In the aftermath of hurricanes, salinity intrusion events cause tree damage and death in coastal forests and wetlands, but are overshadowed by wind and flooding impacts.
After Hurricane SandyÛªs ravaging winds and crashing waves subsided in 2012, another danger lay hidden in the soils of forests and wetlands along the eastern coast of the United States.
Often, hurricane damage is discussed in terms of human infrastructure like trees downing power lines and homes flooded beyond repair due to high winds and storm surges, but the incoming seawater also brings salt, which lingers long after the hurricane has passed. Unlike SandyÛªs 115 mph winds, which ripped up trees immediately, the surge in soil salinity gradually transformed some trees into standing skeletons. Until recently this type of event, called salinity intrusion, was largely overlooked. Recent work reveals that this unseen stressor poses a threat to the health of vegetation in freshwater forests and wetlands.
Beth Middleton is a senior level research ecologist for the United States Geological Survey (USGS). While studying SandyÛªs impact on the Delmarva Peninsula of Maryland and Delaware, Middleton found damage to cypress trees similar to that which she had witnessed in Texas after droughts. Seeing trees dead in the same way in two separate places after experiencing two different weather events, a hurricane and a drought, triggered MiddletonÛªs curiosity.
ÛÏThey’re standing and dead,Û Middleton said. ÛÏUsually trees affected by hurricanes fall down from the wind, but in this case for cypress ÛÒ which we rarely see damaged by wind ÛÒ there was a lot of damage by salinity intrusion. It was a different kind of event than IÛªve seen before. It was more similar to what we’re seeing in Texas with the drought.Û
To determine how Hurricane Sandy affected the Delmarva Peninsula, Middleton and her team surveyed plots in six freshwater swamps over a two-year period. In each plot, Middleton and her team categorized trunks and branches as standing live or dead, recent tipping and down rooted in the plot or outside the plot. Standing dead trees were attributed to salinity intrusion, whereas broken branches and tipped trees were attributed to wind or storm surge.
Of the six swamps Middleton studied, Hickory Point Cypress Swamp suffered the most damage attributed to salinity intrusion with eight percent of standing trees in the study area dead and up to 25 percent in certain plots. In 2014, soil samples from the top 5 cm of the soil surface were taken at each swamp. After comparing all of the data from the swamps, Middleton found the soil in Hickory Point was saltier than the rest. She also analyzed salinity levels of the Pocomoke River at points upstream and downstream from Hickory Point, finding saltier levels during the storm as compared to non-hurricane times before and after Sandy. These higher salinity levels provided important evidence for MiddletonÛªs hypothesis that a salinity intrusion event occurred.
When Middleton first wrote up her findings, the paper was rejected because a reviewer was unconvinced about the cause of the tree death event and commented that her case would be stronger if she had soil salinity data.
Middleton explained, ÛÏSoil salinity is an uncommon measurement ÛÒ most ecologists measure surface or pore water salinityÛ_I didn’t anticipate that the soil salinity would be different than the pore water salinity, because I had no experience with it.Û
Pore water is just under the surface of the ground in gaps between sediment particles, and it changes salinity concentration with respect to surface water. Soil, however, retains salinity. Therefore, soil salinity provides stronger evidence that salinity led to tree mortality because it better demonstrates where, when and for how long the salt accumulated.
Fortunately, Middleton was able to provide the requested additional evidence from soil samples she stored in a refrigerator because her team also does seed bank studies. She found higher soil salinity in the plots where trees were dying as compared to the other plots.
Analyzing the soil samples was a eureka moment. Middleton said, ÛÏThe soil hangs on to salinity for much longer, and acts as a Û÷memoryÛª of salinity intrusion events. Now that I know about soil salinity, I think that it should be a regular part of coastal ecology work.Û
Trees dying from salinity intrusion may transform the area by weakening the wetlandÛªs strength against erosion, limiting habitat availability and changing what vegetation is present. Research like MiddletonÛªs can help provide a more accurate picture of weather event damage and its specific causes.
Middleton was enthusiastic about this discovery because it could greatly improve understanding of how salinity may contribute to plant mortality. Soil may hold on to the salinity for years, depending on the amount of incoming freshwater rainfall to flush the salt out of the soil, and even with a large amount of fresh rain, salt may still remain. Scientific understanding of how long it takes for an area to recover from a salinity intrusion event is a gray area, in part because of the scarcity of detailed salinity intrusion studies. Middleton explained that itÛªs difficult to get funding to study hurricane impacts beyond a year or two, but she believes understanding the long-term impacts is critical.
