Ecotypic differentiation for flood-tolerance and its morphological correlates in Chamaecrista fasciculata

Chamaecristafasciculata is widespread in eastern North America and commonly found on dry sandy soils. In contrast, large seeded C. fasciculata var. macrosperma is restricted to fresh-water tidal marshes of the central coastal plain of Virginia, USA. Greenhouse experiments were conducted to determine if var. macrosperma represents a flood-tolerant ecotype and if flood tolerance is associated with decreased tolerance to dry conditions. Morphological measurements on root and shoot growth were made to determine correlates and possible mechanisms of tolerance to flood and dry conditions. Flood tolerance in var. macrosperma was confirmed. This ecotype had greater seedling survivorship and greater accumulation of biomass under flooded conditions than the upland form. Vegetative performance of the two types was comparable under well-watered but drained conditions. Flood tolerance is associated with increased seed mass, a two-fold greater rate of shoot elongation, and increased shallow lateral root growth. The evolution of flood tolerance does not result in a decrease in vegetative tolerance to dry conditions even under competitive conditions. The large seeded, flood-tolerant ecotype may be absent in drier habitats because of a trade-off between individual seed mass and seed number.