RESPONSE OF 2 OLIGOHALINE MARSH COMMUNITIES TO LETHAL AND NONLETHAL DISTURBANCE

Severity is recognized as an important attribute of disturbance in man y plant communities. However, the effects of disturbances of different severity on patterns of regeneration in oligohaline marsh vegetation have not been experimentally examined. In these communities, a critica l difference in the effects of disturbance severity may be whether the vegetation dies as a result of the disturbance or is merely damaged a nd hence capable of resprouting. We described the regeneration of vege tation in two Louisiana marsh community types, one dominated by Sagitt aria lancifolia L. and the other by Spartina parens (Ait.) Muhl., foll owing three levels of disturbance: no disturbance, a nonlethal disturb ance, and a lethal disturbance. In the nonlethal disturbance, abovegro und vegetation was clipped to simulate common disturbances such as fir e and herbivory that remove aboveground vegetation but leave rhizomes intact. In the lethal disturbance vegetation was killed using herbicid e to simulate disturbances causing plant mortality such as wrack depos ition, sedimentation, scouring, and flooding following fire or herbivo ry. Regeneration was assessed over a 2-year period by measuring plant species richness, relative abundance, relative dominance, cover, and f inal biomass. To elucidate mechanisms for observed responses of vegeta tion, the species composition of the seed bank, light penetration, wat er level, salinity, and soil redox potential were evaluated. Despite d ifferences in the structure of undisturbed vegetation in the two commu nity types, they exhibited the same overall pattern of regeneration. F ollowing nonlethal disturbance, the dominant species resprouted and qu ickly reestablished the structure of the vegetation. In contrast, reco lonization following lethal disturbance occurred primarily via seedlin g recruitment, which resulted in marked shifts in community structure that persisted throughout the study. While the two communities respond ed similarly overall to disturbance, the response of individual specie s was not uniform; abundance, dominance, biomass, or cover increased f or some species but decreased for others in response to disturbance. S eed bank species occurred in the vegetation following lethal disturban ce in the Spartina community and in both disturbed and undisturbed plo ts in the Sagittaria community, indicating that the seed bank is a sou rce of propagules for regeneration and maintenance of oligohaline mars hes. Of the environmental variables measured, light level was most clo sely related to the effect of disturbance severity on community struct ure. Our results suggest that lethal and nonlethal disturbances have d ifferential effects on regeneration of vegetation that can create patt ern in oligohaline marshes communities.