EFFECTS OF ELEVATED CO2 AND PHOSPHORUS ADDITION ON PRODUCTIVITY AND COMMUNITY COMPOSITION OF INTACT MONOLITHS FROM CALCAREOUS GRASSLAND

We investigated the effects of elevated CO2 (600 mu l 1(-1) vs 350 mu l 1(-1)) and phosphorus supply (1 g P m(-2) year(-1) vs unfertilized) on intact monoliths from species-rich calcareous grassland in a greenh ouse. Aboveground community dry mass remained almost unaffected by ele vated CO2 in the first year(+ 6%, n.s.), but was significantly stimula ted by CO2 enrichment in year two (+26%. P<0.01). Among functional gro ups, only graminoids contributed significantly to this increase. The e ffect of phosphorus alone on community biomass was small in both years and marginally significant only when analyzed with MANOVA (+6% in yea r one, +9% in year two, 0.1 greater than or equal to P > 0.05). Belowg round biomass and stubble after two seasons were not different in elev ated CO2 and when P was added. The small initial increase in abovegrou nd community biomass under elevated CO2 is explained by the fact that some species, in particular Carex flacca, responded very positively ri ght from the beginning, while others, especially the dominant Bromus e rectus, responded negatively to CO2 enrichment. Shifts in community co mposition towards more responsive species explain the much larger CO2 response in the second year. These shifts, i.e., a decline in xerophyt ic elements (B. erectus) and an increase in mesophytic grasses and leg umes occurred independently of treatments in all monoliths but were ac celerated significantly by elevated CO2. The difference in average bio mass production at elevated compared to ambient CO2 was higher when P was supplied (at the community level the CO2 response was enhanced fro m 20% to 33% when P was added, in graminoids from 17% to 27%, in legum es from 4% to 60%? and in C. flacca from 120% to 298% by year two). Ba sed on observations in this and similar studies, we suggest that inter actions between CO2 concentration, species presence, and nutrient avai lability will govern community responses to elevated CO2.