ÛÏThe soil hangs on to salinity for much longer, and acts as a Û÷memoryÛª of salinity intrusion events. Now that I know about soil salinity, I think that it should be a regular part of coastal ecology work.Û ÛÒåÊBeth Middleton, USGS senior level research ecologist
Middleton is not alone in her conviction that there is a need for better understanding of the impacts of hurricanes. Samantha Hauser, a doctoral fellow at University of Louisiana at Lafayette, is another researcher who studied the impact of Hurricane Sandy on wetlands. Her work looks at the different ways a hurricane damages coastal environments by wind, water and salt. Hauser explained that marsh dieback is evident when patches of wetland turn brown.
ÛÏThey were under water too long or there was too much saltwater, and so the vegetation dies,Û said Hauser.
Depending on the concentration of the salt and how long the elevated soil salinity levels persist, salt burning ÛÒ a browning of leaves ÛÒ standing tree death or marsh dieback occur in the coastal wetlands and forests where salinity intrusion events have occurred. Prolonged exposure to elevated salinity can harm freshwater trees in two ways: When there are elevated levels of salinity in the soil, it is harder for the vegetation to uptake water, which plays a critical role in photosynthesis. For plants that are not tolerant of increased salt concentrations in their cells, the salt ions can also directly inhibit metabolic processes. Soil salinity as it pertains to salinity intrusion is only beginning to receive increased attention, and researchers have not defined the length of time it takes for trees exposed to increased salinity to suffer damage.
Hauser explains that the vegetation, once dead, leads to erosion and puts the entire wetland at risk of more severe erosion. When coastal wetlands are eroded, inland areas, which are likely to be more populated, are left vulnerable to salinity intrusion, storm surge and further erosion.
As part of her senior thesis project at Rutgers University, Hauser categorized the severity of SandyÛªs damage to New JerseyÛªs coastal wetlands in order to prioritize remediation efforts. She compared pre- and post-Sandy aerial photography of wetlands along the storm surge path from the New Jersey Geographic Information Network. She developed a degradation index in which certain types and amounts of damage received a score, with the sum of damage scores for a wetland giving the overall score for that particular wetland. The more damage a wetland suffered, the higher the score it received and thus could be prioritized for remediation over wetlands with less damage.
Taking her index a step further, Hauser quantified the damage in monetary terms of ecosystem service value (ESV) loss. Ecosystem services are the direct or indirect benefits to people provided by wildlife and nonliving components of the environment. ESV is the economic appraisal of beneficial ecosystem functions to humans and the environment.
She said, ÛÏIn working with governments and policy makersÛ_ there are a lot of people who are not necessarily ecologists, and so it’s hard to convey to them the importance or how to prioritize certain wetlands.Û Hauser believes ecosystem services ÛÏworks as a common language among everyone.”
Of the $9.4 billion value New Jersey freshwater wetlands provide annually, Hauser calculated a possible loss of $4.4 billion due to Hurricane SandyÛªs storm surge. This value includes wind, flooding and salinity damage. The impact of hurricane damage to wetlands, both coastal and inland, affects more than just people by the coast.
ÛÏWetlands work in nutrient cycling, recreation, habitats for wildlife, water purification, which are all quite important to what happens upstream,Û Hauser said. ÛÏIf these wetlands donÛªt act as a buffer, the storm surge and impact of damage that the hurricane can unleash onto an area can go more inland as well.Û
Wetlands face challenges at both their inland and coastal ends, especially with salinity intrusion. When trees die in freshwater wetlands, more salt-tolerant non-resident vegetation may come in, such as cattail, phragmites and sawgrass. Middleton explained that these species can transform the area into a more saline environment by pulling salt to the surface, making it harder for native freshwater vegetation to thrive.
Hauser said herbaceous wetlands are ÛÏthe first line of defense against hurricanes,Û protecting woody vegetation, which is less salinity tolerant. Disasters like Hurricane Sandy leave wetlands weakened.
ÛÏWithout that vegetation, itÛªs easily eroded by tides, flooding or just no longer hold on to the rest of the wetland and falls into the water, and that portion of the wetland is gone,Û she said.
Hurricanes and other severe weather events are predicted to increase in frequency and intensity because of climate change. Coastal environments weakened by salinity intrusion may not be able to provide protection against future storms. Middleton and HauserÛªs work provide insight into the complexity of hurricane impacts including the phenomenon of salinity intrusion to coastal forests and wetlands. Their research can be used as a guide to understanding the full scope of damage to areas in order to better plan for future hurricanes and remediation efforts.
Dani Leviss is a writer based in New Jersey. She recently graduated from Drew University with a bachelorÛªs in chemistry and minors in writing and art.