VARIATION IN THE BIG BLUESTEM (ANDROPOGON GERARDII)-SAND BLUESTEM (ANDROPOGON HALLII) COMPLEX ALONG A LOCAL DUNE/MEADOW GRADIENT IN THE NEBRASKA SANDHILLS

Twenty four quantitative characteristics of 95 specimens of the big bluestem (Andropogon gerardii Vitman)-sand bluestem (Andropogon hallii Hack.) complex collected from six topographic positions along a local dune to meadow topographic gradient in the eastem Nebraska Sandhills were examined to determine the relationship between morphological variation and habitat variation in adjacent bluestem populations. All morphological characters showed intergradation between big bluestem and sand bluestem but based on awn length, ligule length, cilia length on pedicels and rachises, rhizome length, and hybrid indexes, as well as comparisons with isolated bluestem populations, dune and meadow bluestems were found to be morphologically distinct. Sand bluestem types occupied dry, upland sand dunes, big bluestem types were restricted to adjacent, subirrigated meadows and plants intermediate in morphology were found only in narrow dune/meadow transition zones. For all bluestems, four characters (awn, ligule, cilia and rhizome) were significantly correlated with each other but correlations were most non-significant when only intermediates were considered; pollen stainability, however, was highly variable and was not related to morphology. Variation in leaf thickness, adaxial stomatal frequency and leaf absorptance to photosynthetically active radiation was found to exist in bluestems, which previously has not been quantitatively described in this complex. Habitat assortment in bluestem types along this dune to meadow topographic gradient was closely related to soil moisture levels which, in these sandy soils, were controlled primarily by the depth of the underlying water table. INTERGRADATION BETWEEN big bluestem (Andropogon gerardii Vitman) and sand bluestem (Andropogon hallii Hack.), two closely related, native, perennial North American prairie grasses, is well known. Natural hybridization between these bluestems was first suggested by Romberg (1954) and statewide surveys of plants in Nebraska by Satterwhite (1970) and Kestner (1 973) confirmed that morphological intergradation was common and widespread in this complex. The bluestems have been recognized as distinct species (Hitchcock and Chase, 1951; Fernald, 1970) and as varieties in a single I Received for publication 19 June 1985; revision accepted 8 August 1985. This research was supported by the Center for Rural Affairs, Lower Loup Natural Resources District and the University of Nebraska (A. W. Sampson Fellowship and J. A. Lee Award). I thank J. Ehleringer for the leaf spectral analyses, M. Bolick for her help in pollen analysis, D. Sutherland for his suggestions throughout the study, D. Demer for providing a research location and precipitation data, and M. Barnes for her field assistance. Robert Kaul, A. T. Harrison, K. Keeler, J. McClendon, M. Potvin and D. Sutherland all provided helpful comments on the manuscript. 2 Present address: Dept. of Range Science, Utah State Univ., Logan, UT 84322-5230. species (Gould, 1975). Artificial hybridization experiments by Peters and Newell (1961) and Newell and Peters (1961) showed that the bluestems were completely interfertile and their work eventually led to the production of the synthetic hybrid, "Champ bluestem" (Newell, 1968b) which is commercially marketed. Despite the morphological intergradation in this complex, there is a very clear divergence in habitat utilization: big bluestem, a dominant of tallgrass prairie (Weaver, 1954), is broadly distributed in moist, prairie sites in North America (Hitchcock and Chase, 1951), while sand bluestem is restricted to sandy soils in the Great Plains (Hitchcock and Chase, 1951) although it has been reported to occur on sandy soils as far east as Illinois (Birkenholz, Anderson and Katz, 1980). Both bluestems are found in Nebraska, but sand bluestem is restricted to coarse textured soils in the Sandhills region, while big bluestem is confined to mesic Sandhills subirrigated meadows or to finer textured soils in higher precipitation areas outside the Sandhills (Pool, 1914; Tolstead, 1942; Satterwhite, 1970; Kestner, 1973). Both Satterwhite (1970) and Kestner (1973) found a high correlation between bluestem morphology and