Duke, Norman C., Field, Colin, Mackenzie, Jock R., Meynecke, Jan-Olaf, and Wood, Apanie L. (2019) Rainfall and its possible hysteresis effect on the proportional cover of tropical tidal wetland mangroves and saltmarsh-saltpans. Marine and Freshwater Research, 70. pp. 1047-1055.

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Abstract

Mangroves and tidal wetlands are highly dynamic ecosystems, responding and adapting to climate and physical conditions that vary at all spatial and temporal scales. In these times of rapidly changing climatic conditions, identification of previously unrecognised large-scale ecosystem processes influenced by climate variables are highly relevant. This applies in particular, to a more enlightened understanding of the corresponding influences on respective beneficial ecosystem services. In deference to the many factors thought to influence tidal wetland ecosystems, we confirm that average annual rainfall (AAR) (250-5000 mm, 20-30 year average) has had a dominant influence on the vegetative cover and relative abundances as well as the composition and biomass of tidal wetlands. And, that the different condition states were predictable. Based on 205 unmodified, tropical and subtropical estuaries of predominately northern Australia, a sigmoidal relationship was derived between rainfall and the relative amounts of high-biomass mangroves and low-biomass saltmarsh-saltpan vegetation. The presence and probability of the observed combinations of these plant types were usefully quantified using the Wetland Cover Index (WCI), being the ratio of total mangrove area compared to the mangrove area plus the area of upper intertidal saltmarsh and saltpans. For tidal wetland sites well-within the latitudinal range of mangroves, 1368 mm average annual rainfall marked a centroid inflection point in the probability of the transition between the alternate dominance of respective vegetation types of high and low biomass states. In the range of 1066 mm AAR and 1651 AAR the rate of change of the probability of mangrove dominance per 100 mm AAR was greater than 5%. These findings were consistent with rainfall having a significant effect on relative abundances of key vegetation types within tidal wetlands. Furthermore, periodic fluctuations are likely manifest as either encroachment or dieback of mangroves occurring along the ecotone separating mangroves from upper tidal saltmarsh-saltpans. To explore this concept further, a new conceptual framework and model was developed to display these ecosystem-scale processes taking place in tropical and subtropical tidal wetlands.

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