Summary 1We measured sediment elevation and accretion dynamics in mangrove forests on the islands of Guanaja and Roatan, Honduras, impacted by Hurricane Mitch in 1998 to determine if collapse of underlying peat was occurring as a result of mass tree mortality. Little is known about the balance between production and decomposition of soil organic matter in the maintenance of sediment elevation of mangrove forests with biogenic soils. 2 Sediment elevation change measured with the rod surface elevation table from 18 months to 33 months after the storm differed significantly among low, medium and high wind impact sites. Mangrove forests suffering minimal to partial mortality gained elevation at a rate (5 mm year − 1 ) greater than vertical accretion (2 mm year − 1 ) measured from artificial soil marker horizons, suggesting that root production contributed to sediment elevation. Basin forests that suffered mass tree mortality experienced peat collapse of about 11 mm year − 1 as a result of decomposition of dead root material and sediment compaction. Low soil shear strength and lack of root growth accompanied elevation decreases. 3 Model simulations using the Relative Elevation Model indicate that peat collapse in the high impact basin mangrove forest would be 37 mm year − 1 for the 2 years immediately after the storm, as root material decomposed. In the absence of renewed root growth, the model predicts that peat collapse will continue for at least 8 more years at a rate (7 mm year − 1 ) similar to that measured (11 mm year − 1 ). 4 Mass tree mortality caused rapid elevation loss. Few trees survived and recovery of the high impact forest will thus depend primarily on seedling recruitment. Because seedling establishment is controlled in large part by sediment elevation in relation to tide height, continued peat collapse could further impair recovery rates.

Recruitment patterns in clonal plant populations are predicted to vary with seed dispersal capability and disturbance regime, such that species with small, widely dispersed seeds will become increasingly dominated by vegetative recruitment on disturbed areas following early colonization. Subsequent mortality due to competitive or stochastic effects is then predicted to cause a gradual decline in both clonal diversity and the ability of surviving clones to avoid geitonogamous mating and possible inbreeding depression. We tested predictions of these hypotheses by comparing four adjacent populations of the salt marsh plant, Spartina alterniflora, ranging in age from 2 to ∼50 yr, by measuring fine‐scale genetic structure at the level of both ramets and genets, and the rate of inbreeding. For this purpose, we sampled maternal tissue and seeds from discrete patches in the field and then genotyped both maternal and seedling tissue (germinated in a growth chamber) using standard molecular protocols. As predicted, we observed an increase in clonal diversity (measured as the complement of the Simpson Index corrected for finite sample sizes, 1 − D) up to a maximum of 0.71 within 3‐m2 patches at 16 yr, declining to 0.55 by ∼50 yr. Local recruitment of seedlings was evident as genetic structure occurring at the level of patches, as measured by the fixation index, θ, which was inversely correlated with diversity (R2 > 0.90 at all patch scales). Outcrossing rates were positively associated with clonal diversity, with the highest level (89%) at an intermediate level of 1 − D. The greatest selfing (32%) occurred in young (2‐yr‐old) patches with low diversity. Biparental inbreeding was minimal in all populations, never exceeding 1%. Inbreeding depression was inferred to be severe, as evidenced by near‐zero adult inbreeding coefficients. These results suggest a possible fitness trade‐off between clonal growth and the opportunity for outcrossing. We recommend that restoration plantings of clonal species with limited sexual recruitment capabilities should be designed to ensure adequate clonal diversity for the avoidance of inbreeding and the ability to adapt to subsequent environmental disturbances.

Summary1 Genetically based phenotypic and ecotypic variation in a dominant plant species can influence ecological functions and patterns of recruitment by other species in plant communities. However, the nature and degree of importance of genotypic differences is poorly understood in most systems. 2 The dominant salt marsh species, Spartina alterniflora , is known to induce facilitative and competitive effects in different plant species, and the outcomes of interactions can be affected by nutrients and flooding stress. Clonal genotypes, which maintained their different plant architecture phenotypes throughout 31 months of a field experiment, underwent considerable genet-specific senescence in their centres over the last 12 months. 3 Different clonal genotypes and different locations (robust edges vs. senescent centres) permitted significantly different levels of light penetration of the canopy (14.8-77.6%), thus establishing spatial heterogeneity for this important environmental factor. 4 S. alterniflora clonal genotype influenced the degree of suppression of the previously dominant Salicornia bigelovii as well as facilitation of recruitment and growth by other plant species. Aster subulatus and Atriplex patula performed better in Spartina clone centres, and experienced reduced growth in Salicornia -dominated areas. 5 Four other high marsh species ( Borrichia frutescens , Aster tenuifolius , Iva frutescens and Limonium carolinianum ) colonized only into Spartina clones but not into the Salicornia -dominated area. 6 These results suggest that differences in clone size, centre senescence, stem density, height, total stem length and biomass in different genotypes of a dominant marsh plant species can influence recruitment and growth of other plant species. The spatial pattern of habitat heterogeneity is, at least in part, dependent on the genotypic diversity, and possibly the genetic diversity, of such foundation species. 7 We hypothesize that as genotypic diversity increases in populations of a dominant plant species like S. alterniflora , the number and diversity of interactions with other species will increase as well.

We collected naturally recolonizing Spartina alterniflora (smooth cord grass) from each of three restored sites and one undisturbed reference site in southwestern Louisiana to assess the impact of wetland restoration on genetic diversity. We used amplified fragment length polymorphisms (AFLPs) to produce 94 polymorphic genetic markers, which were used to characterize genetic diversity as average heterozygosity Ͻ H Ͼ and the proportion of polymorphic loci Ͻ P Ͼ . Overall our findings indicate that restored populations of S. alterniflora maintain levels of genetic diversity comparable to natural populations, which should provide some measure of resistance against environmental disturbances. Diversity estimates were lowest for the natural reference site ( Ͻ H Ͼ ϭ 0.1059; Ͻ P Ͼ ϭ 0.2763), whereas estimates for the three restored sites ranged from < H > ϭ 0.1148 to 0.1256 and Ͻ P Ͼ ϭ 0.3114 to 0.3202. All sites maintained sufficiently high diversity levels to suggest significant rates of outcrossing. Overall, genetic differentiation among populations was small (Weir and Cockerham's ⌰ ϭ 0.0645), with the values from each pairwise comparison among the populations increasing with the geographic distance between sites (range ϭ 0.0490-0.1101). These values indicate an average migration rate of 3.6 migrants, either pollen or seeds, per generation.