Small-scale heterogeneity in a semi-arid North American grassland. I. Tillering, N uptake and retranslocation in simulated urine patches.
In trials on native semiarid grassland at Cheyenne, Wyoming in 1986-87, the response of Agropyron smithii [Elymus smithii] and Bouteloua gracilis to simulated cattle urine deposition was studied. Simulated urine increased tiller density of both species, with B. gracilis increasing in the first growing season while A. smithii was not increased until the 2nd year. Both species had increased tiller densities and aboveground N concentrations in a ring outside the area wetted by urine. The percentage of retranslocated leaf N decreased and standing dead litter N concentration increased in both species after urine deposition. Aboveground biomass, N yield, and tiller height of A. smithii increased in urine patches. Leaf production was not affected by treatment. Both the increase in the live:dead biomass ratio and the higher number of live leaves at the end of the first growing season suggested urine deposition delayed senescence of A. smithii. The implications of plant growth form in the spread of urine, the potential duration of urine effects in semiarid grasslands, and the role of herbivore urine deposition in promoting grassland structural and functional heterogeneity are discussed.