Microtopography in tidal marshes: Ecosystem engineering by vegetation?

Hummock-hollow microtopography is characteristic of many freshwater wetland systems. It is comprised of elevated, vegetated hummocks and lower elevation hollows; the latter are usually unvegetated, with reducing conditions in sediments unfavorable for plant growth. This microtopography is also often found in interior regions of brackish marshes, where flood duration is high and salinity fluctuations are prominent. Previous investigation showed this spatial patterning to be relatively stable over time and suggested that these microenvironments are produced by the plants themselves. This study investigates the possible mechanisms and controlling factors of this microtopography and considers the effect of different salinity regimes. We examined microtopographic variability of vegetation and sediment biogeochemistry in two interior tidal marshes, a freshwater-oligohaline marsh and a mesohaline marsh, both of which exhibited fine-scale spatial variability. Within a 2-yr period, the freshwater-oligohaline site demonstrated a labile response of both vegetation and sediment chemistry to interannual variability in salinity and sulfide concentrations, whereas the microscale spatial variability of the mesohaline system persisted. Geochronological assessment of the mesohaline marsh, where microtopographic variability was relatively stable, supported the hypothesis that the formation of the hummock-hollow topography is driven by the plants, rather than developing as a result of underlying physical variability. We propose that brackish marsh vegetation alters the sedimentary environment in such a way as to maximize growth under high-stress, variable conditions. The adaptive advantage of this strategy was illustrated in the accretion rates measured at the higher salinity marsh, which were indistinguishable between the interior hummock sediments and those of an adjacent homogeneous bank marsh.