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 70 cm depth, revealed a significant positive linear relationship and strong correlation between hummock height and water depth (r2 = 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, 0.0010 d−1 and 0.0008 d−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.

► One of the few studies that have investigated the role of water regime in hummock formation.
► Found a strong, positive correlation between water depth and hummock height.
► Water regime influenced decomposition rates of organic material on hummocks.
► Results may have wider implications for the management of water regime in wetlands to maintain topographical heterogeneity.