A new study led by UNSW Sydney researchers has found why mangroves thrive on some coastlines and die on others: it comes down to ocean waves.

Published in Nature, the research shows that open-coast mangrove distribution is controlled by wave conditions. Even small shifts in wave direction or height can change whether a site offers a window of opportunity for mangrove seedlings to establish.

The finding matters because mangroves are natural coastal defences. A forest just 100 metres wide can cut storm wave heights dramatically, protecting the coastline behind it. They also store carbon, shelter wildlife and clean water.

The model suggests that as wave climates shift with climate change, some coastlines may lose their mangroves while new sites become suitable.

For coastal planners, the study offers a way to pick restoration sites that will survive. Planting mangroves where waves are right could save billions in engineered sea defences.

Researchers say protecting and restoring mangroves where wave windows exist is among the cheapest coastal defences available.