Interacting effects of elevated carbon dioxide, nitrate, and phosphate on plant growth and spectral reflectance of a California grassland

In the Jasper Ridge Global Change Experiment, which studies long term grassland responses to multifactor global change treatments, plant community responses have interacted with interannual variability to produce changes in species abundances, functional diversity, and net primary production that have varied in magnitude and even direction across years.  During years eight and nine of the field experiment, we conducted a parallel experiment to evaluate the role of species effects and limitation by phosphorus in shaping responses to simulated changes in nitrogen and CO₂.  We used monocultures and communities planted in soil monoliths that were extracted from grassland adjacent to the field site and examined their responses to factorial additions of 6g nitrogen (as calcium nitrate)/m², 5.25g phosphorus (as superphosphate)/m² and elevated CO₂ (ca. 750 ppm).  The community consisted of annual grasses, an annual legume, an annual forb, a perennial grass, and a summer-active annual forb.  Early in the growing season we examined aboveground biomass responses nondestructively using the spectral reflectance of individual pots to calculate the normalized difference vegetation index (NDVI), and at peak standing biomass, we harvested and dried all aboveground plant material.  NDVI and biomass showed similar responses to the factorial treatments.  Each species responded to the N, P, and N+P treatments differently in monoculture and community, and within communities, each species had a unique response pattern.  Although two species showed positive interactions between CO₂ and either N or P when grown in monoculture, the same species were unresponsive to elevated CO₂ when grown in communities, as was the community as a whole.