Spartina alterniflora genotype influences facilitation and suppression of high marsh species colonizing an early successional salt marsh

1 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.