IMPACTS OF GLOBAL CHANGES AND BIOLOGICAL CONTROL OF INVASIVE WEEDS ON WESTERN RANGELANDS
Location: Grassland, Soil and Water Research Laboratory
Title: Dominant species constrain effects of species diversity on temporal variability in biomass production of tallgrass prairie
Submitted to: Oikos
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 10, 2007
Publication Date: October 17, 2007
Citation: Polley, H.W., Wilsey, B.J., Derner, J.D. 2007. Dominant species constrain effects of species diversity on temporal variability in biomass production of tallgrass prairie. Oikos. 116:2044-2052.
Interpretive Summary: Human activities are reducing plant diversity on many grasslands by reducing the number of plant species present per unit of area and by creating greater inequity in abundances of species that remain. The reliability of grassland biomass production from one year to the next is thought to depend partly on species diversity, implying that biomass will vary more among years in severely human-impacted than in more-diverse grasslands. An alternative view is that variability in biomass depends on traits of dominant species rather than on species diversity alone. We compared inter-annual variability of aboveground biomass in response to natural variation in rainfall between restored and remnant tallgrass prairies at two locations in central Texas, USA. Species diversity is greater in remnant than restored prairies, so by comparing remnants to restorations we sought to determine how diversity affected inter-annual variation in grassland biomass. Variability in biomass was reduced by 34-55% by the greater diversity in remnant than restored prairies when calculated assuming each species responds similarly to year-to-year differences in rainfall. However, actual variability in biomass over 5 years did not differ between prairie types because biomass of the dominant species in restored prairies (the perennial grass, little bluestem) varied less than did biomass of other dominant and sub-dominant species in remnant prairies. Our results indicate that the response of grassland biomass to natural variation in rainfall depends more on characteristics of dominant species than on species diversity per se.
Species diversity is thought to stabilize functioning of plant communities, although diversity-stability studies have focused on species richness to the neglect of the second component of diversity, species evenness (equitability with which biomass or abundances are distributed among species). An alternative view is that traits of dominant species constrain effects of diversity on stability. We compared inter-annual variability (inverse of stability) of aboveground biomass in paired restored and remnant tallgrass prairies at two locations in central Texas, USA to test the hypothesis that greater richness and evenness in remnant than restored prairies would reduce variability in aboveground biomass in response to natural variation in rainfall and weather. Remnant and restored prairies at each location are dominated by a common group of perennial grasses. By comparing remnants and restorations, therefore, we sought to determine how diversity affected temporal variability in biomass of grasslands with common dominant grasses. Variability was measured as the coefficient of variation among years (square root of variance/mean), where variance in community biomass equals the sum of variances of individual plant species plus the summed covariances between species pairs. The sum of species variances in biomass was reduced by 34-55% when calculated using the more equitable distribution of biomass observed in remnant than restored prairies, but was reduced by <4% by the greater richness observed in remnant prairies. Still, the CV of community biomass during spring and CV of annual biomass production did not differ consistently between prairie types. Neither the sum of species covariances nor total community biomass differed between prairie types. Rather, biomass of restored prairies varied relatively little in response to natural variation in rainfall because biomass of the dominant species in restored prairies (Schizachyrium scoparium) varied less than did biomass of other dominant and sub-dominant species in more-diverse remnant prairies. In these grasslands, biomass response to natural variation in precipitation depended more on characteristics of dominant species than on differences in diversity.