CO2: Effect on Plant Growth

Each year during the last week in July and the first week in August, at the peak of the growing season, we count shoots, measure lengths and widths of stems in each of the study plots. We use these measures to asses the effects of elevated CO2 on shoot density, leaf area index, and biomass. Over the course of the 17 years we have collected these data, there has been a steady decline in the numbers of C3 shoots on the marsh (Figure 1a). However, the decline was much less in the chambers exposed to elevated CO2 than in the un-chambered control plots or those chambers exposed to normal ambient CO2. This effect caused an increase in relative shoot density (Figure 1b) which was smallest in the first four years of the study at about 16% but thereafter increased to 120% in 2003.

FIGURE 1: Shoot density in the C3 Scirpus -dominated community from 1987 to present (left panel) and the elevated CO 2 stimulation of shoot density over the same period (right panel). Stimulation calculated as [(A-E)/A]*100.


FIGURE 2: The relationship between periods of drought (as indicated by peaks in salinity) and the stimulation in C3 shoot density caused by elevated CO2.

Inter-annual variation, mainly a relative increases in the CO2 effect, was associated with salinity during drought years of 1998, 1992, 1995, 1999, and 2002 (Figure 2). There was a general increase in salinity throughout the period and high levels of salinity occurred in years of low rainfall. It is not clear whether the mechanism regulating shoot density responds to rainfall or to the absence of salt in the soil. Both rainfall and salinity alter the water balance of shoots. Expansive shoot growth is very sensitive to water potential. The relationship of biomass production to rainfall is shown in Figure 3, below, which clearly indicates that in years of high rainfall there was not only relatively more biomass but that from about 1992 onwards, the last 10 years of data, the correlation with rainfall was very strong
(r 2 = 0.92).


A. Shoot biomass at the peak of the growing season and total annual rainfall for the 17 year period, 1988-2003. B. (right panel) Correlation between shoot biomass and annual rainfall, 1992-2000 (elevated, r 2 = 0.92; ambient, r 2 = 0.80) .
Research Questions