What is the future for coastal freshwater wetlands in a changing climate?

By Rebekah Grieger, PhD Candidate

Climate change poses a significant threat to coastal freshwater wetlands. Our new study finds that rising sea levels and changes in rainfall patterns could dramatically alter these important coastal ecosystems.

“Coastal freshwater wetlands are often overlooked for protection and research”

Rebekah Grieger

The response of mangrove and saltmarsh ecosystems to climate change is relatively well known, however coastal freshwater wetlands are often overlooked for protection and research. These areas are typically seen as easy to develop and less beneficial for society compared to estuarine wetlands. The lack of a basic understanding of the function and benefits of these wetlands has also meant that they have slipped under the radar in terms of climate change research and conservation.

Casuarina swamp in Boondall Wetlands Reserve (Photo: R. Grieger).

Our research investigated the responses of vegetation within a well-known Brisbane wetland, the Boondall Wetlands, to the impacts of flooding and saltwater intrusion. This replicated the effect of sea level rise and change in rainfall patterns, as predicted by the Intergovernmental Panel on Climate Change.

This study firstly found that there was a large difference in the vegetation community of the soil seed bank compared to the standing vegetation. More importantly, we found that germination of seeds was greatly reduced by flooding and salinity, particularly when combined. This means that of seeds which were present in the soil, only very few were actually able to germinate under these altered saline flood conditions, suggesting that climate change has the potential to significantly alter the vegetation of our wetlands.

If regeneration in the wetlands does not come from the soil seed bank, where does it come from? Especially in wetlands that are experiencing salt water intrusion from rising sea levels?

The most likely method is through vegetative regeneration, where individuals are able to sprout new plants from broken stem segments and from lateral roots. This mechanism is common in salt marsh species such as Sporobolus virignicus and allows for rapid growth in disturbed areas, enabling them to withstand periods of increased salinity and flooding.

Root growth of Sporobolus virginicus in flooded conditions could be important for this species survival with climate change (Photo: R. Grieger).

But what does this all mean for wetlands in a future where climate change is predicted? This study suggests that wetlands in SEQ may not be able to exist in their current state under the threat of rising sea levels, with more and more salt tolerant species likely with increasing saltwater intrusion events.

We know that vegetation communities can migrate inland, away from rising sea levels, from research conducted in tidal freshwater wetlands in U.S.A. To preserve the future of coastal freshwater wetlands in Australia, undeveloped areas that border onto the wetlands should be protected so that these communities have space to migrate and can continue to exist, providing their many ecosystem services.

This work appeared in Regional Environmental Change (Grieger et al. 2018).

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