Hydrologic regime is an important control of primary production in wetland ecosystems. I investigated the coupling of flooding, soil salinity and plant production in northern prairie marshes that experience shallow spring flooding. Field experiments compared whitetop (Scolochloa festucacea) marsh that was: (1) nonflooded, (2) flooded during spring with 25 cm water and (3) nonflooded but irrigated with 1 cm water · day-1. Pot culture experiments examined whitetop growth response to salinity treatments. The electrical conductivity of soil interstitial water (ECe) at 15 cm depth was 4 to 8 dS· m-1 lower in flooded marsh compared with nonflooded marsh during 2 years. Whitetop aboveground biomass in flooded marsh (937 g · m-2, year 1; 969 g · m-2, year 2) exceeded that of nonflooded marsh (117 g · m-2 year 1; 475 g · m-2, year 2). Irrigated plots had lower ECe and higher aboveground biomass than nonflooded marsh. In pot culture, ECe of 4.3 dS · m-1 (3 g · L-1 NaCl) reduced total whitetop biomass by 29 to 44% and ECe of 21.6 dS · m-1 (15 g · L-1 NaCl) reduced biomass by more than 75%. Large reductions of ECe and increases of whitetop growth with irrigation indicated that plants responded to changes in soil salinity and not other potential environmental changes caused by inundation. The results suggest that spring flooding controls whitetop production by decreasing soil salinity during spring and by buffering surface soils against large increases of soil salinity after mid-summer water level declines. This mechanism can explain higher marsh plant production under more reducing flooded soil conditions and may be an important link between intermittent flooding and primary production in other wetland ecosystems.
Keywords: Flooding; Freshwater marsh; Primary production; Soil salinity; Wetland hydrology.