SPECIES‐RICH PLANTINGS INCREASE BIOMASS AND NITROGEN ACCUMULATION IN A WETLAND RESTORATION EXPERIMENT

Our test of the hypothesis that biomass and nitrogen would increase with more species-rich plantings simultaneously vegetated a salt marsh restoration site and dem- onstrated that on average, randomly chosen, 6-species plantings accumulated more biomass and nitrogen than the mean for 0- and 1-species assemblages, with the mean for 3-species assemblages being intermediate. In addition, we found that individual species (from the pool of eight native halophytes) differed in their functional capacity, with Salicornia vir- ginica (Sv) and Jaumea carnosa contributing the greatest biomass when planted alone, while Triglochin concinna had the highest tissue N concentrations. When planted alone, Sv accumulated comparable amounts of biomass and nitrogen as in the multispecies plots, indicating that individual species can have a large effect on particular functions. Soil TKN in the surface 0-5 cm was greater in 6-species plots than unplanted plots in 1999, while both 3- and 6-species plots were greater than unplanted plots in 2000; however, there were no differences at 5-20 cm depth and no species-specific effects. Root and shoot biomass both increased with species richness, with total biomass of 6-species plots averaging 995.6 ? 120.5 g/m2 in 2000, compared to the mean for 1-species plots (572.1 + 90.3 g/m2) and unplanted plots (164.5 ? 24.7 g/m2). Still, at the age of three years, root biomass was only about one-third that of the species-rich reference site, and shoot biomass was one-half to one-fifth the maxima reported for reference salt marshes. Species-specific effects were found for Sv, which had high biomass of both roots and shoots in the multispecies plots (55% of aboveground biomass in 3-species plots and 41% in 6-species plots) and the highest pool of N (52% of the N pool in 3-species plots and 42% in 6-species plots), even though only one-eighth of the initial plantings were Sv. However, when plots with this species were excluded from the analysis, the species-richness effect persisted. Thus, ecosystem function, as measured by biomass and N accumulation, increased with species richness regardless of dominance by the highly productive Sv. We conclude that manipulating the richness and composition of plantings offers ecosystem restorationists an effective tool for accelerating the rate of functional development.

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