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Monoliths of a fertile, N limited, C3 grassland community were subjected (or not) to an atmospheric CO2 enrichment (600 µmol mol‐‐1) using a Mini‐FACE system, from August 1998 to June 2001 and were subjected to two contrasting cutting frequencies (3 and 6 cuts per year). We report here the effects of the CO2 and cutting frequency factors on the plant community structure and its diversity. Species‐specific responses to elevated CO2 and cutting frequency were observed, which resulted in significant changes in the botanical composition of the grassland monoliths. Elevated CO2 significantly increased the proportion of dicotyledones (forbs + legumes) and reduced that of the monocotyledones (grasses). Management differentiated this response as elevated CO2 increased the proportion of forbs when infrequently and of legumes when frequently defoliated. However, among the two dominant forbs species only one was significantly enhanced by elevated CO2. Moreover, not all grass species responded negatively to high CO2. At a low cutting frequency, the observed decline under ambient CO2 in species diversity (Shannon‐Weaver index) and in forb species number was partly alleviated by elevated CO2. This experiment shows that the botanical composition of temperate grasslands is likely to be affected by the current rise (+ 0.5% per year) in the atmospheric CO2 concentration, and that grassland management guidelines may need to be adapted to a future high CO2 world.

Climate extremes can ultimately reshape grassland services such as forage production and change plant functional type composition. This 3-year field research studied resistance to dehydration and recovery after rehydration of plant community and plant functional types in an upland perennial grassland subjected to climate and cutting frequency (Cut+, Cut-) disturbances by measuring green tissue percentage and above-ground biomass production (ANPP). In year 1, a climate disturbance gradient was applied by co-manipulating temperature and precipitation. Four treatments were considered: control and warming-drought climatic treatment, with or without extreme summer event. In year 2, control and warming-drought treatments were maintained without extreme. In year 3, all treatments received ambient climatic conditions. We found that the grassland community was very sensitive to dehydration during the summer extreme: aerial senescence reached 80% when cumulated climatic water balance fell to -156 mm and biomass declined by 78% at the end of summer. In autumn, canopy greenness and biomass totally recovered in control but not in the warming-drought treatment. However ANPP decreased under both climatic treatments, but the effect was stronger on Cut+ (-24%) than Cut- (-15%). This decline was not compensated by the presence of three functional types because they were negatively affected by the climatic treatments, suggesting an absence of buffering effect on grassland production. In the following 2 years, lasting effects of climate disturbance on ANPP were observable. The unexpected stressful conditions of year 3 induced a decline in grassland production in the Cut+ control treatment. The fact that this treatment cumulated higher (45%) N export over the 3 years suggests that N plays a key role in ANPP stability. As ANPP in this mesic perennial grassland did not show engineering resilience, long-term experimental manipulation is needed. Infrequent mowing appears more appropriate for sustaining grassland ANPP under future climate extremes.

Times Cited: 0Pilon, R. Picon-Cochard, C. Bloor, J. M. G. Revaillot, S. Kuhn, E. Falcimagne, R. Balandier, P. Soussana, J. -F.[ 2 ] INRA, UR341, F-78350 Jouy En Josas, France[ 3 ] Irstea, Res Unit Forest Ecosyst EFNO, F-45290 Nogent Sur Vernisson, France[ 4 ] INRA, PIAF UMR547, F-63100 Clermont Ferrand, FranceWe examine how root system demography and morphology are affected by air warming and multiple, simultaneous climate change drivers. Using minirhizotrons, we studied root growth, morphology, median longevity, risk of mortality and standing root pool in the upper soil horizon of a temperate grassland ecosystem for 3 years. Grassland monoliths were subjected to four climate treatments in a replicated additive design: control (C); elevated temperature (T); combined T and summer precipitation reduction (TD); combined TD and elevated atmospheric CO2 (TDCO2). Air warming (C vs T) and the combined climate change treatment (C vs TDCO2) had a positive effect on root growth rate and standing root pool. However, root responses to climate treatment varied depending on diameter size class. For fine roots (<= 0.1 mm), new root length and mortality increased under warming but decreased in response to elevated CO2 (TD vs TDCO2); for coarse roots (> 0.2 mm), length and mortality increased under both elevated CO2 and combined climate change drivers. Our data suggest that the standing roots pool in our grassland system may increase under future climatic conditions. Contrasted behaviour of fine and coarse roots may correspond to differential root activity of these extreme diameter classes in future climate

