Abstract - Modification of Individual Plant and Community Water and Nitrogen Relations by Grassland Herbivores

Day, Thomas A. 1988. Modification of Individual Plant and Community Water and Nitrogen Relations by Grassland Herbivores. PhD Dissertation. Colorado State University, Ft. Collins, CO. 134 p.


Changes in plant water and nitrogen (N) relations following urine deposition and intensive herbivore grazing were examined in a Northern mixed-grass prairie in South Dakota. Total aboveground biomass and root mass were higher on large herbivore urine patches than in the surrounding community. Although Schizachyrium scoparium (C4) dominated the surrounding community, Poa pratensis (C3) dominated urine patches. Following urine deposition, aboveground N concentrations of Poa and Schizachyrium were higher on urine patches relative to conspecifics, and N concentrations of the former species increased more than the later. Poa on urine patches had higher leaf conductances and lower water potentials that Poa off patches and Schizachyrium, regardless of site. Positive turgor and higher leaf conductances of Poa on urine patches were usually maintained by increased tissue elasticity. Under severe water stress turgor was maintained by decreased tissue elasticity and osmotic adjustment. Large increases in Poa production on urine patches relative to Schizachyrium appeared to result from differential effects of increased N availability on photosynthetic pathway and drought tolerance. Although urine patches covered <2 % of the study area, higher herbivore feeding preference for these patches led to patches contributing >15 % of the N consumed by aboveground herbivores. Urine patches were probably even more important forage and N sources early and late in the growing season when surrounding vegegation was dormant. The effects of intensive grazing on individual plant and community water status were investigated by comparing a heavily grazed black-tailed prairie dog Cynomys ludovicianus colony with an adjacent lightly grazed uncolonized site. Agropyron smithii and Bouteloua gracilis leaf conductances and water potentials were higher on colony, especially late in the growing season and day. Afternoon community evapotranspiration rates were generally higher off colony early in the season but higher on colony in July and August. Changes in canopy microclimate led to higher evaporative demand on plants on colony throughout the season. The major impact of intensive grazing on community water status appeared to be a reduction in transpiring leaf area which controlled community water loss through soil moisture availability.

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