Effects of elevated CO and phosphorus addition on productivity and community composition of intact monoliths from calcareous grassland.

We investigated the effects of elevated CO (600 μl l vs 350 μl l) and phosphorus supply (1 g P m year vs unfertilized) on intact monoliths from species-rich calcareous grassland in a greenhouse. Aboveground community dry mass remained almost unaffected by elevated CO in the first year (+6%, n.s.), but was significantly stimulated by CO enrichment in year two (+26%, P<0.01). Among functional groups, only graminoids contributed significantly to this increase. The effect of phosphorus alone on community biomass was small in both years and marginally significant only when analyzed with MANOVA (+6% in year one, +9% in year two, 0.1 ≥P > 0.05). Belowground biomass and stubble after two seasons were not different in elevated CO and when P was added. The small initial increase in aboveground community biomass under elevated CO is explained by the fact that some species, in particular Carex flacca, responded very positively right from the beginning, while others, especially the dominant Bromus erectus, responded negatively to CO enrichment. Shifts in community composition towards more responsive species explain the much larger CO response in the second year. These shifts, i.e., a decline in xerophytic elements (B. erectus) and an increase in mesophytic grasses and legumes occurred independently of treatments in all monoliths but were accelerated significantly by elevated CO. The difference in average biomass production at elevated compared to ambient CO was higher when P was supplied (at the community level the CO response was enhanced from 20% to 33% when P was added, in graminoids from 17% to 27%, in legumes from 4% to 60%, and in C. flacca from 120% to 298% by year two). Based on observations in this and similar studies, we suggest that interactions between CO concentration, species presence, and nutrient availability will govern community responses to elevated CO.