Deforestation, land-use change and tree planting
One of the human activities that has the greatest impact on the balance of greenhouse gases (GHGs) in our atmosphere, is the change of land use. Many hectares of natural ecosystems, including rain forests are destroyed when clearing uncultivated land, to gain space to grow crop feeds for livestock or simply to establish new pastures for beef and dairy production. But why is deforestation a problem and why are trees so important?
Thanks to a unique physiological process - photosynthesis - plants support life on this planet by converting sunlight into a useable source of energy. Photosynthesis not only provides the source all of the oxygen in our atmosphere, it also provides a process for absorbing carbon dioxide (CO2). By transforming CO2 into carbohydrates such as sugars, starch and cellulose - fundamental components of living organisms - plants are not only able to grow, but also to maintain the balance of CO2 in the planet’s atmosphere.
A forest is the best CO2 pump on Earth, able to take up and store a considerable amount of carbon. The world’s forests contain more carbon than the entire atmosphere, of which 44% is stored in the biomass (living trees and plants for example), 11% in dead wood and litter and 45% in the soil.
If a forest is cut down to make space for plantations or beef pastures, this has an immediate and negative impact on the GHG balance of the area.
Deforested areas are often cleared by fire as this is the most efficient method for getting rid of unwanted plant material and to prepare the land for agriculture. Burning plants instantly releases most of the carbon stored within them back into the atmosphere as CO2 leading to both the loss of a CO2 sink and the creation of a CO2 source.
All photosynthesising plants absorb CO2. Replacing a forest with a plantation of arable crop, oil palms or a pasture will offer some capacity to take up and store carbon, however this is nowhere near as large as that of a mature forest. If the area formerly occupied by a forest is used for keeping beef cattle which emit methane, the land turns from a permanent GHG sink into a GHG source, depending on the intensity of the system.
Figure 2: Black Angus. Angus cattle are a very popular beef breed. Originally bred in Scotland, they have now been exported all over the world. ©Marie Dittmann
Although the rate of deforestation has decreased in recent years – from a global total of 7.3m hectares per year in 2000 to 3.3m hectares in 2015 – large areas of forest are still lost in South America, Africa and South East Asia. Besides an immediate impact on land productivity, biodiversity and stability, deforestation and other changes in land-use are responsible for as much as 17% of human induced GHG emissions.
Whilst unique forest ecosystems are not easily replaced once the trees are cut down, it is possible to increase the potential for carbon sequestration by planting new trees. Termed reforestation, this process offers several opportunities; both ecological and economic. Carefully designed reforestation schemes can act as a sustainable source of wood, fruits, fibre and animal fodder. In addition, reforestation schemes in many countries are currently eligible for funding generated by payments from CO2 emitters in industrialised regions. By making use of the proven capacity of forests to store carbon, emitters can offset their anthropogenic emission of CO2.
We know that the most diverse and best functioning type of ecosystem in many locations on Earth is a mature forest. However abandoning large areas currently used for agriculture in favour of reforestation is not a feasible option if we are to maintain and increase food production. Agroforestry offers a good compromise and a unique way of making use of the capacity of trees to improve their environment both above and below ground.
Growing large numbers of trees in an agricultural landscape, interspersed with areas of crop production or livestock pasture, maintains some of the CO2 storage capacity of the land whilst still producing food to sustain the human population.
References and further reading:
- Food and Agriculture Organisation of the United Nations (FAO) (2015) Global Forest Resource Assessment 2015. FAO, Rome.
- Lorenz, K., & Lal, R. (2014). Soil organic carbon sequestration in agroforestry systems. A review. Agronomy for sustainable development, 34(2), 443-454.
© University of Reading