Genetic effects of a large-scale Spartina alterniflora (smooth cordgrass) dieback and recovery in the northern Gulf of Mexico

A large-scale dieback event struck marshes along the northwestern Gulf of Mexico coast during summer 2000, in apparent response to a prolonged and severe drought. Along the Louisiana coast, large areas of the dominant marsh species, Spartina alterniflora, turned brown, followed by death of at least the aboveground structures of entire plant mortality. Key ecological and genetic measures were studied in a dieback-affected marsh in southwest Louisiana (C83 marsh, Sabine National Wildlife Refuge), for which existed predieback ecologic and genetic datasets. Effects on genetic diversity only were studied in a second set of sites in southeastern Louisiana (near Bay Junop), where the dieback was more widespread. We hypothesized that stem density, live aboveground biomass, and genetic diversity would be significantly reduced compared to predieback conditions and to nearby unaffected marshes. Stem densities and biomass levels approached predieback conditions 14 months after first observance of the dieback in the Sabine marsh and were similar to or exceeded the same measures for a nearby unaffected marsh. DNA extracted from leaf samples in the Sabine and Bay Junop sites was used to construct genotype profiles using AFLPs and analyzed using the complement of Simpson’s Index (1-D), the richness measure G/N, average heterozygosity , and the estimated proportion of polymorphic genes.

. Genetic diversity was relatively unaffected by the dieback at either the Sabine or Bay Junop sites. Evidence from field observations and the results of the genetic analyses suggest that seedling recruitment is an important factor in the recovery of both the Bay Junop and C83 sites, although re-growth from surviving below-ground rhizomes appeared to dominate recovery at the latter site. Survival of below-ground structures, leading to the rapid recovery observed, indicates a high level of resilience of the Sabine marsh to drought-induced stress. Still, the genetic diversity ofS. alterniflora-dominated marshes may be promoted by occasional disturbance events, which produce open areas in which seedling recruitment can occur.