Mangrove sap flux at Sanibel refuge
Mangrove ecosystems occupy an area of approximately 53,000 square miles globally and provide a number of important ecosystem services to people: storm protection, food provisioning, recreation, aesthetic value, and biological carbon sequestration, among others. In recent years, a concerted scientific effort has been directed toward understanding how landscape management influences ecosystem services, biological carbon sequestration, and long-term habitat vulnerability. Recently, they have expanded this focus to the South Florida mangrove wetlands of the J.N. “Ding” Darling National Wildlife Refuge.
The influence that increased loading of nitrogen (N) and phosphorus (P) from the Caloosahatchee River is expected to impact mangrove persistence into the future. The literature is split on whether increased nutrients will facilitate sediment deposition losses or gains. What is clear is that analyses need to be case specific, especially in mangroves where virtually no studies have been undertaken at the appropriate scale.
In response, the U.S. Fish and Wildlife Service has partnered with the U.S. Geological Survey (USGS) to install surface elevation tables in mangroves in the refuge. Mangrove patches are being experimentally augmented with N and P to levels expected with future Everglades Agricultural Area releases into the river. Along with surface elevation (“land building”) change studies, a team of scientists are tracking tree growth, soil respiration, and shifts in individual tree eco-physiology that may be indicative of lethal or sublethal stress to these ecosystems.
Last summer, scientists from the USGS and Clemson University installed sap flow sensors on mangroves that were receiving N fertilization, P fertilization, and no fertilization (i.e. “control”) over a two-year time period; that data collection went from May through August.
More recently, we returned to install sap flow sensors on a different group of study trees under the same type of treatments, to give insights to winter conditions. The summer 2019, winter 2020, and summer 2020 data collected will be used to calculate the amount of water transpired each day by the individual trees in the study plots. That information will be paired with growth measurements to provide a measure of the amount of mangrove tree growth that is provided by a given amount of water transpired. This is important because trees, even mangroves, cannot tolerate high levels of salt within their cells. Mangroves exclude some portion of the salt at their root interfaces while the remaining salts are released by some mechanism that varies by species. For instance, black mangroves excrete salt on the undersides of their leaves. White mangroves concentrate salts within their leaves, then drop those leaves. This makes water very “energetically expensive.” Knowing how nutrient increases affect the mangroves is vital in predicting the impact of nutrient-rich releases into the Caloosahatchee River basin.
The extensive mangrove research at the refuge is being led by biologist Jeremy Conrad. The Sanibel-Captiva Conservation Foundation’s Marine Lab hosted the scientific team from Clemson at the Wilmeth cottage for visiting scientists in February to facilitate this research on mangrove wetlands.
Dr. Jaime Duberstein is a researcher from Clemson University.