A Comparison of the Mycorrhizal Inoculum Potential of the Hydric Forests of the Kalamazoo River Riparian Ecosystem and Surrounding Mesic Forests in the Context of an Ecological Gradient

Abstract

A riparian ecosystem consists of the floodplain of a river. These ecosystems, found between upland terrestrial habitats and rivers, are diverse and are influenced by a flood-pulse hydrology (Gregory et al., 1990; Bayley, 1995). A distinct ecological gradient exists in these ecosystems (Mitsch and Gosselink, 1993). This gradient ranges from low-lying areas with moist, often peaty and hypoxic soils, almost no herbaceous vegetation, and low diversity of tree species, to a more elevated area with drier soils, more abundant herbaceous vegetation and tree species more characteristic of upland forests (Mitsch and Gosselink, 1993). An essential component of all forests, including floodplain forests, is the cycling of nutrients, in which mycorrhizal fungi playa key role (Brundrett, 1991). Mycorrhizae are obligate aerobes (Killham, 1994); however, Turner and Friese (1998) and Keeley (1980) found mycorrhizae even in completely submerged soils, and Avis (1994) and Foster (1994) found evidence of mycorrhizae in saturated soils of Kalamazoo River floodplain. Mycorrhizae, due to their capacity to enhance the growth of species from every major plant taxon, may help determine the location of plant species within floodplain forests and the potential for the growth thereof. To test the variable abundance of mycorrhizae in the floodplain, the mycorrhizal inoculum potential (MIP), or an indication of the ability of the soil to provide in situ mycorrhizal inocula, of soils from seven sites within the Kalamazoo River floodplain ecosystem and neighboring upland forests was determined. Only the abundance of colonization by arbuscular mycorrhizal fungi (AMF) found on the roots of a bait plant, Sorghum sudanense, was used to indicate MIP. Three vegetative zones corresponding to soil moisture and plant species (a "low," a "transition," and an "upland" zone) were sampled from each site with the expectation that the sparsely vegetated low zone, with its likely hypoxic soil, would have an MIP lower than the transitional and upland zones. The transition zone yielded the highest MIP, and the low and upland zones had equivalently lower MIP values. Since there was not an absence of AMF in the low zones, the lack of herbaceous species is likely due to the physiological tolerances of the surrounding species for hypoxia, high soil organic content, and the hydrology of the ecosystem.