How do marshes migrate?

Environments change and move, but how? Emily Kottler and Keryn Gedan have been looking at a migrating salt marsh by the Chesapeake Bay to see what the processes are that move a marsh.

It might be helpful to start by asked what, exactly, we mean by marsh. Emily Kottler explained: “A habitat is a marsh when it is periodically submerged in water (i.e. it is a wetland) and it is dominated by non-woody plants, often grasses.” So a proper marsh isn’t just damp, it also has marsh plants there.

Currently, sea-levels are rising in many places around the world, so we should expect salt marsh to be on the move. Kottler and Gedan looked to see how that happens. It could be that the moving saltwater clears land to be colonised. However, plant seeds get everywhere, so are marsh plant seeds lying in wait inland with plant ready to succeed if saltwater arrives?

Kottler and Gedan collected seeds across transects in Blackwater National Wildlife Refuge, Maryland, USA. They then watered the samples with either freshwater or seawater solution and looked to see what they had in the samples. What they found was that salinity affected seed bank diversity, but they also found salt-tolerant plants were depositing seeds up to 15m into the forest. Kottler said that the seeds were using a variety of methods to get inland. “Seeds are often dispersed on the wind, but can also be carried by water in flooding and tidal flow events, and some larger seeds by birds eating their fruit. ”

The research into shifting marsh is something that Kottler took up after graduating. “After finishing my bachelor’s degree, I worked for the Conservation Land Management in coastal areas of the Mid-Atlantic,” she said. “I collected native seeds to be used for restoration projects in areas that had been affected by Hurricane Sandy. I fell in love with these beautiful natural areas. But, when speaking with locals who saw me at work, I became increasingly aware of the changes that were occurring: shrinking of marshes with increased flooding, roads submerged, invasive species pushing out native ones. I was fascinated by how rapidly this system was changing and wanted to understand the processes involved in this change.”

“Due to its flat topography and position within the Mid-Atlantic, the Chesapeake Bay is a hotspot of sea-level rise and marsh migration. One third of the marsh land in the Chesapeake today was formed in the last 100 years due to marsh migration into upland habitats. The progress of sea-level rise is very visible to people living on the Bay, with marsh plants growing into their back yards and farm fields. Our lab also looks at the impacts of sea-level rise on agriculture, in the hopes of mitigating the negative impacts of saltwater intrusion on coastal farms.”

While Kottler and Gedan conclude that seed banks predict future communities, they also note that other forms of dispersal, like clonal propagation need to be examined, along with other factors like herbivory. It’s a complex challenge that attracts Kottler. “I find it fascinating how the habitats that I work in are so visibly changing with sea-level rise. For example, Blackwater NWR, the site where we did our work, used to be all marsh habitat. It now has a large brackish lake in the middle, because the center of the marsh died as a result of sea-level rise. It is surrounded by ghost forests of bleached white dead tree trunks, that look quite dramatic on the landscape.
While this change can be alarming, I think its so important to understand how plants and natural communities can cope with rapid environmental change. It’s very motivating for me.”

“After doing this work, I’m definitely interested in how the varying environmental conditions along this habitat border affect marsh plants at multiple stages in their lives. I’ll be conducting more research in the field and in the greenhouse to study their response and resilience to novel conditions as part of my PhD dissertation.”

While the study may be rooted in the marsh of Maryland, the lessons apply to other environments around the world, Kottler said. “Anyone looking at habitat borders that are shifting with climate change should take a look, and consider the role of the germinable seedbank in that transition.”

Understanding more of how wetlands move could help protect communities from some of the damage of rising sea levels. In their paper Kottler and Gedan conclude:”If we are to maintain marshes, inland migration will be an important aspect of preserving marsh area and ecosystem services.”