What is a tree island, and how does it help with carbon storage?

We explored how coastal areas are capable of storing carbon at a great capacity in a recent blog. The Everglades in southern Florida are also known as a reserve of carbon storage. Some areas of the Everglades have “tree islands” and a recent study showed they are capable of storing more carbon and other nutrients than their marsh neighbors.

The study was published in the Soil Science Society of America Journal last year (2021).

The Everglades contain a diverse set of habitats, including prairies and mangrove swamps. Other areas of woody vegetation embedded within the marsh landscape are known as tree islands. The variety of trees can vary from willows to cypress to various hardwood varieties.

An aerial view of a tree island within Loxahatchee National Wildlife Refuge. A recent study showed that tree islands can store more carbon, nitrogen, and phosphorus than their marshy neighbors. Credit: US Fish and Wildlife Service

The Everglades is a managed area. Water flow is controlled through a series of canals. Stormwater treatment areas were constructed in the 1980s to control damage from stormwater surges. There are three areas that scientists are currently focusing on to determine best practices within the Everglades:

1. sediment control best management practices.
2. nutrient management control best management practices.
3. water management control best management practices.

The current study by a group of scientists at Florida International University researched whether tree islands were less affected by changing water levels than neighboring marshes.

To study the soil, the researchers took “soil cores.” They place long tubes into the soil and pull them out, to capture the soil composition at various depths. Tree islands’ soils were composed of low-organic peat, heavy peat, and peat soils. These types of soil store large amounts of carbon in our ecosystems throughout the world (including cold areas like the arctic and subarctic).

The study found that tree islands could store nearly five times more organic matter (carbon) than marshy areas. They could store over twice the amount of nitrogen. Finally, they stored almost six times the amount of phosphorus.

Soil cores taken in a recent study of the Everglades comparing nutrient storage between marshes and tree islands. Notice the darker color of the soil from left to right – on the left are marshy soils and on the right are soils made of peat, which stores carbon better than other soils. Credit: Scinto et al https://doi.org/10.1002/saj2.20270

The authors attribute these observed differences in nutrient storage capacity between the marshes and tree islands to two factors. First, the quality and quantity of litterfall and root biomass between marshes and tree islands are different. The leaf litter from marshes decomposes faster than leaf litter from trees. And the relationship between the roots and soil microbes differs, too. This allows for more nutrient buildup in the tree islands.

Second, water flows differently in the marshy areas and tree islands. Tree islands tend to be at a higher elevation, due to the way they developed. This means they are buffered more from the ebbs and flows of tides, as well as storm surges. It could be that some leaf litter in marshes is carried away from the marsh with flowing water – so it decomposes elsewhere.

This study illustrates that within the coastal Everglades landscape, different habitat types exist closely together, and each has their own distinct soil properties. These differences in nutrient storage and water levels between habitat types can have ramifications when considering the stressors affecting the coastal landscape in the Everglades. They can affect how these soils respond to sea level rise, saltwater intrusion, and woody encroachment, each of which is currently affecting this landscape.

Compiled by Susan V. Fisk from a Soil Science Society of America Journal paper by Scinto et al.

For more information about the Everglades, visit the Friends of the Everglades site.

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