Sz. Czóbel

Chamber series and space-scale analysis of CO2 gas-exchange in grassland vegetation: A novel approach

Significant part of our work was developing a new type of CO2 and H2O gas exchange chambers fit for measuring stand patches. Ground areas of six chambers (ranged between 0.044–4.531 m2) constituted a logarithmic series with doubling diameters from 7.5 to 240.0 cm. We demonstrate one of the first results for stand net ecosystem CO2 exchange (NEE) rates and temporal variability for two characteristic Central European grassland types: loess and sand. The measured mean NEE rates and their ranges in these grasslands were similar to values reported in other studies on temperate grasslands. We also dealt with the spatial scale dependence from ecophysiological point of view. Our chamber-series measurement was performed in a perennial ruderal weed association. The variability of CO2-assimilation of this weed vegetation showed clear spatial scale-dependence. We found the lowest variability of the vegetation photosynthesis at the small-middle scales. The results of spatial variability suggest the 0.2832 m2 patch size is the characteristic unit of the investigated weed association and there is a kind of synphysiological minimi-area with characteristic size for each vegetation type.

Comparison of CO2 and H2O fluxes over grassland vegetations measured by the eddy-covariance technique and by open system chamber

Measurements of CO2 and H2O fluxes were carried out using two different techniques—eddy-covariance (EC) and open system gas exchange chamber (OC)—during two-years’ period (2003–2004) at three different grassland sites. OC measurements were made during fourteen measurement campaigns. We found good agreement between the OC and EC CO2 flux values (n = 63, r2 = 0.5323, OC FCO2 = −0.6408+0.9508 EC FCO2). The OC FH2O values were consistently lower than those measured by the EC technique, probably caused by the air stream difference inside and outside the chamber. Adjusting flow rate within the chamber to the natural conditions would be necessary in future OC measurements. In comparison with EC, the OC proved to be a good tool for gas exchange measurements in grassland ecosystems.

Seasonal CO2-exchange variations of temperate semi-desert grassland in Hungary

CO2 exchange components of a temperate semi-desert sand grassland ecosystem in Hungary were measured 21 times in 2000–2001 using a closed IRGA system. Stand CO2 uptake and release, soil respiration rate (Rs), and micrometeorological values were determined with two types of closed system chambers to investigate the daily courses of gas exchange. The maximum CO2 uptake and release were −3.240 and 1.903 μmol m−2 s−1, respectively, indicating a relatively low carbon sequestration potential. The maximum and the minimum Rs were 1.470 and 0.226 μmol(CO2) m−2 s−1, respectively. Water shortage was probably more effective in decreasing photosynthetic rates than Rs, indicating water supply as the primary driving variable for the sink-source relations in this ecosystem type.

Correlation between stand photosynthesis and composition at micro-scale in loess grassland

Introduction Compositional and functional patterns and trends in micro-scale, that of the organization of grassland communities, are more difficult to explain than phenomena of the population or macro ecological level (Lawton 1999). To observe rules and forces, mechanisms that govern them, selection of appropriate scales is necessary (Bartha 2000, Elmore 1980). Small-scale spatial heterogeneity of CO2 gas exchange in grasslands partly depends on plant community compositional traits. Species richness, species composition, horizontal and vertical structure and green leaf area index (LAI) of grasslands are diverse in space, and affect net ecosystem CO2 exchange through its components (gross photosynthesis, soil respiration etc.). Study of the rapport of diversity and productivity has long history. LAI and NDVI, considered here with reference to compositional richness (Csillag et al. 2001, Kertesz et al. 2001) are usually related to photosynthetic performance. Papers published on the relationships of the CO2 exchange and diversity (Craine et al. 2001, Spehn et al. 2000, Stocker et al. 1999) report that diverse ecosystems perform better both aboveand below-ground, than monocultures, other articles on the topic of photosynthesis and species dominance or cover (LeCain et al. 2002, McAllister et al. 1998) emphasize that dominant species with high cover are more effective in their gas exchange, than rare species, but these studies mainly refer to leaf level photosynthesis and CO2 enrichment experiences. We investigated the effect of species richness and surface cover on photosynthesis at the same spatial scale: stand-level gross photosynthesis and composition of the measured stand patches have been compared to: (i) find the relationships between photosynthesis and vegetation cover, (ii) reveal the rapport between species number and photosynthesis and the importance of the dominant species.