Most of earth’s terrestrial plant life evolved around 500 to 400 million years ago, when the atmospheric CO2 concentration was possibly 10 to 20 times higher than it is today. Hence, the biochemical pathways and enzymes involved in carbon fixation would be expected to be better adapted to significantly higher-than-present atmospheric CO2 levels; and such has been demonstrated to be the case. In fact, as the atmosphere’s CO2 content has dropped from that early point in time, it has caused most of earth’s vegetation to become less efficient at extracting carbon dioxide from the air. Fortunately, however, the ongoing rise in atmospheric CO2 concentration caused by the burning of fossil fuels is gradually increasing photosynthetic rates and stimulating vegetative productivity all around the globe. In this summary, we review some of the recent evidence for this phenomenon in various grassland species.
To provide an overview of the effects of elevated atmospheric CO2 concentrations on photosynthesis in grasses, we turn to the review paper of Wand et al. (1999), who compiled and analyzed the pertinent peer-reviewed literature published between 1980 and 1997. These authors determined that a doubling of the air’s CO2 content increases photosynthetic rates of C3 and C4 grasses by 33 and 25%, respectively. Importantly, their results also demonstrated, contrary to some circulating opinions, that C4 plants can – and do – respond positively to increases in the air’s CO2 content.
In the study of Szente et al. (1998), the authors grew two grass and two broad-leaved species common to loess grasslands of Budapest in open-top chambers for 231 days at atmospheric CO2 concentrations of 350 and 700 ppm. To their surprise, the elevated CO2 caused tremendous photosynthetic enhancements of 136 and 486% in the grass and broad-leaved species, respectively.
Greer et al. (2000) studied the effects of elevated air temperature and CO2 concentration on photosynthesis in five pasture species grown for approximately one month in controlled environment chambers and observed that the CO2-induced photosynthetic enhancement rose with increasing air temperature. In fact, at twice-ambient levels of atmospheric CO2, average photosynthetic rates were 36 and 70% greater than they were for control plants grown under ambient CO2 concentrations at air temperatures of 18 and 28 °C, respectively.
When grasses are exposed to water stress, the CO2-induced increase in photosynthesis is typically greater than it is under well-watered conditions. In the long-term study of Adams et al. (2000), for example, tallgrass prairie plots exposed to atmospheric CO2 concentrations of 350 and 700 ppm exhibited minimal differences in their photosynthetic rates during relatively wet years. During relatively dry years, however, CO2-enriched grasses displayed significantly greater rates of photosynthesis than plants grown at ambient CO2 concentrations. Similarly, Clark et al. (1999) grew New Zealand grassland species for 15 months at ambient and elevated (700 ppm) CO2 concentrations and reported that elevated CO2 increased photosynthetic rates in all four of the species studied. Moreover, CO2-induced increases in photosynthesis were greater under conditions of water stress. In Trifolium repens, for example, the CO2-induced increases in photosynthesis were approximately 50 and 300% for well-watered and water-stressed plants, respectively.
Because many grasslands are subject to grazing pressure from herbivores, it is also important to see how this stress phenomenon impacts photosynthetic responses to atmospheric CO2 enrichment. In the study of Rogers et al. (1998), swards of perennial ryegrass were grown in a FACE experiment utilizing atmospheric CO2 concentrations of 360 and 600 ppm. In addition, swards were supplied with low and high levels of soil nitrogen and were further subjected to cutting treatments to simulate herbivory. Under these conditions, elevated CO2 stimulated photosynthetic rates by approximately 35%, regardless of soil nitrogen supply or cutting treatment. In a similar FACE experiment, two forbs and one grass species common to chalk grassland swards of Europe were grown for 14 months at 355 and 600 ppm CO2 to study the influence of simulated grazing on their photosynthetic responses to atmospheric CO2 enrichment (Bryant et al. 1998). Prior to simulated grazing, the CO2 -induced photosynthetic response of the grass and one forb species were both around 28%, while the other forb was non-responsive to elevated CO2. After grazing, however, both forbs exhibited a much larger 40% increase in photosynthesis, while the grass slightly increased its positive response to 30%. The data from these two studies thus suggest that grazing pressure from herbivores will not reduce CO2-induced increases in photosynthesis, but may actually cause them to rise.
In summary, as the air’s CO2 concentration continues to increase, grassland species should respond positively by exhibiting increased rates of photosynthesis. In addition, such increases in photosynthesis will likely occur even under unfavorable growing conditions characterized by less-than-adequate soil moisture, inadequate soil nutrition, elevated air temperature, and physical stress imposed by herbivory. Thus, earth’s grassland species will likely grow ever more robustly in the future, thanks to the ever increasing atmospheric CO2 concentration produced by the burning of ever larger quantities of fossil fuels.
References
Adams, N.R., Owensby, C.E. and Ham, J.M. 2000. The effect of CO2 enrichment on leaf photosynthetic rates and instantaneous water use efficiency of Andropogon gerardii in the tallgrass prairie. Photosynthesis Research 65: 121-129.
Bryant, J., Taylor, G. and Frehner, M. 1998. Photosynthetic acclimation to elevated CO2 is modified by source:sink balance in three component species of chalk grassland swards grown in a free air carbon dioxide enrichment (FACE) experiment. Plant, Cell and Environment 21: 159-168.
Clark, H., Newton, P.C.D. and Barker, D.J. 1999. Physiological and morphological responses to elevated CO2 and a soil moisture deficit of temperate pasture species growing in an established plant community. Journal of Experimental Botany 50: 233-242.
Greer, D.H., Laing, W.A., Campbell, B.D. and Halligan, E.A. 2000. The effect of perturbations in temperature and photon flux density on the growth and photosynthetic responses of five pasture species. Australian Journal of Plant Physiology 27: 301-310.
Rogers, A., Fischer, B.U., Bryant, J., Frehner, M., Blum, H., Raines, C.A. and Long, S.P. 1998. Acclimation of photosynthesis to elevated CO2 under low-nitrogen nutrition is affected by the capacity for assimilate utilization. Perennial ryegrass under free-air CO2 enrichment. Plant Physiology 118: 683-689.
Szente, K., Nagy, Z. and Tuba, Z. 1998. Enhanced water use efficiency in dry loess grassland species grown at elevated air CO2 concentration. Photosynthetica 35: 637-640.
Wand, S.J.E., Midgley, G.F., Jones, M.H. and Curtis, P.S. 1999. Responses of wild C4 and C3 grass (Poaceae) species to elevated atmospheric CO2 concentration: a meta-analytic test of current theories and perceptions. Global Change Biology 5: 723-741.