Species distribution and community organization in a Nebraska Sandhills mixed prairie as influenced by plant/soil-water relationships.

Plant and soil water relationships in a typical nebraska Sandhills prairie were examined to 1) explain the observed distribution patterns of several dominant grasses along a topographic gradient, and 2) show how spatial and temporal variations in soil moisture are critical to community organization on a sandy substrate. An experimental transect encompassing the major community and soil types along a steep, west-facing vegetated dune was established. Maximum available water was shown to be significantly higher in the fine textured surface soils of the lowland sites than the coarse textured sands of the dune sites. Seasonal (1979) patterns of available soil moisture of the sampling sites on the transect showed that in the upper elevation dune sands, moisture was available in the entire profile with surface depletions not occurring until mid to late summer. In contrast, moisture in the surface 60-80 cm in the fine textured lowland soils was exhausted by early to mid-summer with the entire profile nearly dry by late summer. Deep-rooted, C4 species, Andropogon hallii and Calamovilfa longifolia which are common on upper, coarser sandy soils showed significantly greater water stress on fine textured soils than on dune sands. C3, shallowrooted species, Agropyron smithii, Stipa comata, and Koeleria cristata always experienced lower mid-day and predawn leaf water potentials than the C4 species. The C3 species, with the exception of Koeleria are most abundant on finer textured soils that provide substantial moisture during their peak activity in the spring. It appears that the C4 species show more conservative water use patterns than the C3 species as significantly lower leaf conductances in the C4's were measured when soil water was abundant. The C3 species appear to be opportunistic with available water and rapidly deplete surface soil moisture as a result of high transpiration rates. These data suggest that the temporal and spatial distribution of available water along this gradient controls species distribution according to rooting morphology, photosynthetic physiology, and water deficits, incurred by transpirational losses. Competitive interactions between species that utilize soil moisture differently may be an important factor in community organization.