Climate change threatens the first line of defence against coastal storms

Changes in climate are expected to increase the likelihood of extreme storm events. Plants on the coast line can play a big role in mitigating the damage from storms, but the same changes that are driving the storms are also putting plants under more stress. Mick Hanley, Tjeerd Bouma and Hannah Mossman have examined how these stresses and the increased storm damage on ecosystems interact.

Dr Mick Hanley, Associate Professor (Reader) in the School of Biological and Marine Sciences at the University of Plymouth, led the research. He said: “It has been suggested that by 2050, it could cost well over $50billion to protect the world’s largest cities from coastal flooding. In contrast, coastal vegetation can offer natural protection against erosion and flooding for a fraction of the costs associated with constructing so-called hard defences like concrete walls. Society is only just beginning to appreciate this, but estuarine and coastal ecosystems can be integrated into a dynamic, low-cost flood defence strategy to meet the ever increasing challenges posed by rising sea-levels and storms.”

The review in Annals of Botany highlights a combination of rising sea levels, increased sea surface temperatures and enhanced wave forcing as a combination of stresses that interact with estuarine and coastal ecosystems. Throw in the impact of a major storm, and damage can be sudden and dramatic. It is however, difficult to get the data as the damage is also local, so you need a storm to strike somewhere where there is already good survey data.

Hanley and colleagues write, “[W]here sufficient ‘pre-event’ data are available, studies show major changes in coastal geomorphology and vegetation for many years afterwards. Carter et al. (2018), for example, used a time series of remotely sensed images to show major breaching, land area reduction, and vegetation loss throughout the Mississippi–Alabama barrier islands in the first 10 months after Hurricane Katrina made landfall. These changes were, however, site specific, depending on sediment removal or accretion, underscoring the more general problem that it is difficult to predict exactly how and when storms affect particular coastlines.”

The authors note that storm surges are hard to predict, leading to a relative paucity of knowledge, but some factors are conspicuous. A storm surge can cover rarely inundated land with seawater, leading to a combination of flooding, leading to anoxia (a lack of oxygen) and also to salt stress. But Hanley and colleagues write that the two problems have very different effects. “In fact the ‘salt stress’ associated with coastal flooding seems to be much more important to plant response and recovery than anoxia. In experiments where supra-littoral plants have been simultaneously exposed to freshwater and seawater immersion, the former has never resulted in any noticeable impact on plant ecophysiology compared with untreated (no immersion) controls.”

“Most importantly perhaps, the ability of plants to tolerate, and recover from, seawater flooding seems to be species specific. Long-term observation of Arctic tundra following a major storm surge in the Mackenzie Delta, Canada, shows that dwarf shrub tundra had a much reduced regenerative capacity compared with graminoids or upright shrubs.”

For plants in the inter-tidal region, inundation is a regular event, but even here Hanley and colleagues find evidence that increasing inundation will reduce plant productivity. As storms hit the shore they will reshape coasts, leading to losses in plant cover in places that they impact. Even beneath the waves, sub-tidal plants such as kelp can be damaged at wave energy and flow speeds strip at the fronds.

The damage matters, say the authors, because the plants often help stabilise a physical barrier between surging waves and settlement inland. They note that mangroves are shown to reduce wave height and lower water levels during storm surges. It is also possible that kelp can reduce the energy in storm waves before they even reach the shore. 

The review concludes with suggested directions for future research, with four priorities:

1. Effects of storm damage and flooding on plant reproductive performance and recruitment
2. Coastal plant responses to multiple stressors associated with sea level rise and storm damage
3. Plant community interactions and post-disturbance recovery
4. Better prediction of where and how storm events and sea level rise impact estuarine and coastal ecosystems and the delivery of essential ecosystem services

Dr Hanley, who also co-edited the Annals of Botany special edition focusing on the vulnerability to, and management of, plant communities in the face of increased flood risk, added: “The realisation that coastlines globally are now facing increased threats provides the impetus for understanding how hurricanes, typhoons, cyclones and other extreme weather events can affect coastal vegetation. That is critical to ensure we can effectively manage risk over the coming decades, but considerable work is needed to ensure the potential power of plants to defend our coastlines is not lost before it’s fully understood.”

The special issue is free access for three months.