Intraspecific diversity effects on population dynamics, community processes, and ecosystem functions have been demonstrated using a variety of taxa in aquatic and terrestrial ecosystems. Yet relatively little is known about the relationships among different diversity metrics, nor which of these metrics best predicts population, community, and ecosystem-level responses to changing environmental conditions. In salt marshes, the foundation plant species Spartina alterniflora forms the habitat for diverse invertebrate and vertebrate species, in addition to providing ecosystem services such as coastal protection, carbon sequestration, and water purification. Spartina exhibits two growth forms along an elevation gradient, with tall plants in the low marsh closer to the tidal creek and short plants in the high marsh farther from the tidal creek. Metrics of intraspecific diversity in Spartina include genotypic identity, ecotypic differentiation, and phenotypic variation, as well as integrative measures like genetic relatedness and functional trait diversity. To address how different metrics of Spartina diversity relate to each other as well as to ecosystem function, we combined a field survey of three marshes with a controlled greenhouse experiment assessing plant traits, with the goal of predicting the capacity of this foundational plant to respond to climate change stressors, including sea level rise.
Results/Conclusions
In both the field survey and the greenhouse experiment, we found effects of Spartina ecotype (tall versus short) on Spartina stem density, stem height, and leaf number, as well as elemental content (percent carbon and percent nitrogen). In the field, we also found differences in stable isotope composition between ecotypes, indicating variation in water use efficiency. In the greenhouse experiment, tall and short Spartina differed in above- and below-ground biomass, as well as the ratio of root to rhizome biomass. Respiration rates of tall and short Spartina also differed in the greenhouse common garden, with important implications for ecosystem function. Within ecotypes, there was often two-fold or greater variation in morphological and physiological traits among Spartina genotypes, indicating that genotypic identity, and consequently genetic relatedness, can impact population and community dynamics, as well as ecosystem function. Further, the degree of trait variation within ecotypes differed: for example, there were pronounced differences in net and gross ecosystem exchange rates for short, but not tall, Spartina genotypes. This suggests that relationships between genetic versus trait metrics of intraspecific diversity may vary across environmental gradients; thus, attempts to generalize the nature of these relationships may require consideration of population scale and environmental heterogeneity.