How do mangrove forests recover from cyclones?

Mangroves provide protection for coasts against extreme weather. While they may protect shorelines from cyclones, how does a hit from a hurricane affect the mangroves? The literature has been surveyed by Ken Krauss and Michael Osland in a new paper Tropical cyclones and the organization of mangrove forests: a review.

“Land managers in the National Park Service and program coordinators in the USGS asked us if such a review existed after Hurricanes Irma and/or Maria ravaged the US Virgin Islands, Puerto Rico, and Florida,” said Ken Krauss. “We realized that such a review did not exist, but the fear of managing mangroves that are hit by repetitive cyclones has prompted folks to wonder what can be expected in their mangroves after such storms. What is normal? We attempted to answer that.”

For the review, Ken Krauss drew on over 25 years of experience. “For me, one of my first mangrove projects as an intern was to view aerial images of Hurricane Andrew’s (1992) damage to the mangrove forests of Everglades National Park. I was working in Louisiana viewing VHS-style video from helicopter surveys. At the time, I had never been to a mangrove forest, and I was amazed at the amount of damage caused by the hurricane and the spatial patterns of that damage based on tree height, proximity to open water, and distance and azimuth from the storm’s path.”

“From about 2001-2004 (during my dissertation studies), I finally visited many of those same forests in Everglades National Park along the Shark, Lostmans, Harney, Broad, and Chatham Rivers that I viewed during those original VHS-style image analyses. In some cases, it was difficult to even tell that a storm had passed some ten plus years earlier. I found the recovery potential of mangroves striking.”

Mangrove regeneration is a major challenge for the plant. Firstly, the seedlings have to survive a cyclone strike that rips down older trees. “Surge and high tides can serve to buffer and protect seedlings and saplings from wind effects during tropical cyclones,” write Krauss and Osland in their review. “When Cyclone Tracy affected the mangrove forests around Darwin, Australia in 1974, forests were on a rising tide, leaving most saplings and small trees covered in water and unaffected by winds (Stocker, 1976). Ferwerda et al. (2007) suggested that flooding of the regeneration pool contributed significantly to a near-complete recovery of the mangrove forest within ten years of Cyclone Tracy.”

Once the cyclone passes, the seedlings are exposed. This change dramatically increases the light levels for saplings to grow. However, they are still in a saline environment, meaning using water for photosynthesis is not a simple matter. As well as harbouring seedlings in the hit location, mangroves can release propagules, plants ready to grow, when a cyclone hits. These propagules are scattered and can survive for months at a time. If they arrive in a suitable location, they can settle and regenerate a new mangrove forest. The timing of propagule production coincides with some of the hurricane season in the Gulf of Mexico. It is tempting to ask if mangroves are using cyclones as a reproduction opportunity, but Krauss is sceptical.

“Some have suggested that the reproductive delivery of some species, including Rhizophora mangle, have evolved to peak during tropical cyclone season in the Neotropics. The idea being that longer distance dispersal is assisted by these storm winds, surge (especially dispersal into interior marsh areas), and current anomalies. It is difficult to visualize the fitness benefits gained by such interaction, and if they do exist, they still need to be sorted. All of this is confounded by temperature. Maximum average temperatures in the tropics (and sub-tropics) that would facilitate reproduction happen to coincide also with the best conditions for tropical cyclone intensification. From reading papers during the development of this review, several authors have suggested some creative benefits of co-evolution potentially, but I believe that further testing is required to make such links.”

It’s not just light levels that can change, following a cyclone. The soil that a tree is in can change dramatically too. Feher and colleagues found an immediate gain in soil surface elevation from storm-deposited sediments along the Shark and Lostmans rivers immediately following Hurricane Wilma, but that gain was partially negated over time as those sediments compacted in the years following the sediment pulse. Hurricane Irma later added more sediments, lifting the elevation further.

It’s not just land that can rise, so can the sea. In order to survive rising seas, mangrove forests often need to catch or deposit material in order to rise themselves. “Sea‐level rise is a special concern for coastal land managers, especially as many mangrove ecosystems are unable to migrate inland because of artificial barriers, such as sea walls and dikes,” Krauss and colleagues wrote.

Study of mangrove response to cyclones relies on a little luck and a lot of survey work. Knowing exactly where a cyclone will land in advance is difficult, even just a few days out. So ecologists looking at shoreline damage have to move in afterwards.

“For forest injury assessment, ecologists typically rely on three approaches,” Krauss said.

1. re-surveying forest plots that have by been previously measured and by chance have been impacted by a cyclone,
2. surveying stands after impact and re-creating previous and current forest structure from damaged and standing trees, and
3. surveying stands affected by storms versus those that may be separated by distance (or mountains) and not affected by the storm.

Understanding how mangroves regenerate has been recognised as more of a priority since Krauss started his career. “I find a much greater sense of urgency to protect and restore mangroves globally, and with this, folks are also starting to understand better how mangrove restoration should proceed. Globally, millions of dollars have been wasted on mangrove plantings in the name of “restoration” when site selection was ill-conceived, and basic underpinnings of mangrove hydrodynamic requirements are overlooked. This is unfortunate but changing. Mangrove restoration is contributing to vastly reduced annual overall mangrove area losses globally, although certain countries still serve as mangrove loss hot-spots (e.g., Indonesia, Malaysia, Myanmar).”

“Mangrove managers and scientists are gradually starting to realize that often when mangroves do not recover from storms, it is because something is problematic before the storm. Mangroves can survive under various forms of stress for decades without showing obvious signs of that stress to the casual observer. Under stress – such as changes in hydrology – cyclone recovery can be impeded, and the storm often gets the blame for the destruction and lack of recovery when in fact, it is that road, causeway, or bulkhead that was constructed 30 years ago that caused the mortality. A preeminent mangrove restoration ecologist, Robin Lewis, has referred to these sites as “mangrove heart attacks” (it is worth reading about this further in Marine Pollution Bulletin, v. 109, p. 764-771, 2016).”

“This is not always the case, however. Cyclones can cause mangrove loss even when mangroves are relatively un-impacted. There are examples of mangrove “backstepping” or collapsing in the Everglades over the Holocene, and certainly additional evidence for this can be found globally.”

Looking to the future Krauss would like to see work on how the strength of a cyclone contributes to the impact. “One of the primary needs is to relate wind energy realized in mangrove forests to the damage incurred by the storm, but this is not a simple task. We are decades out from this, I think.”

“I think that forest simulation storm recovery models for mangrove ecosystems that could eventually be applied globally would be a useful topic. This is reasonable given that there are just 70 mangrove species globally, and there is some available work from the neotropics to build from. Such storm recovery models could include sea-level rise and carbon sequestration to bridge past research on disturbance ecology in mangroves, including lots of good papers and observations, with our growing recognition that mangroves are incredible places to sequester carbon from CO₂ and are ecosystems that protect coastlines in various ways.”

The review is fascinating, and sometimes sobering reading. While cyclones may become more dangerous in the future, Krauss and Osland caution that it is one factor of a connected web of anthropogenic activity. In the conclusion, they write, “[I]f society is to become increasingly dependent on the ecosystem services and recovery of mangrove ecosystems during and after tropical cyclones, more effort must be placed into identifying mangrove stress on large spatial scales far in advance of tropical cyclone impact so that they can be rehabilitated and remain resilient to storm effects… Stress comes in many forms, intended and unintended, but all chronic stress affects recovery to acute events in mangroves.”