Relationship between water regime and hummock-building by Melaleuca ericifolia and Phragmites australis in a brackish wetland

We investigated the relationship between hummock height and depth of inundation in a permanently inundated wetland in south-eastern Australia. Our survey of 318 hummocks, in water ranging from 0 to 70cm depth, revealed a significant positive linear relationship and strong correlation between hummock height and water depth (r(2) = 0.53 and 0.79 for Melaleuca ericifolia and Phragmites australis hummocks respectively). We also investigated whether water regime affects the decomposition rate of litter on hummocks; specifically, whether constant inundation slows decomposition to an extent that would promote accumulation of litter and hummock-building. On the contrary, we found that constantly submerged M. ericifolia litter decomposed faster than dry litter, but at a similar rate to litter that experienced intermittent inundation (decay rates (k) 0.0015 d(-1), 0.0010d(-1) and 0.0008d(-1) for submerged, intermittent and dry treatments respectively). Submerged P. australis litter also decomposed faster (k = 0.0024 d(-1)) than dry litter (k = 0.0011 d(-1)). We discuss the interaction of water regime and decomposition of organic material and implications for the maintenance of hummock and hollow topography.