Mangroves on the March: climatic and hydrologic variables influence southern-most mangrove encroachment into saltmarsh

Ms Ashley Whitt1, Dr Rhys  Coleman2, Dr Chris Gillies3, Dr Catherine Lovelock4, Dr Pete Macreadie1

1Deakin University , Melbourne, Australia, 2Melbourne Water , Melbourne, Australia, 3The Nature Conservancy , Melbourne, Australia, 4The University of Queensland , Melbourne , Australia


Unprecedented rates of climate change have contributed to shifts in species distributions, such as mangrove encroachment into saltmarshes near latitudinal range limits. Western Port Bay (WPB) in Victoria, Australia, nears the southern-most range of mangroves and is home to a diverse assemblage of saltmarsh species. We used a semi-automated object-based imagery analysis to classify wetlands in aerial photographs spanning 1970-2017 and assessed whether changes in mangrove extents were related to climatic variations and site hydrology.

Mangrove coverage increased whereas saltmarsh coverage decreased overtime; furthermore, mangrove encroachment was more likely to occur landward than seaward. Mangrove coverage was significantly impacted by an increase in drought like conditions, specifically an increase in maximum temperatures and decrease in rainfall days. Additionally, our best-fit model showed a very strong hydrologic dependence of mangrove coverage. Though there was no significant effect of wave fetch or sea level rise, the percent of inundation was strongly significant. The probability of mangrove encroachment significantly increased with higher densities of and shorter distances to tidal creeks. 87% of mangrove encroachment occurred within 50m of the marsh-mangrove ecotone, of which 20% occurred within 50m of a tidal creek.

In summary, our study revealed mangrove encroachment and thus loss of salt marsh biodiversity, was not only impacted by climate change but also hydrological features such as tidal creeks. Understanding regional variability in mangrove encroachment is paramount to effectively implementing proactive management of climate change impacts to Victoria’s coastlines, such as assisting in the prioritization of land parcels to conserve or restore.


Ashley is a PhD candidate at Deakin University’s Blue Carbon Lab in Victoria, Australia. She works with Melbourne Water and The Nature Conservancy to understand climate change impacts on coastal wetlands and to develop management strategies.

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