Study Finds “Portfolio Approach” Can Help Protect Restorations From Extreme Climate Events

Knowing how to protect their communities from climate change has become a staple for local governments. Restoration projects—be they forests, wetlands or coral reefs—are no exception. However, many projects only account for gradual impacts, like sea level rise. They are often woefully unprepared for extreme events, like hurricanes or droughts.

A new study published Feb. 15 offers evidence for a promising solution: Treat restoration projects like stock investments. Creating a “portfolio effect” by diversifying certain aspects, like species or location, could give them a better shot at withstanding unpredictable extremes.

“You need to not put all your eggs in one basket,” said Chela Zabin, lead author and marine biologist at the San Francisco branch of the Smithsonian Environmental Research Center (SERC). “Because we just don’t know, really, how extreme an event’s going to be, what form it’s going to take, where it’s going to hit, how severe the impact might be.”

Zabin and her coauthors witnessed this firsthand during the winter and spring of 2017, when extreme rainfall devastated one of their oyster restorations with the San Francisco Bay Living Shorelines Project. The downpours topped anything seen since California records began in 1895. The water’s saltiness plummeted to five parts per thousand—a level Olympia oysters can withstand for days but not months. Not a single oyster at the site survived.

“For Northern California, 2017 was the wettest winter on record,” said Andy Chang, a SERC biologist and coauthor. “Given our changing climate, record-breaking, once-in-a-lifetime wet years like 2017 are now projected to occur much more often.”

However, the following fall, oysters began growing there again naturally. New oyster larvae had drifted in, likely from other parts of the bay not as affected by the extreme rainfall. The discovery that other sites could buffer an ailing restoration inspired the team to look at other restorations around the world. In the new literature review, published in Frontiers in Ecology and the Environment, they discovered the San Francisco oyster project was no exception: Diversity can make restorations more resilient to all manner of climate extremes.

Diversifying the Portfolio

To get more data, Zabin, Chang and others on the team scoured the scientific literature for restoration projects hit by climate extremes. They also talked to restoration practitioners on the ground in places that recently suffered an extreme event. They found 24 projects, from coral reefs in the Philippines to pine forests in France.

Most of the projects (20 out of the 24) incorporated diversity to some degree. And diversity almost always paid off. Though extreme climatic events routinely took their toll, 85% of the diversity-including projects saw some treatments perform better than others when faced with those extremes.

In the Philippines, some portions of a coral restoration were able to better withstand multiple extremes—including a Category 4 typhoon, a bleaching event and a monsoon—because the designers included different species and different restoration techniques. At a wetland restoration in Texas, spacing out planting over multiple years ensured some plants survived Hurricane Harvey in 2017.

“Wetland plants that went in the summer before Harvey didn’t have time to grow roots and stabilize the soil, so acres were just washed away,” said Laura Jurgens, coauthor and marine biologist at Texas A&M University at Galveston. “But sections planted earlier survived.”

Even restorations with only a single species found ways to vary things up. A wetland restoration in Wisconsin, focused primarily on the tussock sedge, took advantage of natural variation in the landscape. They planted some sedges in deeper depressions, and others in slightly higher peat soils. The project suffered two extremes—a drought in year one and extreme rainfall in year two. Diversity ensured that some parts of the project survived either event.

Overcoming the Obstacles

However, making diversity a key component of restorations will take more than a shift in planning. The barriers are real: from securing funding to managing public expectations. The authors hope the new study will help convince funders and policymakers to build more flexibility into projects.

“Sometimes we’re so focused on getting this one species back in this one area, but the reality of our changing climate means that we really need to think about the long-term resilience of projects,” Jurgens said. “Success is way more likely if we’re planting more species, at more sites, over more years.”

The paper suggests some low-hanging fruit restoration planners could use. As the Wisconsin wetland study showed, even small variations in a landscape can make the difference between surviving and perishing. The San Francisco Bay Living Shorelines Project highlighted the benefits of multiple species. In addition to oysters, the project also sought to boost eelgrass. While oysters returned naturally after the downpour of 2017, the eelgrass needed extra help. Coauthor Katharyn Boyer of San Francisco State University’s Estuary & Ocean Science Center led the eelgrass replanting effort. In the process, she discovered diversifying the timing and spacing of plantings can hedge bets that some portions will escape extreme events.

“Around a large estuary, we have seen vulnerable new eelgrass plantings experience different degrees of impact from the same extreme event, emphasizing the need to spread the risk of losses over multiple restoration sites,” Boyer said.

Washington College and the University of California at Davis also contributed to this article. It will be available on the journal’s website at https://doi.org/10.1002/fee.2471 after publication. For photos, an advance copy of the article or to speak with the authors, contact Chela Zabin at zabinc@si.edu, or Kristen Goodhue at goodhuek@si.edu.

# # #

SI-53-2022