Abstract

Aboveground plant community dynamics in an oligohaline marsh at Big Branch Marsh National Wildlife Refuge, Louisiana, U.S.A., were assessed in response to nutrient loading (3 N × 3 P factorial) and disturbance (both planned herbicide treatment and stochastic tropical storm or hurricane disturbances). Sampling was conducted seasonally from April 2004 to September 2006. Spartina patens and Schoenoplectus americanus are codominant plant species in this marsh. Although Spartina patens displayed increased aboveground cover under the low N addition (20 g N m−2 y−1) relative to ambient conditions or high N addition (40 g N m−2 y−1), increased N or P loading did not result in a shift in plant community composition or species richness during the study period. Schoenoplectus americanus consistently had higher leaf tissue N and generally higher leaf tissue P than Spartina patens regardless of treatment. Our results indicate that Schoenoplectus americanus is more resilient than Spartina patens to disturbances that do not increase marsh surface elevation, such as minor disturbances (e.g., prolonged flooding events) or prescribed burning, which is often utilized as a management technique to increase the relative abundance of Schoenoplectus americanus in this marsh type. Similarly, Schoenoplectus americanus was able to recolonize the herbicide treatment plots to some degree during the study via a combination of seed bank and rhizome tillering, whereas Spartina patens remained essentially absent. Hurricane Katrina deposited significant amounts of sediment (average of +27 cm) into plots that survived the storm (August 29, 2005). By 2006, this elevation increase resulted in a significant increase of both Spartina patens cover and species richness, suggesting that a shift in the relative abundance of the two codominants is mitigated by disturbance type and the resultant effect of disturbance on the abiotic environment, particularly marsh surface elevation.

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