Submitted to: Oikos
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/18/2005
Publication Date: 8/4/2006
Citation: Polley, H.W., Wilsey, B.J., Derner, J.D., Johnson, H.B., Sanabria, J. 2006. Early-successional plants regulate grassland productivity and species composition: a removal experiment. Oikos. 113:287-295. Interpretive Summary: Vegetation in many grasslands has been altered by heavy grazing and other disturbances and by plant invasions. Yet, we understand little of how changes in the composition of vegetation affect plant productivity and other of the services that grasslands provide. We removed two groups of annual plants (annual broomweed, annual grasses) from a heavily-grazed grassland in central Texas to determine whether these temporary residents of disturbed sites influence the delivery of ecological services (plant productivity for grazers, regulation of the nitrogen cycle) on this grassland either directly by affecting process rates or indirectly by regulating the composition of perennial plant species. Biomass production and nitrogen accumulation by vegetation declined in the year following the removal of annual grasses in approximate proportion to the contribution of these plants to biomass production and nitrogen accumulation in unmanipulated grassland. The decline in production was short-lived, however, because biomass production and N accumulation by other species increased during the second year to compensate for the loss of annual plants. Compensation was greatest among perennial species of broad-leaf herbaceaous plants (forbs). Because forbs differ from the perennial grasses that usually dominant in this grassland in functioning and in value as a forage for cattle, annual plants may influence the services derived from this grassland more by regulating the composition of vegetation than by directly affecting productivity or nitrogen accumulation by plants.
Technical Abstract: J.P. Grime's '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 in 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 nitrogen) and composition. Removals had little impact on N dynamics 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 nitrogen (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 in the longer term influence grassland functioning more by regulating the composition of vegetation than by directly affecting process rates.