Abstract.We have set up a facility allowing steady state 13 CO 2 labeling of short stature vegetation (12 m 2 ) for several years. 13 C labelling is obtained by scrubbing the CO 2 from outdoors air with a self-regenerating molecular sieve and by replacing it with 13 C depleted (−34.7±0.03‰) fossilfuel derived CO 2 The facility, which comprises 16 replicate mesocosms, allows to trace the fate of photosynthetic carbon in plant-soil systems in natural light and at outdoors temperature. This method was applied to the study of soil organic carbon turnover in temperate grasslands. We tested the hypothesis that a low disturbance by grazing and cutting of the grassland increases the mean residence time of carbon in coarse (>0.2 mm) soil organic fractions.Grassland monoliths (0.5×0.5×0.4 m) were sampled from high and low disturbance treatments in a long-term (14 yrs) grazing experiment and were placed during two years in the mesocosms. During daytime, the canopy enclosure in each mesocosm was supplied in an open flow with air at mean CO 2 concentration of 425 µmol mol −1 and δ 13 C of −21.5±0.27‰. Fully labelled mature grass leaves reached a δ 13 C of −40.8 (±0.93) and −42.2‰ (±0.60) in the low and high disturbance treatments, respectively, indicating a mean 13 C labelling intensity of 12.7‰ compared to unlabelled control grass leaves. After two years, the delta 13 C value of total soil organic matter above 0.2 mm was reduced in average by 7.8‰ in the labelled monoliths compared to controls. The isotope mass balance technique was used to calculate for the top (0-10 cm) soil the fraction of 13 C labelled carbon in the soil organic matter above 0.2 mm (i.e. roots, rhizomes and particulate organic matter). A first order exponential decay model fitted to the unlabelled C in this fraction shows an increase in mean residence time from 22 to 31 months at low compared to high disturbance. A slower decay of roots, rhizomes and particulate organic matter above 0.2 mm Correspondence to: K. Klumpp (kklump@clermont.inra.fr) is therefore likely to contribute to the observed increased in soil carbon sequestration in grassland monoliths exposed to low disturbance.

Abstract. We have set up a facility allowing steady state 13CO2 labeling of short stature vegetation (12 m2) for several years. 13C labelling is obtained by scrubbing the CO2 from outdoors air with a self-regenerating molecular sieve and by replacing it with 13C depleted (−34.7±0.03‰) fossil-fuel derived CO2 The facility, which comprises 16 replicate mesocosms, allows tracing the fate of photosynthetic carbon in plant-soil systems in natural light and at outdoors temperature. This method was applied during 2 yrs to temperate grassland monoliths (0.5×0.5×0.4 m) sampled in a long term grazing experiment. During daytime, the canopy enclosure in each mesocosm was supplied in an open flow (0.67–0.88 volume per minute) with modified air (43% scrubbed air and 57% cooled and humidified ambient air) at mean CO2 concentration of 425 µmol mol−1 and δ13C of −21.5±0.27‰. Above and belowground CO2 fluxes were continuously monitored. The difference in δ13C between the CO2 at the outlet and at the inlet of each canopy enclosure was not significant (−0.35±0.39‰). Due to mixing with outdoors air, the CO2 concentration at enclosure inlet followed a seasonal cycle, often found in urban areas, where δ13C of CO2 is lower in winter than in summer. Mature C3 grass leaves were sampled monthly in each mesocosm, as well as leave from pot-grown control C4 (Paspalum dilatatum). The mean δ13C of fully labelled C3 and C4 leaves reached −41.4±0.67 and −28.7±0.39‰ respectively. On average, the labelling reduced by 12.7‰ the δ13C of C3 grass leaves. The isotope mass balance technique was used to calculate the fraction of "new" C in the soil organic matter (SOM) above 0.2 mm. A first order exponential decay model fitted to "old" C data showed that reducing aboveground disturbance by cutting increased from 22 to 31 months the mean residence time of belowground organic C (>0.2 mm) in the top soil.