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 eastern 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.