Drought is a normal, recurrent feature of climate. In order to understand the potential effect of increasing atmospheric CO2 concentration (Ca) on ecosystems, it is essential to determine the combined effects of drought and elevated Ca (EC) under field conditions. A severe drought occurred in Central Florida in 1998 when precipitation was 88 % less than the average between 1984 and 2002. We determined daytime net ecosystem CO2 exchange (NEE) before, during, and after the drought in the Florida scrub-oak ecosystem exposed to doubled Ca in open-top chamber since May 1996. We measured diurnal leaf net photosynthetic rate (PN) of Quercus myrtifolia Willd, the dominant species, during and after the drought. Drought caused a midday depression in NEE and PN at ambient CO2 concentration (AC) and EC. EC mitigated the midday depression in NEE by about 60 % compared to AC and the effect of EC on leaf PN was similar to its effect on NEE. Growth in EC lowered the sensitivity of NEE to air vapor pressure deficit under drought. Thus EC would help the scrub-oak ecosystem to survive the consequences of the effects of rising atmospheric CO2 on climate change, including increased frequency of drought, while simultaneously sequestering more anthropogenic carbon. and J. H. Li ... [et al.].
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. and J. Balogh ... [et al.].
Among the most extended ecosystems of the temperate zone, the seminatural, dry grasslands constitute a substantial proportion in the Carpathian Basin. The aim of our present study was to investigate the short-term effect of extensive fertilization on the species composition and CO2 exchange of loess grassland at community level. The in situ investigation of the latter parameter have not been yet carried out in Pannonian loess grasslands. Most of the parameters studied showed a considerable interannual variation both in the fertilized and in the control stands. As a result of the treatment, the average species number of the fertilized stand decreased by 22%, which was more significant in the autumn (26%) than in the spring. Diversity values, including Shannon index and species richness, increased by nearly 1.5 times in the year with adequate rainfall compared with the initial values. In general, species richness and the ratio of dicots decreased, while the ratio of therophytes, alien competitors, and C4 plants increased with the addition of fertilizers. Significant carbon sequestration potential was only detected during wet periods in the fertilized grass. The rate of CO2 uptake was found to be nearly five times higher in the fertilized stand and nearly three times higher in the control stand during the wet year compared with the previous, extremely dry year. The CO2 uptake potential of the fertilized grassland exceeded that of the control stand by 12% in the year with high rainfall, while the rate of CO2 exchange dropped by 50% in the dry year in the fertilized stand. Our study reinforced the idea that the decline in species richness was not necessarily followed by the reduction of stand level carbon uptake in a short period due to an insignificant change in ecophysiological functional groups. and S. Z. Czóbel ... [et al.].