16.3: The Role of Plants in Carbon Storage and Greenhouse Gas Emissions

Plants as conduits and reservoirs

When it comes to the global carbon cycle, plants are the ultimate carbon-capturing champions. While artificial carbon capture technologies are being developed to address climate change, photosynthesis can be thought of as a “technology” that has been perfected and deployed for over 2 billion years. The rate of atmospheric carbon uptake by plants via photosynthesis dwarfs all other forms of carbon capture. Land plants pull an estimated 120 petagrams of carbon from the atmosphere each year—over 15% of the total atmospheric carbon pool.

Plants, in addition to being the primary conduits for CO₂ removal from the atmosphere, can sequester carbon. As mentioned above, the global terrestrial plant carbon pool stores approximately 560 petagrams of carbon. It is easier to estimate aboveground plant carbon pools than it is to estimate carbon in soils. Plants, and more specifically the reflectance of the chlorophyll in leaves, can be measured from satellites in space. This technique is called remote sensing and is a powerful tool for assessing the amount of plant biomass on the land surface. Changes in reflectance over time are correlated with changes in leaf area, which in turn is directly related to rates of plant growth. Thus, satellite imagery repeated over time can be used to estimate how much plant growth is occurring on land. Belowground plant parts, namely roots, are much harder to estimate than aboveground plant parts, as roots are hidden from view. Roots can extend deep into soils. Roots can be large, like the structural roots of trees, or small and ephemeral, like the main absorptive roots of grasses and herbs. Although measuring root biomass is difficult, it is important for understanding the carbon cycle. In some ecosystems there is as much root carbon below ground as is stored in plant biomass above ground. Roots are also important because they are the main conduits for soil organic matter formation. Roots are generally assumed to be a greater contributor to soil carbon stocks than aboveground plant tissues because roots are already buried below ground and can be more easily captured and sequestered in the soil than aboveground tissues. Thus, roots play a key role as transmitters of carbon into soils.

Forests as carbon sinks and sources

Overall, trees store more carbon than any other plant type. Forests cover about 30% of the total land area on Earth and account for approximately 80% of the terrestrial plant biomass. This means that forests store an estimated 350 petagrams of carbon in their tissues, most of which is in wood. Forests are vulnerable to natural and human-caused disturbance events such as fire, logging, pests, and weather-related disturbances. Some estimates suggest that 60% of the world’s forests are in some stage of recovery from the last disturbance event. Forest disturbance often leads to the emissions of greenhouse gases. Deforestation is a big contributor to global greenhouse gas emissions, especially in the tropics. At a global scale, tropical deforestation accounts for about 10% of all greenhouse gas emissions annually. Some of the greenhouse gas emissions from deforestation result from disturbance to soils. Tree cutting and removal can break up soil aggregates, exposing previously trapped carbon to microbial decomposition and providing fuel to microbes that produce CO₂, methane, and nitrous oxide. Greenhouse gases are also released during the decomposition of the plant litter produced from deforestation.

Forest fire and biomass burning is another large source of greenhouse gas emissions. Fires consume biomass and produce CO₂, methane, and nitrous oxide, among other gases. Globally, 2 to 3 petagrams of carbon are emitted to the atmosphere annually from fires. Over 80% of this comes from tropical regions, with approximately 1 petagram of carbon per year coming from savannas (wooded grasslands). Climate change is increasing the frequency and severity of drought in some regions, and this can in turn increase the occurrence of fires.