of the rainforests are now within protected areas such as national parks and sanctuaries. But wood encroachment is also rampant in the park, as a February 2023 study by researchers from the Indian Institute of Science Education and Research (IISER), Kolkata; Hainan University in China; and the Durrell Conservation Trust in England, it was reported. Using data from remote sensing satellites, they found high levels of timber encroachment in several national parks in India and Nepal over the past three decades. Grass habitat cover decreased by 34%, while tree cover in the area increased by 8.7%.
Human hands
Woody encroachment is a direct result of human-driven factors that change the disturbance regime open ecosystems need to develop. Suppression of grassland practices must evolve from colonial conservation and management policies. Colonial officers in tropical countries are known to consider open ecosystems as “empty land” because they take up space where trees can grow, and provide wood. The classification allowed the habitat to be converted into plantations and also provided land for the colonial government to criminalize communities practicing grazing and fire management.
A different kind of colonialism threatens open ecosystems today: instead of wood, many see trees as providers of carbon sequestration potential, and open ecosystems as encroachments that reduce human access to that potential.
Refusal to acknowledge the existence of historical grasslands and savannahs also results in failure to protect them. Woody encroachment on open ecosystems has altered biodiversity in various ways. There has been a great decline in grassland birds due to timber encroachment.
In the South African countries of South Africa, Eswatini, and Lesotho, scientists used citizen science data from the ‘South African Bird Atlas Project 2’ to find dramatic declines in open ecosystem bird populations. Of the 191 species analyzed from 2007 to 2016, the trend of population decline is present in 121. Of these, the decline of 34 species is related to wood encroachment.
A six-year-old cheetah in the Masai Mara Savannah cautiously approaches its prey hiding in the grass. Under the Cheetah Project, the next few cheetahs from Africa can be sent to a cheetah breeding and conservation center in the Banni grasslands, the Hindus reported on July 25 | Photo Credit: RAMESH SUSARLA
“The succession of woody species changes soil conditions, which changes grass species and faunal associations. Woody species invite increased predation especially of specialist bird nests,” said Hem Sagar Baral, ornithologist and former head of the Zoological Society of London (Nepal chapter).
For the same reason, timber encroachment has affected the population of grassland specialist rats in the grasslands of Banni Kutch. This species also suffers from a penalty for survival: the grass allows them to hide from predators, but when the trees grow, they spend more time on alert and less time feeding.
Invasion by trees
Woody encroachment on grass has also received a leg-up from large-scale tree plantation programs. In Banni grass, studies have found that the spread of invasive species Prosopis juliflora – planted by the Gujarat Forest Department since 1961 to combat desertification and provide communities with firewood – has since transformed swaths of grass into Prosop forest.
In fact, many open ecosystems in India have stories of destruction led by artificial plants. In the Shola grasslands, eucalyptus plantations have collapsed, while Malabar silk cotton trees have stormed the wet terai grasslands of the Himalayas.
To combat the growing threat of timber encroachment on grasslands, we need to build more evidence of its impact. Many studies have shown the negative effects of wood encroachment on biodiversity in grasslands, but it is also the tip of the proverbial iceberg.
We also need long-term ecological monitoring in open ecosystems because it provides fine-scale information. “More science is needed before action and policy change,” said Ashish Nerlekar, a presidential postdoctoral fellow at Michigan State University who studies the ecology and conservation of open ecosystems in India.
In India itself, we also need to dismantle colonial terminology like “empty land”, which swims in the misclassification of open ecosystems and passively promotes activities that transform them to be used for other purposes.
Sutirtha Lahiri is a doctoral student in conservation science at the University of Minnesota and a recipient of an Interdisciplinary Center for the study of Global Change (ICGC) fellowship.
