Allocation patterns contribute to the distributional limits of a flood tolerant ecotype of Chamaecrista fasciculata

• We examine factors underlying the distributional limits of a xeric and mesic ecotype.
• We investigated the role of ecological and genetic mechanisms as delimiting factors.
• Negative genetic pleiotropy underlies trade-offs in allocation patterns.
• These allocation patterns limit the ecotypes to their respective environments.

To determine factors limiting the distribution of two ecotypes of the annual legume Chamaecrista fasciculata along an abrupt flooding gradient, we quantified the relative importance of tolerance, intra-specific competition, and allocation patterns to vegetative and reproductive functions and their genetic basis. The ecotypes grow in dry upland and freshwater tidal marsh habitat, respectively. Intolerance to flooding limited the upland ecotype from the marsh site, whereas the marsh ecotype had high survival in the upland site. Competition experiments revealed no competitive hierarchy between the two ecotypes under upland conditions. However, the upland ecotype allocated double the proportion of total above ground biomass to reproductive structures compared to the marsh ecotype. In addition, the marsh ecotype produced larger but fewer seeds per fruit than upland ecotype plants. These allocation patterns resulted in the upland ecotype having much greater seed production in the upland habitat. Using a crossing experiment, we quantified the genetic correlation between seed mass and seed number. A negative genetic correlation between seed mass and seed number, as well as a positive genetic correlation with seed mass and shoot elongation of flooded plants suggests that the trade-off in allocation patterns between these two components of reproductive output represents a pleiotropic genetic constraint associated with adaptation to a flooded environment. Thus resource allocation patterns leading to striking trade-offs between above ground vegetative and reproductive allocation appear to limit the marsh ecotype from invading upland habitat.