PARENT SESSION
Contributed Oral Session 33: Grassland and Shrubland Communities
Monday, August 8, 1:30 PM - 5:00 PM, Meeting Room 522 A, Level 5, Palais des congrès de Montréal
Early-successional plants regulate grassland productivity and species composition.
Polley, Wayne*,1, Wilsey, Brian2, Derner, Justin3, Johnson, Hyrum1, Sanabria, Joaquin4, 1 USDA/Agricultural Research Service, Temple, Texas, USA2 Iowa State University, Ames, Iowa, USA3 USDA/Agricultural Research Service, Cheyenne, Wyoming, USDA4 Texas Agricultural Experiment Station, Temple, Texas, USA
ABSTRACT- The mass-ratio hypothesis holds that ecosystem processes depend in the short term on functional properties of dominant plant species and in the longer term on how resident species influence the recruitment of potential dominants. The latter of these effects may be especially important among early-successional species in disturbed ecosystems, but experimental tests are few. We removed two groups of early-successional species, an annual forb Gutierrezia dracunculoides (common or annual broomweed) and annual species (mostly grasses) that complete growth early during the growing season (early-season species), from a heavily-grazed grassland in central Texas, USA that was dominated by a C4 perennial grass. We sought to determine effects of these annuals on grassland functioning (productivity, water balance, and soil and plant nitrogen (N) content) and composition. Removals had little impact on N retention in the soil/plant system during the two years of this study, but removing early-season annuals increased the amount of water present in the 30 to 120 cm increment of the soil profile during the early part of each growing season. Immediate effects of removing annuals on productivity of the grassland were consistent with predictions from the mass-ratio hypothesis. Biomass production and N accumulation by vegetation declined following the removal of early-season annuals in approximate proportion to the contribution of annuals to aboveground biomass and N. Longer-term effects of annuals on grassland composition were evident in a dramatic increase in the biomass of perennial forbs after annuals were removed. Because perennial forbs differ from the dominant grass in this grassland in traits that influence ecosystem functioning, early-successional annuals may influence grassland functioning more by regulating the composition of vegetation than by directly affecting process rates.
Key words: annual plant species, removal experiment, nitrogen retention