Jaramillo, Victor J. 1986. The Effect of Grazing History on Competition on the Response of Bouteloua gracilis to Defoliation. M.S. thesis. Colorado State University. Ft. Collins, CO.
The goal of this study was to compare, under controlled conditions, nitrogen uptake and allocation in two populations of Bouteloua gracilis (H.B.K.) Griffithes with contrasting grazing histories in response to defoliation and intraspecific competition. Plants were collected in a heavily grazed prairie dog (Cynomys ludovicianus) colony (ON-town plants).
OFF-town plants were significantly taller and had greateer biomass production than ON-town plants, supporting existing evidence that heavy grazing pressure promotes morphological differentiation in plant populations, which is apparently genetically based. Furthermore, OFF-twon plants had greater nitrogen yield per plant and per plant part than plants from the prairie dog colony. Defoliation increased nitrogen uptake per unit of root biomass, total leaf nitrogen content, and decreased total biomass production in both population. Such similar response to defoliation in OFF- and ON-town plants suggests that the short and more prostrate ON-town plants may have evolved as a "grazing avoidance" rather than as a "grazing tolerance" response to heavy grazing pressure. The similar shoot-nitrogen concentrations found in both populations in this laboratory study suggests that the differences observed in the field may be due to either the indirect effect of herbivores, through urine an dung deposition, or to higher mineralization rates in the prairie dog colonies.
ON-town plants allocated a greater percentage of biomass and nitrogen to roots, while OFF-town plants allocated a greater percentage of resources to sheaths and reproductive structures. A greater allocation of resources to roots may allow the ON-town population to cope with a seemingly droughtier environment and with the heavy grazing pressure exerted by both above- and belowground herbivores on the prairie dog colonly. Greater allocation to sheaths and reproductive structures may allow OFF-town plants to more successfully face the predominately light-competitive environement in the sites off the prairie dog town.
Both OFF- and On-town populations increased their relative biomass and nitrogen allocation to blades in response to defoliation. This plasticity in the allocation patterns allows them to more rapidly restore photosynthetic area and photosynthetic capacity.
Competition by neighbors of the same population drastically reduced biomass production and nitrogen yield of individuals in both populations. The effect of competition did not depend on the level of defoliation or on the population.