Temporary floating island formation maintains wetland plantspecies richness: The role of the seed bank
Julia A. Cherry a,*, Laura Gough b
a Department of Biological Sciences, University of Alabama, P.O. Box 870206, Tuscaloosa, AL 35487, USA
b Department of Biology, University of Texas at Arlington, P.O. Box 19498, Arlington, TX 76019, USA
Received 14 July 2005; received in revised form 16 January 2006; accepted 20 January 2006
Abstract
In wetlands, fluctuating water levels create opportunities for recruitment of new individuals from seed banks, and drawdown periods often favor the establishment of species adapted for life in shallow water. In this study, floating island formation functioned similarly to drawdowns in water level by creating patches of sediment that were less inundated relative to the surrounding deep water marsh. The disturbance of floating island formation (i.e., mats of sediment and vegetation rising vertically in the water column) also resulted in reduced cover of the dominant rooted, floating-leaved macrophyte, thereby creating temporary gaps for the establishment of other species. To assess how floating island formation influences species richness and composition of wetland plant assemblages relative to surrounding deep water marsh, field surveys of plant percent cover on and off of islands were conducted over 2 years, along with a controlled greenhouse seed bank experiment in which levels of inundation were manipulated. Five plant species were present in deep water marsh compared to 22 in surrounding on floating islands. Plant assemblages on floating islands consisted primarily of emergent species, while floating-leaved perennials dominated the deep water marsh. Species richness and assemblage composition in the greenhouse seed bank experiment differed among water level treatments in a manner consistent with differences observed in field surveys. Assemblages germinating under minimal inundation treatments were more species rich (3.5–4.3 species per sample) and contained more emergent species (>450 individuals m_2) than did those germinating under flooded conditions (2.8–2.9 species per sample and <405 individuals m_2). This study, in addition to being the only reported seed bank study of temporary (i.e., seasonal) floating islands, demonstrates that islands altered levels of inundation favoring the germination of more species-rich, emergent wetland plant assemblages. Because these islands persisted long enough for several species to set seed, their formation may be one mechanism by which the seed bank is replenished and populations of otherwise uncommon species are maintained.
Keywords: Deep water marsh; Drawdown; Floating island; Percent cover; Seed bank