The effects of elevated concentrations of carbon dioxide on individual plants, populations, communities and ecosystems

Changes in the atmospheric concentration of CO2, over periods of millennia, are positively correlated with the temperature of the world. It is expected that this positive correlation will be manifested in the future, warmer 'greenhouse world' with higher concentrations of CO2. The predicted changes in temperature and precipitation are expected to cause significant changes in the distribution patterns of the world's terrestrial vegetation (Woodward and McKee, 1991). In addition to this indirect effect, CO2 influences plants directly and an increase in the concentration of CO2 may increase the rate of photosynthesis in plants with the C3 pathway of fixation. Experimental observations often differ in the degree and length of this stimulation, reflecting the stronger impact of other photosynthetic limitations. Where photosynthetic stimulation does occur there is a general decrease in leaf protein, which may stimulate rates of leaf herbivory. The well established and associated increase in the C/N ratio of individual leaves should reduce rates of leaf decomposition. However the few community experiments at elevated CO2 suggest little change in the rate of nutrient cycling in communities. Stomatal opening is generally reduced as CO2 concentration increases. This feature scales up through to the community level, however, it appears that the total volume of water used by a community is unlikely to alter with CO2 alone, because plants tend to develop leafier canopies. This change, plus enhanced rates of root development, indicate a greater potential for carbon sequestration by terrestrial ecosystems. Monthly observations of atmospheric CO2 concentration above the tundra over the last 14 years indicate these expected increases in the rates of CO2 drawdown by the northern ecosystems of the tundra and the boreal and temperate deciduous forests. However, some of this change may be due to interactions with the warmer climate of the 1980s and perhaps an increased aerial supply of pollutant nitrogen.