How Trees Prevent Soil Erosion

About 30 % of the world’s arable land has become unproductive as 60 % of its soil has been washed away and deposited in rivers, streams and lakes, that has made these water bodies prone to flooding and contamination with pesticides present in soil. Once soil is lost, those areas become desertified. It takes many centuries of careful work to once again make these lands productive.

Examples of this can be seen around the world. For example, the region along the Mediterranean in North Africa was known as the “Granary of the Roman Empire” two thousand years ago. However, this region suffered significant “land degradation during the Roman period itself, as a result of their agricultural techniques and expansion.”

The Great Dust Bowl in the American and Canadian prairies in the 1930s was a result of improper agricultural practices leading to loss of soil. Extensive deep plowing of the virgin topsoil of the American Great Plains during the previous decade displaced the native vegetation, which allowed the top soil to be swept off by winds during an extensive drought in the 1930s. This led to massive dust storms in American cities such as Washington DC and New York, and the collapse of agriculture in 400,000 sq kms in Texas, Oklahoma, New Mexico, Kansas and Colorado.

Today, a giant dustbowl is forming in Northwestern China and Mongolia. The yearly dust storms that happen in spring and engulf Beijing and other Chinese cities are due to these dustbowls that have formed because of over ploughing and overgrazing. The dust from these areas also enters South Korea every year, and sometimes travels across the Pacific and into Western US and Canada.

Trees minimize soil erosion from surface water runoff and flooding. This helps maintain soil health and prevents silt deposition in water bodies. Studies have shown that the more closely an agricultural system resembles a natural forest in its canopy structure, tree spacing and ground cover, the less chance there is of soil erosion.

Every year, India loses about 5.3 billion tons of soil or 16.4 tons per hectare due to erosion. Of India’s 3.2 million sq km, about 25% or 820,000 sq km is facing desertification. Another 40% or 1.26 million sq km is facing water erosion. Almost 20% of this is very severe erosion with over 40 tons of soil lost per hectare per year. 10% of this land is facing extreme erosion, with 80 tons of soil lost per year. 80 tons of soil is about 5mm of topsoil a year.

Erosion is especially serious in the Himalayas and Eastern and Western Ghats. The entire Himalayan region is prone to soil loss. For example, within just the state of Himachal Pradesh, about 280 million tons of soil is lost every year.

In the Eastern and Western Ghats, 40-50 tons are lost per hectare each year. This is about 3mm of topsoil a year. It takes 30-40 years to form a millimeter  of topsoil, and it takes about 150mm of topsoil to grow a good crop. Average topsoil depth in India is only about 180mm. At an erosion rate of 5mm per year, the entire topsoil will disappear in 35 years.

And soil erosion is not a recent phenomenon in India. It has been a problem for many decades. Which is why 170,000 sq km of land is considered to have already suffered topsoil loss in India. In 2010, the country is estimated to have lost INR 28,500 crore on account of degraded lands. This is a loss of 12% of the total value productivity of these lands.

Soil loss per hectare in agricultural land is five to ten times more than land under forests. During episodes of very heavy rainfall, agricultural land may lose up to 26 times more soil than forests. This susceptibility under heavy rain is especially significant considering that climate change models predict more spells of heavy rain on the Indian subcontinent.

Tree cover plays an important role in reversing soil erosion, and additionally preventing landslides in hilly regions. In a study done in the Alaknanda Valley of the Uttarakand Himalayas, it was found that the landslide occurrence for open and degraded pine forests was 2-3 times higher than for healthy pine forests. The density of the pine forest was found to be the slope-stabilizing factor.

Soil is protected under tree cover because of the litter – the leaves and branches that our shed by the tree and fall to the ground. When the ground is free from litter and cover, water flows freely and can take away the soil with it. So, along with the planting of trees, ensuring that the ground – the understorey and leaf litter – beneath the trees is not disturbed is also critical. According to the UN’s Food and Agriculture Organization, forest and tree crop plantations in which ground cover was disturbed are more susceptible to erosion than areas where ground cover was undisturbed.

Soil restoration through tree plantation

In the Loess Plateau in China, nearly one million hectares of degraded land were reforested between 1994 to 2005, in the Wei and Yellow river basins. During the 1980s and 1990s, the degradation of the region’s soils and vegetation had reduced food production, affected water transport and air quality in nearby as well as distant cities. The erosion of soil was at such a serious level that almost 90% of the sediment entering the Yellow River – one of China’s largest rivers – was due to the plateau.

Working with the World Bank, China invested in six forestry programs, targeting around 1100 small rivers and streams in the plateau. Over a ten-year period, this initiative led to greater food security, increased per capita grain production, 89,600 hectares of new agricultural land and reduction of sediment in the water by 99%.

Similar work has been done around the world. For example, starting from 1935, the United States government worked with farmers to restore the Coon valley rivershed. By planting trees and other initiatives, tree cover went up from 37% in 1939 to 50% in 1993. Results included better dry season water levels in the river, reduced floods and healthier soil.