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Accueil du site → Doctorat → États-Unis → 1989 → Small-scale heterogeneity in a semiarid grassland : The role of urine deposition by herbivores

Colorado State University (1989)

Small-scale heterogeneity in a semiarid grassland : The role of urine deposition by herbivores

Jaramillo, Victor Joaquin

Titre : Small-scale heterogeneity in a semiarid grassland : The role of urine deposition by herbivores

Auteur : Jaramillo, Victor Joaquin

Université de soutenance : Colorado State University

Grade : Doctor of Philosophy (PhD) 1989

I examined morphological and ecophysiological responses of two grass species with different growth forms and photosynthetic pathways to simulated cattle urine deposition. The species were Agropyron smithii Rydb. a C$\sb3$, spreading, rhizomatous grass, and Bouteloua gracilis (H.B.K.) Griffiths a C$\sb4$, cespitose species. Tiller density of both species increased in simulated urine patches. However B. gracilis responded in the first growing season while A. smithii did not respond until the second year after application. Both species had increased tiller densities and aboveground N concentrations in a ring around, and outside, the area wetted by urine. Percent leaf N retranslocated decreased and standing dead litter N concentration increased in both species in response to urine deposition. Aboveground biomass and N-yield and tiller height of A. smithii increased in urine patches. Leaf production, however, 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. I also studied the effects of artificial urine on the frequency and intensity of grazing of A. smithii tillers by cattle under moderate and heavy stocking rates in a continuous grazing system. Up to July, a greater percentage of tillers was grazed in urine patches than in controls, although this difference disappeared later in the growing season. The odds of tillers being grazed was substantially greater when growing in urine patches than in control patches. Tillers were grazed to a lower height in urine patches under both grazing pressures. Urine deposition increased aboveground N concentration of tillers and my estimates indicated that steers removed 3 to 4 times more N per unit area from urine patches than from controls. Under heavy stocking tillers were defoliated to a lower height, had fewer leaves and shorter blades than under moderate stocking. Grazed tiller height was more variable under moderate than under heavy grazing. Results from both studies indicated that the duration of urine effects in semiarid grasslands may be longer than has been reported for other pastures and grassland ecosystems. The changes in structural (e.g., tiller density and height) and functional (e.g., nutrient uptake and retranslocation) properties of the vegetation, and in diet selection by herbivores suggested urine deposition promotes heterogeneity in this grassland.

Mots clés : Ecology, Biological sciences

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