Structure and composition of oligohaline marsh plant communities exposed to salinity pulses

The response of two oligohaline marsh macrophyte communities to pulses of i ncreased salinity was studied over a single growing season in a greenhouse experiment. The plant communities were allowed a recovery period in freshwa ter following the pulse events. The experimental treatments included: (1) s alinity influx rate (rate of salinity increase from 0 to 12 g l(-1)); (2) d uration of exposure to elevated salinity; and (3) water depth. The communit ies both included Sagittaria lancifolia L,; the codominant species were Ele ocharis palustris (L.) Roemer and J.A. Schultes in community 1 and Schoenop lectus americanus (Pers.) Volk. ex Schinz and R. Keller in community 2. Eff ects of the treatments on sediment chemical characteristics (salinity, pH, redox potential, and sulfide and ammonium concentrations) and plant communi ty attributes (aboveground and belowground biomass, stem density, leaf tiss ue nutrients, and species richness) were examined. The treatment effects often interacted to influence sediment and plant comm unities characteristics following recovery in fresh water. Salinity influx rate per se, however, had little effect on the abiotic or biotic response v ariables; significant influx effects were found when the 0 g l(-1) (zero in flux) treatment was compared to the 12 g l(-1) treatments, regardless of th e rate salinity was raised. A salinity level of 12 g l(-1) had negative eff ects on plant community structure and composition; these effects were usual ly associated with 3 months of salinity exposure. Water depth often interac ted with exposure duration, but increased water depth did independently dec rease the values of some community response measures. Community 1 was affected more than community 2 in the most extreme salinity treatment (3 months exposure/15-cm water depth). Although species richness in both communities was reduced, structural changes were more dramatic in community 1. Biomass and stem density were reduced in community 1 overall a nd in both dominant species. Structural changes in community 2 consisted of reduced biomass and stem density in the community overall and in S. lancif olia; S. americanus was not affected by salinity. in this most extreme trea tment, community 2 tended to change to a monospecific S. americanus stand w hile community 1 was reduced to a few surviving stems of secondary species. Our results suggest that vegetation recovery or establishment of new speci es following a temporary increase in soil water salinity will vary with exp osure duration and water depth. (C) 2000 Elsevier Science B.V. All rights r eserved.