Coastal wrack changes hydrology
Featured Image: Coastal forest hammock in Perdido, Florida. Source: Author.
Van Stan II, J.T., S.T. Allen, T. Swanson, M. Skinner, and D.A. Gordon. Wrack and ruin: Legacy hydrologic effects of hurricane deposited wrack on hardwood-hammock coastal islands. Environmental Research Communications 2 061001.
Hurricane activity is picking up in the northern hemisphere as we approach the historical peak of the season in late August. Hurricanes not only cause devastating damage to human infrastructure but can disturb ecological communities on a large scale. These disturbances can play out over a long period of time, years after the main effects of a hurricane have passed. One such disturbance is the deposition of thick layers of dead salt marsh grass, largely of the species Spartina alterniflora, in the southeastern United States. A recent study details how wrack deposition may change local vegetation communities through altering the local site hydrology of coastal island forests (called ‘hammocks’) by reducing the amount of rainfall entering into soils (a process referred to as infiltration) and causing excess salt to build up (a condition known as ‘salinization’).
Coastal tree hammocks
Small islands of salt intolerant trees surrounded by marsh are generally referred to as coastal tree hammocks (Fig. 1). Hammocks are typically located on ground that for various reasons sits higher than the rest of the surrounding marsh. It is thought that trees in hammocks rely on shallow infiltration of local rain as a water source. This infiltrated rain water sits on top of a deeper more saline water table that the trees cannot use and thus avoid. Although tree species common to coastal hammocks are salt intolerant, they can typically tolerate short pulses of seawater that may occur with infrequent tidal surges. Soils of non-impounded areas that are subject to hurricane surge typically flush relatively quickly after surge events. High rainfall prior to and after hurricanes means soils are usually wet prior to surge events. This limits the amount of soil subject to salt water intrusion (soils that are already wet won’t absorb as much water) and soils flush relatively rapidly after the surge event due to frequent rain. But what happens if detritus material such as marsh grass wrack is washed up and deposited on the soils of coastal hammocks?
Wrack is a common feature of coastal areas and generally refers to any type of coastal vegetation marine debris that washes onshore (Fig. 2). Wrack deposition can influence coastal geomorphology and productivity and has been shown to help build land in some areas. However, large amounts of wrack deposition can smother existing coastal vegetation, potentially leading to coastal erosion in some instances.
It is well known that organic detritus layers such as leaf litter can influence soil hydrology. Leaf litter generally absorbs rain and reduces the amount of rain that infiltrates deeper into the soil. However, only recently in a study by Van Stan and others were the effects of wrack deposition on coastal hammock hydrology investigated.
In 2017, Hurricane Irma deposited up to 0.5 m thick layers of Spartina wrack all along the Atlantic coastline. Researchers in Georgia used instrumentation at a long term coastal hammock study site to investigate the effects of the wrack-deposition disturbance on site hydrology. The authors quantified the amount of throughfall or the amount of rain that makes it through the tree canopy to the forest floor and calculated the percentage of throughfall that was being intercepted by the wrack and evaporated back into the atmosphere. The study found that the wrack layer reduced the amount of precipitation infiltrating into the soil by 71%! This rate of interception is much higher than that of leaf litter and the authors hypothesized that it might be related to the tubular structure of the Spartina that made up the majority of the wrack.
Using soil moisture and conductivity (a measure of the degree to which a material conducts electricity that is closely related to salinity) measurements, the authors also observed that soil water infiltration only occurred during intense storms. When infiltration was observed, soil salinity also increased substantially. The authors hypothesized that salt marsh wrack likely had a high concentration of precipitated salts when deposited and this only increased as salt was concentrated on the wrack surface through wetting and subsequent evaporation during small storms. When storms of high intensity caused infiltration, salt from the wrack was flushed into the soil.
The overall observed effect of the wrack was to slow down the flushing process after a surge event by reducing the amount of fresh water moving through the shallow soils and by acting as a long duration salt source for soil (Fig. 3). The reduction in rain infiltration can also reduce the thickness of the freshwater soil water layer and leave trees vulnerable to salt water intrusion from below. The authors hypothesize that changes in the hydrology of coastal hammocks from wrack deposition may negatively impact tree growth rates and potentially lead to a reduction in woody biomass in the long term. This study is an interesting example of how a seemingly discreet disturbance event such as a hurricane can have long lasting impacts on local hydrology and even lead to changes in vegetation communities.