Document: SHA-3-42-30

Grass-shrub competition, water and grubs: How important is root herbivory on the nether world of competition?

BRUMBAUGH, M.S. ¹ and R.B.JACKSON ²

The University of Texas at Austin, Austin, TX 78712 USA ¹
Duke University, Durham, NC USA ²

Abstract:
In semi-arid grasslands root herbivory may be an important factor in plant competition by altering the ability of plants to take up water. Results from a previous experiment showed that a high density of June beetle (Phyllophaga spp.) larvae (4 per plot) significantly decreased plant survival and growth for both mesquite (Prosopis glandulosa) and little bluestem (Schizachyrium scoparium) and that root herbivory significantly altered the competitive interaction between grass and shrub species. Here we examine the effect of root herbivory under a lower range of larval densities on water availability and belowground competition between mesquite and little bluestem. We conducted a 3 by 3 factorial garden transplant experiment in Austin, Texas, growing individual mesquite seedlings and little bluestem grasses alone and together in tubes (15-cm diam. x 60-cm deep) placed in the ground. We introduced June beetle larvae into tubes at densities of 0, 1 and 2 per tube. The low level (1 larva) represents the typical density in the field. In addition to plant growth, we measured volumetric soil moisture and plant water potential biweekly August-October. Moderate and low larval density had no effect on mesquite and little bluestem survival and total biomass. However, root herbivory did produce a 40% reduction in mesquite fine root (<2 mm diam.) biomass. There was also no difference in volumetric soil moisture or in little bluestem water potential for the three larval densities. In comparison, competition with little bluestem reduced mesquite growth by 70% and volumetric soil moisture was 25% higher in plots with mesquite alone. Plots with little bluestem alone and with mesquite were not statistically different. These results, along with those of our previous study, suggest that the importance of root herbivory on grass-shrub interactions is dependent on larval density; under low to moderate larval densities competition with grass limits mesquite seedling performance, whereas under high larval densities root herbivory plays the greater limiting role.

Keywords: belowground competition, root herbivory, water, Schizachyrium scoparium, little bluestem, Prosopis glandulosa, mesquite, Phyllophaga, June beetle.

This abstract is being presented at: 3:30 PM in session:
ROOT PROCESSES