Professor University of Minnesota; Institute for Global Change Biology, University of Michigan, Minnesota, United States
Abstract: Global change may influence ecosystem function by impacting distinct functional groups by different magnitude and/or by changing functional group composition of plant communities. Additionally, responses of ecosystem function may be affected by species interactions that occur in mixtures but not monocultures and can vary temporally. But relatively little is known about the role of functional groups and their interactions in influencing long-term ecosystem response to global change factors due to the limited number of relevant decadal studies. To address this gap, we explored temporal variation of total aboveground biomass for four functional groups (C3 grass, C4 grass, legumes and non N-fixing forbs) in response to Diversity, N and CO2 manipulation, by leveraging data from a 24-year long grassland experiment at Cedar Creek, Minnesota. The BioCON experiment consists of 359 2 m X 2 m plots that spans two soil N availability treatments, two CO2 treatments and four grassland plant diversity levels (1, 4, 9 and 16 species). Species-specific biomass, measured for each plot every year since 1998, was used to study temporal variation in functional groups. We used structural equation modeling (SEM) to better understand the effects of N and CO2 on community-scale aboveground biomass via changing species richness and functional group composition.
Functional groups differed in their responses to N and CO2 treatments and these responses were often amplified by species interactions, which occurred in mixtures but not monocultures. Furthermore, plant community composition changed over time in the mixture plots, even under ambient conditions in the control plots. Interestingly, we did not find evidence that N and CO2 consistently influenced total aboveground biomass by changing functional group composition, except for the impact of N addition on legume proportions. N enrichment reduced legume proportions over all years, but it indirectly reduced community-scale biomass in the early years of the experiment and increased community-scale biomass in later years due to the decreasing effect of legume proportions on aboveground biomass. Finally, functional group proportions also influenced total aboveground biomass independently of the experimental manipulations: C3 proportions were negatively associated and C4 proportions had temporally variable relationships with biomass. Overall our results highlight the varying sensitivity of functional group responses to N and CO2 over time and the community-mediated effects of global changes on plant biomass production.