Carbon consequences when trees invade

The scent of an oak forest wafts out of the office of Sophie Higgs, a graduate student in the Division of Biology at Kansas State University. Inside her office, the floor is completely covered with black bags stuffed with oak leaves that she had collected over the past week. No, these leaves are not an extreme attempt to freshen the air after she had microwaved fish for lunch that day. The bags of leaf litter are important tools for conducting her research.

Sophie’s current research takes place in tallgrass prairie streams in the area around Manhattan, KS. The majority of tallgrass prairie in the United States has been plowed for agricultural use, which makes it one of the most endangered ecosystems in the world. Humans have also played a role in suppressing natural wildfires that historically occurred every few years in the tallgrass prairie. This has led to a greater abundance of woody shrub and tree species that can outcompete grasses. Over time, this can transform the prairie into a shrubland and potentially even a forest. This can have negative consequences for the animals that rely on prairie grasses for habitat and food.

Sophie is an aquatic biogeochemist which means that she is interested in how energy and nutrients flow and cycle in aquatic environments. Carbon is the most important building block for the cells in all living things and can enter streams in various ways.

Algae and leaves
Algae (left) release simple carbon compounds, while tree leaves (right) release more complex carbon compounds that are not as easy for microorganisms to use for energy.

One way that carbon enters streams is when photosynthetic organisms, particularly algae, capture carbon dioxide from the atmosphere and turn it into simple carbon compounds, such as sugars. Another is by leaves falling into the water from plants growing at the edge of the stream. Once in the stream, the carbon can be taken up by microorganisms or become stuck to the sediments at the bottom of the stream.

As woody species take over stream banks in the tallgrass prairie, the major source of carbon in streams could shift from photosynthetic algae to leaf litter. As more trees grow along the stream, they produce more leaves that drop into streams while blocking the sunlight that photosynthetic algae rely on. This is important because the carbon in leaf litter is contained in more complex compounds than the carbon that comes from photosynthetic algae. The difference in complexity of the carbon can affect how quickly it is taken up by aquatic microorganisms or how likely it is to get stuck to sediments on the streambed. After dumping the oak leaves that she has collected into the stream, Sophie is able to track how the amount of the specific carbon compounds from the leaves changes at different points down the stream. When she compares carbon compound amounts at downstream locations to upstream locations, she can determine how quickly the carbon from the leaves she dumped in the stream is being used by microorganisms or getting stuck to sediments.

Upstream and downstream on the prairie
A section of prairie stream surrounded by grasses and shrubs (left) and a second section with tree canopy cover (right). Sophie conducts her experiments in both types of habitats to look for differences in carbon processing.

Much to her expectations, Sophie has found that the more complex carbon compounds contained in leaves are used more slowly than simple carbon compounds. She hypothesizes that this is because simple carbon compounds are of a higher nutrient quality and are used more readily by aquatic microorganisms. “Since different qualities of carbon are used at differing rates, the replacement of grasses with trees and shrubs could lead to changes in carbon processing,” Sophie says. “Changes in how nutrients cycle can influence how an entire ecosystem functions.”

Sophie says that her research can help us predict the consequences of ecosystem changes such as woody encroachment and potentially help with tallgrass prairie conservation efforts.


This post was written by Kent Connell. Kent is a Ph.D. student in the KSU Division of Biology studying the interactions between plants and microorganisms in the soil.



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