Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Biological Sciences

First Advisor

James B. Grace


Iris fulva and Iris hexagona have overlapping geographic ranges in Louisiana; in areas of overlap hybrids are fairly common. Iris hexagona occupies the borders of freshwater marshes of southern Louisiana while I. fulva can be found farther north along edges of natural levees, canals and swamps. An area of hybridization (hybrid zone) can be found where bayous penetrate the freshwater marsh. The objective of this research was to investigate some of the factors affecting habitat differentiation of I. fulva, I. hexagona, and their natural hybrids ("Hybrid Purple" and "Hybrid Red"). A three year field study was conducted to determine the survival, growth, and reproductive characteristics of these taxa. Additionally, three greenhouse experiments were undertaken to examine shade tolerance, competitive ability, and salt tolerance. Results from field data found that hybrids had the highest survival rate while I. fulva had the lowest. Iris hexagona generally had the highest clonal growth rate. Hybrids were generally intermediate morphologically. Iris fulva and I. hexagona were not significantly different in terms of reproductive fitness and both had greater reproductive capacity than the hybrids. The shade tolerance experiment revealed I. fulva to be more tolerant of shading than I. hexagona and the two hybrids. Iris hexagona was greatly affected by all levels of shade. The competition experiment demonstrated that I. fulva was a weaker competitor than either I. hexagona or a natural hybrid. Iris hexagona and the hybrid were equally competitive. Results from the salt tolerance experiment indicated that I. fulva and "Hybrid Red" were less salt tolerant than I. hexagona and "Hybrid Purple." In sum, results indicate that the taxa are segregated across a complex environmental gradient. It is hypothesized that I. fulva with its greater shade tolerance, weaker competitive ability, lesser salt tolerance, and reduced clonal growth is displaced from the marsh by I. hexagona and confined to the bayous and canals. Iris hexagona with its greater clonal growth, competitive ability, and salt tolerance but lesser shade tolerance appears to be best adapted to represent an ecotone between parental habitats where there may be a selective advantage for the intermediate taxon, the hybrid.