Establishment of Native Grass Seedlings along a Topographic/Moisture Gradient in the Nebraska Sandhills

-This study examines the effect of differential survival of seedlings on the distribution of native grasses along topographic gradients in the Nebraska Sandhills. In 1981, seeds of three ridge species (Andropogon hallii, Sorghastrum nutans, Panicum virgatum) were planted into rototilled, irrigated, rototilled and irrigated, or untreated soils on three sites: one ridge site (ridge) and two valley sites (valley and lowland). The three sites represented a gradient of topography and soil moisture-holding capacity. For all species, seedling survival was greatest (86%) in the rototilled and irrigated treatment on the lowland site. No seedlings survived in untreated plots along the gradient. In 1982 and 1983, seeds of a ridge species (Andropogon hallii) and a valley species (Bouteloua gracilis) were planted in replicated ridge and valley plots. Seeds were planted into either untreated plots, or plots where plants received weekly clipping treatments around seedlings. Although clipping significantly increased survival of both species in both sites, it had a significantly greater effect in valley sites. In untreated plots, seedling survival was greatest (20%) in ridge sites in a year of average precipitation (1982) and in valley sites (49%) in a year that was exceptionally wet (1983). In 1983, soil moisture explained 98% of the variation in the survival of A. hallii seedlings and 82% in B. gracilis seedlings. Competition from existing vegetation severely limited the probability of seedling establishment. The distribution pattern of ridge species likely results from differential survival of juveniles and adults along the gradient, rather than the ability of seedlings to have higher establishment success on ridge sites. Significant establishment of the valley species (Bouteloua gracilis) occurred only during an exceptionally wet year when the pattern of survival of seedlings reflected the distribution pattern of adult plants. Establishment of prairie grass seedlings appears to depend on the availability of soil moisture, which may only be adequate in relatively large disturbance areas (> 1 m2), or in years of higher than normal precipitation.