Document: VAN-3-38-24

Are changes in belowground competition the cause of alteration in composition of semi-arid grasslands?

VAN AUKEN, O.W.*

The University of Texas at San Antonio, San Antonio, TX 78249-0662 USA ¹

Abstract:
The composition and structure of the semiarid grasslands of southwestern North America have changed over the past 150 years. Brush encroachment by native species has occurred over large areas. Mesquite, is one of the major woody species that has increased in density in these semiarid grasslands. Factorial field competition experiments in grasslands in central Texas suggest that the woody plants are not the cause of the changes in these semiarid grasslands, but they are the result of changes in other variables. Mesquite seedlings grown with established grasses are suppressed but some survive for two years. Clipping or simulated herbivory of the aboveground grass biomass reduced their belowground drymass. All woody seedlings in the grass clipping treatment survived and mesquite dry mass was 4-6 times higher compared to the no herbivory treatment. When competition was partitioned between above- and belowground, using netting and root excluders to reduce the grass interaction with the woody plants, belowground competition was more important in suppressing woody plant growth. Total mesquite dry mass was significantly, inversely related to grass root dry mass. Although grass plants reduce the growth of mesquite, apparently other factors such as fire are necessary to completely suppress the growth of mesquite in these grasslands.

Keywords: shrub encroachment, competition, herbivory, fuel reduction

This abstract is being presented at: 8:15 AM in session:
Oral Session #72: Plant Competition.