By Tracey Li*
Much is written about the need to reduce deforestation and replant the forests that have been logged for human use and economic development. This is because trees are needed for fighting climate change and vital to the very survival of the planet. But what is it exactly that makes trees and other plants so special?
Climate change is caused, at least partly, by man-made emissions of greenhouse gases which accumulate in the Earth’s atmosphere and trap heat. The Intergovernmental Panel on Climate Change (IPCC), set up in 1988 by the United Nations Environment Programme (UNEP) and the World Meteorological Organization (WMO), and endorsed by the General Assembly of the United Nations, is the leading international body for assessing this phenomenon.
In their most recent Assessment Report from 2007, the IPCC reported that carbon dioxide (CO2) is the most significant anthropogenic greenhouse gas, both in the sense of the amount of heat it traps and the quantity that is released into the atmosphere (mainly from the burning of fossil fuels). Forests are very effective ‘carbon sinks’, extracting CO2 from the atmosphere and keeping it locked away for long periods of time. Therefore, some of the key strategies to alleviate the causes and effects of climate change recommended by the IPCC include efforts to reduce deforestation, while simultaneously increasing afforestation and reforestation: planting of forests where none were before and replanting areas where forest has been removed, respectively.
So what happens to the CO2 that plants absorb? CO2 is one of the raw ingredients for a process called photosynthesis. This is the method by which plants synthesize the carbon-based molecules that constitute the basic building blocks of all living organisms. Since all plants and animals are carbon-based, a continuous supply of this element is needed in order for growth and repair to occur. Animals and humans cannot obtain carbon by extracting it directly from CO2 like plants do; instead they acquire it indirectly by eating plants and/or other animals which in turn have eaten plants.
So carbon is locked up, or ‘sequestered’, in the molecules that make up the stems, leaves, roots, and all other parts of a plant. It is also locked up in the bodies of all animals. When a plant is eaten, the carbon is transferred up the food chain. Carbon is released back into the ecosystem in the form of CO2 when the plant rots or is burned. If a tree is cut down and the timber used to make a coffee table, the carbon contained within the wood remains sequestered in that piece of furniture until it finally rots, although as the tree is no longer alive it cannot absorb any further CO2.
There are two aspects to consider when studying carbon sequestration in forests. One is: how much carbon is stored in a forest at any given time? This tells us how much CO2 would be released directly if the forest were burned. The second is to ask how much CO2 the forest absorbs each year, which can be measured by studying the plants’ rates of growth – a fast growth rate indicates that a plant can absorb a large amount of CO2 in a short time, transforming it rapidly into new shoots and roots where it is sequestered. This figure can then be compared to the amount of CO2 emitted each year by human actions to calculate the net accumulation in the atmosphere. If the forest is destroyed, the CO2 absorption rate also tells us the amount of CO2 that will now not be absorbed each year, and will instead end up in the atmosphere.
These calculations help us to discover which plants fight climate change best. It is often large trees, such as oaks and ferns, that make for very good carbon sinks. However, some non-tree plants can hold a few surprises too. Click here to find out how much the grassy bamboo can do about CO2.