Adaptation to Water Stress in the Big Bluestem-Sand Bluestem Complex

Reciprocal transplant experiments and comparative water relations studies were con- ducted to determine the mechanisms controlling habitat assortment in the big bluestem (Andropogon gerardii)-sand bluestem (Andropogon hallii) complex along a local dune/meadow-soil moisture gra- dient in the Nebraska Sandhills. For bluestems growing in their natural habitats, midday leaf con- ductances and leaf water potentials during 1982 and 1983 were consistently lower in sand bluestem on dry, upland sand dunes, than in big bluestem in adjacent, naturally subirrigated, wet meadows. Hybrids at dune/meadow transition zones where soil moisture levels were intermediate between dunes and meadows showed intermediate leaf conductances, but midday leaf water potentials were consis- tently lower than other bluestem types in other habitats. In common gardens and under similar drought conditions, sand bluestem transplants maintained significantly higher midday leaf water potentials than adjacent big bluestem and hybrid transplants. A greater drought resistance in sand bluestem appeared to be related to a greater ability to control transpirational losses under drought conditions, such that higher leaf water potentials were maintained for longer periods of time. In contrast, under drought conditions, leaves of big bluestem continued to lose water, even at water potentials of -4 MPa. Data from the field and a laboratory drought experiment indicated that the bluestems did not differ in osmoregulation capabilities or stomatal sensitivity to vapor pressure deficits; there were major differences in leaf rolling characteristics, which together with variation in epicuticular wax accumulation may be important in controlling water loss under drought conditions. These physiological data cor- related well with results from seedling and adult rhizome transplant experiments, which showed rapid and massive mortality of big bluestem and hybrids following brief dry periods on dune sites, and suggest that variation in drought resistance within the bluestem complex could be an important mechanism of segregating bluestem types along local dune-meadow soil moisture gradients.