What is Carbon Sequestration?

Grasslands play a crucial role in mitigating climate change. Before we delve into their unique suitability for reducing greenhouse gas emissions, let’s demystify the phenomenon of carbon sequestration.

Humans need to hunt, grow, forage, or buy their food, but plants create their own nutrients through photosynthesis. In the chloroplasts of plant cells, the interaction of sunlight, carbon dioxide, and water produces energy-rich sugars and oxygen. We know, with every inhalation and exhalation, that we cannot live without oxygen. Unprecedented levels of carbon dioxide in our atmosphere have made us aware that our survival also depends on the other product of photosynthesis - glucose. Glucose is a compound containing carbon, sourced from the carbon dioxide pulled from the air. Carbon sequestration is the process by which plants absorb carbon and incorporate it into their various tissues - leaves, stems, and roots.  By pumping carbon out of the air, and sequestering it in their plant bodies, plants are natural “carbon sinks” that lower the concentration of greenhouse gases, which in turn mitigate the effects of global warming.

Role of Grasslands in Earth’s Carbon Cycle

A grassland is an ecosystem where the predominant vegetation consists of grasses and forbs (wildflowers) with relatively few trees and shrubs. They exist on every continent except Antarctica. Examples of grasslands include North American prairies, the African savannas, the pampas of South America, the steppe of Central Asia, and the alpine grasslands of Northern Europe. Grasslands are typically found in regions with moderate levels of rainfall, which are insufficient to support the growth of forests and also too wet to produce desert. In fact, they often occupy the regions between forests and desert.  

Grasslands support a wide range of wildlife species, and their management and conservation are essential for protecting biodiversity and ecosystem services. One of these essential services is carbon sequestration. According to Norderhaug et al (2023), there are several factors that impact an ecosystem’s potential for soil carbon sequestration. These factors help us understand why grasses, and their soil profiles, are carbon sequestration powerhouses. 

Highly Developed Root Systems

Grasslands have very high root-to-shoot ratios. The tallest grasses in a short-grass prairie may be only waist-high, but they spring from a vast network of roots that can run six to eight feet deep. While trees can store sugars and water in their stems, branches, and leaves, the storage organs of grasses are all underground. Buffalo grass, a common prairie species, is a few inches tall, but has roots that burrow down 4’-6’ (Rosen, 2022). These extensive root systems create vast storehouses of carbon. Roots contribute to soil carbon in numerous ways, and it has been estimated that there is more carbon in the soil than in the world’s vegetation and atmosphere combined (Lehmann and Kleber 2015).

Mean Residence Time of Root Carbon

The mean residence time of root carbon represents the average amount of time that a specific portion of root carbon remains in the soil before it is decomposed or respired back into the atmosphere. Grassland soils contain an intricate network of roots that stabilize soil and reduce erosion. This contributes to a high residence time of root carbon, preventing its release into the atmosphere through erosion. Grasslands are resilient in the face of disturbances such as fire and grazing. Although their aboveground parts burn in a fire, the roots survive, and continue to efficiently cycle carbon as they put out new shoots. If grasslands burn during winter dormancy, the roots are not bothered at all, and the plant remains dormant until the start of the next growing season.

Above and Below Ground Biodiversity

Plant species have different root morphologies, which allow them to access water and nutrients from different horizons in the soil. Diverse plantings will have root systems that each occupy a different below ground niche, limiting competition between species (Lynn, 2020). The deep taproots of tall forbs like Joe-Pye weed do not directly compete with the shallower, fibrous root systems of carexes. They also occupy different aboveground niches, which is why tall grasses and wildflowers can thrive in the same spot as ground-covering perennials (Rainer and West, 2015).

Mycorrhiza

Grassland plants live in symbiosis with mycorrhizal fungi, which ingest plant photosynthate. These mycorrhizal fungi inhabit plant roots and form soil mycelium that scavenge for nutrients. They take up macronutrients like nitrogen, phosphorus, and sulfur, and transport them to the plant’s roots. The factors that impact efficiency of carbon sequestration overlap heavily with the measures of soil ecological health. A thriving soil community of plant roots and their exudates, fungi, bacteria, and other soil microorganisms yields a productive landscape that stores planet-cooling vaults of carbon (Lynn, 2020).

