Water-level fluctuations in North American prairie wetlands

Oscillatory water-level fluctuations are reversible changes in water levels around a long-term mean. Longterm water-level studies in wetlands in the prairie pothole region of North America and proxy data (e.g., tree rings) for water levels in this region indicate that oscillatory water-level fluctuations have occurred for thousands of years. Because there has been no standard set of terms to describe oscillatory water-level fluctuations, some terminology is proposed that is based on previous work on riverine wetlands. Changes in prairie wetland vegetation caused by oscillating water levels are called wet-dry cycles. Field studies indicate that two kinds of vegetation change are common during wet-dry cycles, fluctuations and successions. Fluctuations are changes in the relative abundance of species between the wet and dry phases of the cycles. They occur whenever the range of water levels during a cycle is small (ca. 50 cm), as in seasonal wetlands. Succesions are changes in species composition. They occur wherever the range of water levels is large (ca. 1.5-2.0 in), as in semi-permanent wetlands. During successions, high water levels during the wet phase can typically eliminate most emergent species and low or no water during the dry phase allows emergent species to become re-established from seed and terrestrial annuals to dominate the vegetation. Experimental studies at the ecosystem- and species-level have confirmed observations made during field studies of semi-permanent wetlands, e.g., that water depth tolerance is the primary determinant of distribution of emergent species. Both qualitative and quantitative assembly-rule models of wet-dry cycles have been developed. When adequate data are available, the latest quantitative models can accurately predict changes in composition and distribution of emergent vegetation in semi-permanent wetlands during all or parts of a wet-dry cycle.