Mangroves remember droughts — this could be bad news for our coastlines

For many Australians, memories of the disastrous 2019 and 2020 Black Summer bushfires are front of mind as we prepare for another hot, challenging summer.

It turns out we aren’t the only ones — mangroves also ‘remember’ severe conditions in order to prepare themselves for the future.

It’s a handy quirk of nature, but it might make them vulnerable as our climate changes rapidly. Dr Holly Beckett from the ANU Research School of Biology helps us to unpack the science.

What makes mangroves unique?

Unlike most other trees and shrubs, mangroves live with their roots in salt water in tidal rivers and wetlands. You might think this abundance of water would mean these remarkable plants are spared the worst of droughts, but their salty habitats present a challenge most other trees don’t have to deal with.

“Although a mangrove might have its roots surrounded by water, salt levels actually affect the plant’s ability to absorb water. The saltier the water is, the less water they can take up,” Beckett says.

“During a drought, the water level of a wetland or a river will drop, creating a higher concentration of salt in the water. Then, as mangroves take up the water, they actually leave most of the salt behind, which causes the salt to concentrate even further around their roots.

“Despite this seeming abundance, the actual amount of water a mangrove can use to hydrate during a drought is very small.”

Why are they important?

Mangroves protect our coastlines. They act as a buffer between the land and the ocean, dispersing the energy from storms, cyclones and large swells, reducing damage to the shoreline, reducing economic costs and even saving lives.

They are also a biodiversity hotspot, acting as a nursery for thousands of species, supporting life on coral reefs and providing important food and habitat for an estimated 75 per cent of the commercially important fish in Queensland alone.

“They have a complex root system that can filter out pollutants and sediments in water. Mangroves are capable of holding more carbon than other terrestrial forest types,” Beckett says.

“They also make up some of the oldest forests in the world, such as the Daintree.”

So, do they have a memory?

In a recent study, Beckett and her colleagues studied two mangrove species that both occur along the Daintree River.

“We took measurements before and after a severe drought at the end of 2018 to see how the plants manage hydration in their cells, including taking the water up and transporting it through the plant,” Beckett says.

“After the drought, the mangroves had measurements indicating increased drought and salinity tolerance compared to previous years.

“That finding itself is not that surprising. What was surprising was that our measurements were made six months on from the end of the drought, using leaves that grew in extremely low salinity conditions and had themselves never experienced drought conditions.”

For this to occur, trees must be able to ‘remember’ those drought conditions to program new leaves to grow for conditions of the past, not for the conditions they were actually experiencing. This is called an ecological stress memory.

What does this mean for our upcoming summer?

By remembering the worst conditions of a previous dry season, a tree can prepare itself to face those conditions again in the next season, only making small changes as the dry season progresses, rather than big adjustments.

However, the long period of cool and wet La Niña conditions makes it unlikely that mangroves across Australia have any pre-existing acclimatisation to the dry conditions we are bracing for.

“If the summer progresses slowly, the mangroves are likely to adapt well this season at least,” Beckett says.

“However, if the onset of drought is fast and severe, it may outstrip the capacity of mangroves to adjust, particularly if coupled with other environmental changes such as a sea-level drop. This occurred in 2015 and caused a mangrove die-back in the Gulf of Carpentaria.”

The researchers say further work is needed to confirm ecological stress memory in mangrove species and to better understand the influence on mangrove survival under different climate scenarios.

“As far as we are aware, no other study has investigated ecological stress memory in mangroves, so that is the first call to action,” Beckett says.

“Mangrove survival is critical to the survival of coastal ecosystems and will likely be critical to the survival of coastal populations as the climate emergency progresses.

“If we don’t understand the complexity of factors that influence a mangrove’s capacity to survive, we cannot accurately predict how these forests will be affected in the future.”

Dr Beckett’s work was conducted with support from the Australian Government Research Training Programme and the Australian Research Council.