Plant diversity linked to higher levels of carbon storage in Boulder’s grassland ecosystems
Scientists in Boulder County tested the relationship between plant species richness and carbon storage in the soil in grassland ecosystems. Data show that higher species richness is linked to higher percentages of organic soil carbon, on average, across all grassland types. This suggests that plant biodiversity, particularly in our native grasslands, provides important climate services.
Trees are highly celebrated for their ability to sequester carbon from the atmosphere, but soils play an even larger role in mitigating the feedback between terrestrial carbon and climate. Soils are the unsung hero of the carbon storage world, storing approximately 80% of all terrestrial carbon on Earth.
Carbon gets into the soil when plants and animals die and bring their carbon with them underground. Thus, scientists have begun exploring the connection between biodiversity above ground and carbon storage below. A 2018 study, published in the Proceedings of the National Academy of Sciences, found that plant diversity in grassland ecosystems had strong positive effects on soil carbon storage by increasing carbon inputs by plants and increasing soil microbial activity.
Scientists in Boulder County tested this relationship across three grassland types (xeric tallgrass, mesic big bluestem, and mixed-grass). Their data show that higher plant species richness is linked to higher percentages of organic soil carbon, but the relationship depended on the grassland type. This suggests that plant diversity plays an important role in mitigating atmospheric CO2 and regulating our local and global climate.
Another 2018 study suggests that grasslands are actually more resilient carbon sinks than forests because they are more resilient to droughts and wildfires. Thus, Boulder’s native grasslands, and their diverse plant life, are an increasingly important force fighting for our climate future.
Exploring the data
Plant species richness leads to higher carbon storage in the soil, across grassland types
Figure 1. Percentage of soil organic carbon versus plant species richness for three grassland types. Gray points represents the Mesic Big Bluestem Prairie, blue points represent the Mixed-Grass Prairie, and gold points represent the Xeric Tall Grass Prairie. Note that in grasslands where native species richness is higher, there is also higher soil organic carbon. Source: Data was collected as part of the City of Boulder's Open Space and Mountain Parks grassland monitoring program and 2018-2019 externally funded soil monitoring. Data represents the 07/17/20 version of the data set, with a sample size of 141. Species richness was calculated as a count of all unique species, including native species, non-native species, and those not identified to the species level based on 2016 vegetation monitoring. Total percentage of carbon in the soil sample was determined using LECO furnace combustion CN analyses at the CSU Soil, Water, and Plant Testing Laboratory. Inorganic C was determined by Tracy Halward working at CSU laboratories by pressure transducer analyses.
Get to know your grasslands
Grasslands, also known as prairies, are flat, fertile lands dominated by grasses. Prairie grasses, like those found in Boulder County, hold soil firmly in place, so erosion is minimal. Prairie grass roots are good at reaching water more than a meter deep, and they can live for a very long time. Grains are a type of grass, so the prairie grassland is perfect for growing grains like wheat, rye, and oats.
Learn more about the grassland types included in this study:
Mesic Big Bluestem prairie
Mesic tallgrass plant communities, dominated by big bluestem, little bluestem and switch grass, are associated with floodplains that are irrigated or naturally wet. In the Boulder Valley, mesic big bluestem communities occur in the South Boulder Creek floodplain and along ancient creek terraces. The largest remnants of Mesic Bluestem Prairie in Colorado occur in Boulder, separated from the mesic tallgrass prairie in the eastern Great Plains by hundreds of miles. The robust rodent populations occurring in this habitat type attract Swainson's hawks and other raptors that forage in grassland habitats. These grasslands provide habitat for several rare butterflies, including the Ottoe skipper and Arogos skipper. These species depend on tallgrass plant species and are considered rare and imperiled throughout the Great Plains. These grasslands also support many grassland bird species, such as bobolinks. Photo by Bradford S. Slaughter.
Xeric tall grass prairie
Before the steel plow, tall grass prairies stretched from southern Canada to northern Texas. They flourished in areas with rich soils and moderate rainfall. Some of the largest areas of upland tallgrass plant communities — dominated by big bluestem (Andropogon gerardii) — remaining in Colorado are in the Boulder area. These foothills tallgrass communities share similarities with the tallgrass prairies of the eastern Great Plains, but also have distinctive characteristics of their own. The Xeric Tallgrass Prairie includes several community types occurring in open meadows, savannas at the prairie-forest interface, and as matrix-forming grasslands on prominent mesa tops. Large, unfragmented patches of xeric tallgrass create seasonal habitat for a suite of grassland nesting birds, and are used seasonally by elk. Tallgrass prairie is considered rare and imperiled globally, and is one of the most endangered vegetation types in the world. Photo by Caasi Saari.
Mixedgrass prairie mosaic
At the forest-grassland interface or ecotone, mid- and short-grass prairie species blend with Rocky Mountain species to form a distinctive and localized set of plant associations. Diverse topography, soils and geology combine with climate to create habitat for grassland plant associations characterized by mid-height species such as western wheatgrass, needleand-thread grass, green needlegrass, New Mexico feathergrass, sideoats grama, little bluestem, and Rocky Mountain bluegrass. The mixedgrass mosaic supports a diverse fauna including uncommon species such as the shorthorned lizard, olive-backed pocket mouse, and several rare butterfly species. Large blocks of mixedgrass prairie provide habitat for numerous grassland nesting birds, the American badger, and elk. Much of the land inhabited by black-tailed prairie dogs in Boulder County is mixedgrass prairie. Photo by Jan Huber / Unsplash.
recommendations
Conserve grassland ecosystems, and areas of high plant diversity, to encourage carbon sequestration and storage.
There is a clear relationship between plant diversity and carbon storage, but this relationship varies across grassland type. Mixed-grass prairies store the lowest amount of soil carbon, on average, and the Mesic big bluestem prairies store the most. The Xeric tall grass prairies had the highest levels of species diversity, suggesting their potential for future carbon storage.
These relationships between plant diversity and carbon storage in grasslands mandate conservation action.
Not only are these ecosystems endangered globally, they are providing a valuable service to our community in the form of carbon storage. We need to continue restoring and protecting grassland ecosystems to maintain this service and provide a home for threatened and endangered grassland species.
Learn more about the geologic history, ecology, species diversity, management, and monitoring of Boulder’s grasslands in the Grassland Ecosystem Management Plan and in Boulder County’s management plans and policies.
