Lichens can help save coasts from climate change

This article was originally published by Hakai Magazine.

The clock is ticking for many low-lying coastal areas. Sea levels are rising at historic rates and promise to radically redraw the map. But determining exactly which properties will flood and which will remain dry is a much more daunting task. This effort may have an ally almost nobody would have guessed, one of the smallest and most inconspicuous life forms: lichens.

More than 3,600 lichen species have been identified in North America alone. Each is a community made up of one or more fungal species and an alga or cyanobacteria. This combination has allowed lichens to survive in diverse and often hostile conditions, as diverse as tropical heat and bitter Antarctic cold.

In order to scrape out their niche, different lichens have developed a tolerance to different factors of temperature, light, air quality, and other factors. Because of this sensitivity, lichens are already being used by scientists to measure environmental disturbances such as air pollution. Lichens also differ in their salt tolerance. It’s this property, says Roger Rosentreter, a botanist at Boise State University in Idaho, that makes them so useful for understanding sea-level rise.

“Lichens are a good indicator of site history,” says Rosentreter, who has been studying lichens and related species for more than 40 years. In particular, the lichen species growing in a coastal site can be a powerful indicator of low levels of saltwater intrusion and sea spray, which can be caused by infrequent flooding or storm events. As sea levels continue to rise, any site that has experienced occasional saltwater intrusion in the past is likely to experience more frequent flooding and storms in the future.

Recently, Rosentreter and his wife, Ann DeBolt, a Boise State botanist, surveyed the lichen communities of two state parks near West Palm Beach, Florida. One park on a barrier island is often exposed to salt spray and storm surges, the other is just 500 meters inland. The scientists found two surprisingly different lichen communities at each site. By comparing the two, they began compiling a list of lichen species that can be useful indicators of long-term or historical saltwater presence.

It takes more than just salt sensitivity to make a lichen a good indicator of whether a site has experienced the first effects of sea level rise. The lichen’s own life story also comes into play.

Species like powdery medallion lichen can be killed if exposed to too much salt water from a storm or flood. But the rapid reproduction of this lichen means it will quickly recolonize after the sea recedes. Larger species with slower growth and slower reproduction and low salt tolerance, e.g. such as the ruffled blue jellyskin, can better tell a site’s saltwater history. These salt-intolerant lichens probably would not have been able to survive and grow if a saltwater event such as storm spray or flooding occurred. Because some lichen species can live for decades or longer, the records they provide can be both spatially hyperlocal and temporally extensive.

Of the 48 different lichen species that Rosentreter and DeBolt found at their two Florida survey sites, 11 are potential indicators of the presence of salt water. Seven of the species only like to grow in places with very little saltwater exposure, while four are salt-tolerant. So if they’re growing in one location, it suggests they have a moderate salinity history and a higher risk of being affected by rising seas.

In general, Rosentreter and DeBolt found that the species that best indicate whether a site is relatively safe from sea level rise and saltwater inundation are larger and leafy lichens, many of which are light green or blue. But lichens can be difficult to identify, and some promising indicator species look quite similar to less useful ones. “You have to be at least an advanced plant expert to find out,” says Rosentreter.

“The good thing is that these aren’t unique to Florida. They’re all over the southeast coastal plain,” he says. Reports on iNaturalist, the California Academy of Sciences, and National Geographic’s Nature app place the ruffled blue jellyskin all over the US East Coast and beyond.

Borja G. Reguero, a UC Santa Cruz expert on maintaining natural defenses against sea-level rise who was not involved in the research, sees parallels between the way coastal communities and lichens deal with environmental change. “It makes a lot of sense to find these indicators [species] where the frequency of spray or flooding events is above a threshold beyond which some species can no longer survive,” he says. “The same could be said about people and coastal infrastructure. You reach a tipping point where certain parts of the city are flooded so regularly that they don’t get insurance.”

Modern science offers a range of tools to study sea level rise, including satellite data and groundwater and soil samples. Lichens could be another way to see, on smaller, site-specific scales, where the sea is next and, just as important, where it isn’t.

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