The influence of elevated CO₂ on community structure, biomass and carbon balance of mediterranean old‐fleld microcosms

Abstract

We studied the effects of a doubling of atmospheric CO2 concentration on intact monoliths of Mediterranean grassland in growth chambers where climatic field conditions were simulated. During the six month growing season, changes in community structure were monitored by quantifying species richness and cover. The CO2 exchange of microcosms was measured continuously and the resulting quantity and quality of biomass were evaluated. Species richness and cover did not respond to elevated CO2. After one month of treatment, CO2 exchange measured during the day did not differ between CO2 levels but the night respiration was two‐fold higher under elevated CO2. Stimulations of both day and night CO2 flux by short‐term CO2 enrichment were recorded several times during the growing season. These results suggest that despite some downward adjustment of photosynthesis, net canopy photosynthesis was stimulated by elevated CO2, but this stimulation was compensated for by an increased respiration. The 20% stimulation of final phytomass under elevated CO2 was not significant: it resulted from unchanged live plant matter but a significant, 100% increase in litter accumulation. These results suggest that in low‐productivity Mediterranean herbaceous systems, the greatest effect of CO2 is not on the storage of carbon in biomass but on the turnover of the carbon in the plants.

Effects of elevated CO 2 and phosphorus addition on productivity and community composition of intact monoliths from calcareous grassland

“…Most studies with undisturbed natural grassland communities under elevated CO 2 have yielded only marginally and frequently not signi®cant increases in community-level biomass (Owensby et al 1993;Navas et al 1995;Wolfenden and Diggle 1995). We observed a small increase (+6%) in aboveground community biomass in the ®rst, and a much larger increase (+26% ) in the second year of our experiment.…”

“…Leadley and StoÈ cklin (1996), however, observed signi®cant genetic variation in CO 2 response within this species, including positive-and negative-responding genotypes. In a study by Navas et al (1995) with monoliths from dry Mediterranean grasslands, growth of B. erectus was stimulated under elevated CO 2 . Also during the third year of exposure to elevated CO 2 at the site of origin of our monoliths, a moderately positive e ect of CO 2 enrichment became apparent in this species, exclusively due to enhanced tillering (Leadley et al unpublished).…”

“…The few studies with vegetation in undisturbed lowfertility soils have shown that the direct responses of community biomass to elevated CO 2 are usually much smaller than in agro-ecosystems, in some cases even zero (Owensby et al 1993;Jackson et al 1994;Navas et al 1995;Wolfenden and Diggle 1995;Leadley and KoÈ rner 1996;SchaÈ ppi and KoÈ rner 1996). Results from studies with species-rich model ecosystems indicate that community responses to elevated CO 2 may be low because highly positive responses of some species are dampened by small and sometimes negative responses of other species (Leadley and StoÈ cklin 1996;Roy et al 1996;StoÈ cklin et al 1997).…”

Nutrient relations in calcareous grassland under elevated CO 2

“…However, all these experiments were carried out under fertile, horticultural or agronomic conditions not representative for natural plant communities where other nutrients such as P may limit legume growth. In contrast Navas et al (1995) working with unfertilized intact mediterranean grassland sods found no CO 2 response of legumes.…”

Root production and turnover and carbon budgets of two contrasting grasslands under ambient and elevated atmospheric carbon dioxide concentrations

“…The data we present here add to a growing body of literature that suggest that, when whole communities are examined, rather than individual plants growing in pots in ideal conditions. increases in below-ground production are more likely than effects above ground (Arnone & Korner, 1995, Dhillioii & Roy, 1996Drake et al, 1996), This pattern is not universal-Studies in which no or little response to CO,^ are reported (Navas et al, 1995) are perhaps less often published than those which give clear-cut results, as can be seen by studying the report of a GCTE meeting by Canadell et al (1996), It is difficult to generalize from these results, but when taken with the meta-analysis by Rogers et al (1996) that demonstrates a w^idely-observed tendency for root grow^th to be stimulated under elevated CO^, it seems likely that root bionnass will accumulate in many ecosystems. Even so.…”