A couple weeks ago, I went on a field trip for my Wetlands Ecology class to learn about different types of wetland ecosystems. We followed the Patuxent River along a salinity gradient from freshwater (less than 0.5 parts per thousand (ppt) salinity) near the head of the river to salt water towards the mouth of the river, which drains into the Chesapeake Bay. The first stop was at Patuxent Wetland Park, a freshwater wetland. We saw firsthand the ways that wetland plants adapt to the stresses of a low oxygen environment. Soil is a mixture of particles separated by spaces called pores. In many ecosystems, these pores are filled with a mixture of water and air. In wetlands, these pores are frequently or permanently filled with water and plant roots cannot acquire oxygen through them. Therefore, they have special adaptations to fill their oxygen needs. For example, the water lily uses aerenchyma, or spongy tissue with air channels to pipe oxygen from above the water table down to its roots. Our second stop was at the Clyde Watson Boating area where the salinity ranged from about 5 to 7.5 ppt. We noticed salt buildup on the leaves of many of the plant species here. These plants adapt to saline conditions by extruding salt through specialized glands. It was a hot day and everyone decided to cool off in the river in their full chest-wader getups. The third stop was a salt marsh at Island Creek Marina where the salinity ranged from about 10 to 12.5 ppt. Here I spoke to the class a bit about my work in these ecosystems. We passed around a soil sample for everyone to smell the hydrogen sulfide produced by the specialized microbes that colonize salt marsh soil. I knew the rotten egg smell well from this past summer’s research and I felt right at home as I excitedly nerded-out to the class about redox conditions. At each of these sites, we split into groups and took a vegetation survey using a PVC quadrat. This allowed us to determine the dominant plant species present in each wetland type. We noticed how dramatically the plant species composition shifted with these salinity changes. Specifically, plant biodiversity dropped sharply as salinity increased from site to site. As an impromptu final stop, the group decided to check out Battle Creek Cypress Swamp, which contains one of the northernmost stands of Cypress trees in the United States. We noticed the stubby cypress “knees,” or part of the roots that sit above the water line. Though their function is not entirely confirmed, researchers think that the knees help to bring air down to the lower parts of the plant roots. We took a walk across the swamp on a beautiful newly build boardwalk. I also got to collect some edible fruits from the native Pawpaw tree as we strolled along. Despite being completely covered in mud at the end of the day, I was so happy to spend my Saturday outside learning about these very unique ecosystems!
- By Dani Weissman
Dr. Kate Tully
Kate is an Assistant Professor of Agroecology at the University of Maryland.