Is the growth-enhancing effect of atmospheric CO2 enrichment reduced when light intensities are less than optimal? In a review of the scientific literature designed to answer this question, Kerstiens (1998) analyzed the results of 15 previously published studies of trees having differing degrees of shade tolerance, finding that elevated CO2 caused greater relative biomass increases in shade-tolerant species than in shade-intolerant or sun-loving species. In fact, in more than half of the studies analyzed, shade-tolerant species experienced CO2-induced relative growth increases that were two to three times greater than those of less shade-tolerant species.
In an extended follow-up review analyzing 74 observations from 24 studies, Kerstiens (2001) reported that twice-ambient CO2 concentrations increased the relative growth response of shade-tolerant and shade-intolerant woody species by an average of 51 and 18%, respectively. Moreover, similar results were reported by Poorter and Perez-Soba (2001), who performed a detailed meta-analysis of research results pertaining to this topic, and more recently by Kubiske et al. (2002), who measured photosynthetic acclimation in aspen and sugar maple trees. Low light intensity, therefore, is by no means a roadblock to the benefits that come to plants as a consequence of an increase in the air's CO2 content.
Of course, most general rules do have their exceptions. In one such study, a 200-ppm increase in the air's CO2 concentration enhanced the photosynthetic rates of sunlit and shaded leaves of sweetgum trees by 92 and 54%, respectively, at one time of year, and by 166 and 68% at another time (Herrick and Thomas, 1999). Likewise, Naumburg and Ellsworth (2000) reported that a 200-ppm increase in the air's CO2 content boosted steady-state photosynthetic rates in leaves of four hardwood understory species by an average of 60 and 40% under high and low light intensities, respectively. Thus, even though these photosynthetic responses were significantly less in shaded leaves, they were still substantial, with mean increases ranging from 40 to 68% for a 60% increase in atmospheric CO2 concentration. And that's anything but shabby!
Under extremely low light intensities, the benefits arising from atmospheric CO2 enrichment may be small, but oftentimes they are very important in terms of plant carbon budgeting. In the study of Hattenschwiler (2001), for example, seedlings of five temperate forest species subjected to an additional 200-ppm CO2 under light intensities that were only 3.4 and 1.3% of full sunlight exhibited CO2-induced biomass increases that ranged from 17 to 74%. Similarly, in the study of Naumburg et al. (2001), a 200-ppm increase in the air's CO2 content enhanced photosynthetic carbon uptake in three of four hardwood understory species by more than two-fold in three of the four species under light irradiances that were as low as 3% of full sunlight.
In a final study we have reviewed on our web site, in which potato plantlets inoculated with an arbuscular mycorrhizal fungus were grown at various light intensities and super CO2 enrichment of approximately 10,000 ppm, Louche-Tessandier et al. (1999) found that the unusually high CO2 concentration produced an unusually high degree of root colonization by the beneficial mycorrhizal fungus, which typically helps supply water and nutrients to plants. And it did so irrespective of the degree of light intensity to which the potato plantlets were exposed.
So, whether light intensity is high or low, or leaves are shaded or sunlit, when the CO2 content of the air is increased, so too are the various biological processes that lead to plant robustness also increased. Less than optimal light intensities, therefore, clearly do not negate the beneficial effects of atmospheric CO2 enrichment.
References
Hattenschwiler, S. 2001. Tree seedling growth in natural deep shade: functional traits related to interspecific variation in response to elevated CO2. Oecologia 129: 31-42.
Herrick, J.D. and Thomas, R.B. 1999. Effects of CO2 enrichment on the photosynthetic light response of sun and shade leaves of canopy sweetgum trees (Liquidambar styraciflua) in a forest ecosystem. Tree Physiology 19: 779-786.
Kerstiens, G. 1998. Shade-tolerance as a predictor of responses to elevated CO2 in trees. Physiologia Plantarum 102: 472-480.
Kerstiens, G. 2001. Meta-analysis of the interaction between shade-tolerance, light environment and growth response of woody species to elevated CO2. Acta Oecologica 22: 61-69.
Kubiske, M.E., Zak, D.R., Pregitzer, K.S. and Takeuchi, Y. 2002. Photosynthetic acclimation of overstory Populus tremuloides and understory Acer saccharum to elevated atmospheric CO2 concentration: interactions with shade and soil nitrogen. Tree Physiology 22: 321-329.
Louche-Tessandier, D., Samson, G., Hernandez-Sebastia, C., Chagvardieff, P. and Desjardins, Y. 1999. Importance of light and CO2 on the effects of endomycorrhizal colonization on growth and photosynthesis of potato plantlets (Solanum tuberosum) in an in vitro tripartite system. New Phytologist 142: 539-550.
Naumburg, E. and Ellsworth, D.S. 2000. Photosynthetic sunfleck utilization potential of understory saplings growing under elevated CO2 in FACE. Oecologia 122: 163-174.
Naumburg, E., Ellsworth, D.S. and Katul, G.G. 2001. Modeling dynamic understory photosynthesis of contrasting species in ambient and elevated carbon dioxide. Oecologia 126: 487-499.
Poorter, H. and Perez-Soba, M. 2001. The growth response of plants to elevated CO2 under non-optimal environmental conditions. Oecologia 129: 1-20.