Species-specific responses of plant communities to altered carbon and nutrient availability

In a field microcosm experiment, species-specific responses of aboveground biomass of two California annual grassland communities to elevated CO2 and nutrient availability were investigated. One community grows on shallow, nutrient-poor serpentine-derived soil whereas the other occurs on deeper, modestly fertile sandstone/greenstone-derived substrate. In most species, CO2 effects did not appear until late in the growing season, probably because the elevated CO2 increased water-use-efficiency easing, the onset of the summer drought. Responses of aboveground biomass to elevated CO2 differed depending on nutrient availability. Similarly, biomass responses to nutrient treatments differed depending on the CO2 status. For the majority of the species, production increased most under elevated CO2 with added nutrients (N,P,K, and micro nutrients). Some species were losers under conditions that increased overall community production, including Bromus hordeaceus in the serpentine community (negative biomass response under elevated CO2) and Lotus wrangelianus in both communities (negative biomass response with added nitrogen). Treatment and competitive effects on species-specific biomass varied in both magnitude and direction, especially in the serpentine community, significantly affecting community structure. Individual resource environments are likely to be affected by neighbouring plants, and these competitive interactions complicate predictions of species' responses to elevated CO2.