Water relations and growth of three grasses during wet and drought years in a tallgrass prairie

The water relations and growth of three tallgrass prairie species Panicum virgatum, Andropogon gerardii and A. scoparius were examined in irrigated and unwatered prairie in eastern Kansas (USA). Measurements of the osmotic potential at full turgor, ψ _π¹⁰⁰ , at zero turgor, ψ⁰, and growth of vegetative and reproductive tillers were made in a year with above-normal precipitation and a drought year to evaluate: 1) the ability of these grasses to osmotically adjust in response to water stress and 2) the effect of drought or supplemental water on growth of these species. Although these grasses adjusted osmotically even in the wet year, the degree of adjustment of ψ _π¹⁰⁰ and ψ⁰ in the drought year was relatively large (0.60-0.78 MPa and 0.88-1.34 MPa, respectively) compared to reports for other species. Seasonal minimum values of ψ _π¹⁰⁰ and ψ⁰ for these grasses in the drought year were lower than in most mesic species and seasonal fluctuations in ψ _π¹⁰⁰ and ψ⁰ were greater than reported for most mesic or xeric species. The relatively frequent occurrence of drought in sub-humid tallgrass prairies may partially explain the greater than expected magnitude of osmotic adjustment in these grasses.Irrigation in the wet year increased reproductive biomass in the mesic grass P. virgatum, but had no effect on A. gerardii or the more xeric grass A. scoparius. However, irrigation in the drought year increased maximum shoot biomass in all three grasses significantly with the largest increase in P. virgatum. Reproduction in P. virgatum was also increased more by irrigation in the drought year compared to the other grasses. Irrigation did not increase season's end production of A. gerardii in the wet year, but in the drought year production was 28% greater in irrigated than unwatered prairie. The combination of these water relations and growth responses of the three grasses to wetter than normal and drought years supports their reported distribution along a moisture gradient in tallgrass prairies.