Grazing Pressure

Moderate grazing intensity stimulates accumulation of soil carbon by promoting root growth and turnover through defoliation. Herding animals in the traditional way, where groups of animals have a wide range, and move on from an area that has been grazed, maintains the health of grasslands while preventing forests from taking root. When grasslands are not given the time and space to recover from grazing, they degrade into pasture. 

Sensitivity to Soil Disturbance

Fire is to a grassland as rain is to a rainforest (Rosen, 2022). In grassy ecosystems, disturbances such as fire, grazing, and periods of drought are stabilizing forces. After enduring millions of years of seemingly adverse conditions, grassland plants have evolved to stock up and wait it out. In many parts of the world, there has been an observed increase in the frequency and severity of wildfires. Since grasslands are inherently fire-prone, they are uniquely adapted to the climate change driven increase in wildfires. After the grasses have crackled and burned, the roots continue to store carbon and are ready to push out new growth.

Why Not Just Plant Trees?

In a world where human development constantly encroaches upon ecosystems, grasslands across the continents (with a few exceptions) receive less conservation efforts and funding than forests. The high biodiversity found in grasslands means that their loss has created long and growing red lists of endangered species (Rosen, 2022). Grasslands support everything from large, impressive megafauna like lions, elephants and buffalos, to birds such as raptors, meadowlarks, and sparrows, to humble insects like grasshoppers, dragonflies, and beetles.

The tendency to misjudge and undervalue grasslands can be partly attributed to their relatively low height, and their primarily flat topography. These spaces of open and majestic beauty have been mistaken as vacant lots ready for the plough and bulldozer. Old growth forests possess a sacred, silencing power, with the passage of time embodied in their soaring trees and celestial canopies. In grasslands, hundreds of years of growth appear as waves of grasses and wildflowers. The years they took to evolve are underground, in the form of roots, bulbs, tubers, and a rich network of bacteria and fungi. To the uninformed eye, it is hard to know the difference between former agricultural land, overgrown lawns, and grasslands.

In the finite resource pool of environmental funding, there has been a tendency to value forests over grasslands - a collective forgetting that they are also one of Earth’s precious biomes. The well-intentioned but misguided effort to plant trees everywhere - including on grassland, is alarming conservationists. Trees face increasing threats from drought and wildfire. Although grasslands store less carbon per acre than a forest, in a volatile climate, they can store it longer.

In North America, 70% of grasslands have been lost. Since 2007, nearly 50 million acres of grassland in the United States have been converted to cropland or been consumed by development. Millions more acres are lost each year. In the summer of 2022, the North American Grasslands Conservation Act was introduced. This legislation will create Grassland Conservation Councils that will include representatives from the farming, ranching, and grazing communities as well as representatives from state, Tribal, and federal agencies. By organizing the people who live, work, and recreate on North America’s grasslands, the councils have the best chance of finding long-lasting preservation policies. Grasslands might not have the towering grandeur of old-growth forests, but their role in climate stability is equally vital. It's time to appreciate and protect these unassuming carbon heroes. You can be matched with your lawmakers and can send a pre-written message of support for the North American Grasslands Conservation Act here: https://actforgrasslands.org/take-action/

Literature Cited:

Lehmann, J., & Kleber, M. (2015). The contentious nature of soil organic matter. Nature, 528, 60–68. https://doi.org/10.1038/nature16069

Lynn, D. (2020). Landscape Design for carbon sequestration. Master’s Thesis. Department of Landscape Architecture, University of Portland. https://issuu.com/dmlynn/docs/lynn_mla_project

Norderhaug, A., Clemmensen, K. E., Kardol, P., Thorhallsdottir, A. G., & Aslaksen, I. (2023). Carbon sequestration potential and the multiple functions of Nordic grasslands. Climatic Change, 176(55). https://doi.org/10.1007/s10584-023-03537-w

Rosen, J. (2022, January). Trees are Overrated. The Atlantic. https://www.theatlantic.com/science/archive/2022/07/climate-change-tree-planting-preserve-grass-grasslands/670583/

Rainer, T., & West, C. (2015). Planting in a post-wild world. Timber Press