Effects of Elevated CO₂ on Foliar Phenolics and Condensed Tannins of Three Tree Species

For a period of four years (2000-2003), Kelly et al. (2010) grew twice-weekly-watered six-year-old quaking aspen (Populus tremuloides) clones, two-year-old white willow (Salix alba) clones, and two-year-old sugar maple (Acer saccharum) siblings out-of-doors at the University of Michigan Biological Station in northern Michigan (USA) in open-bottom root boxes enclosed within clear-plastic-wall open-top chambers continuously supplied throughout the growing season (from May until leaf senescence in November) with either ambient-CO₂- air (360 ppm) or elevated-CO₂-air (720 ppm). Then, at the conclusion of the four-year period, fallen leaves were collected, dried, and analyzed for simple phenolic and condensed tannin concentrations.

From Kelly et al.'s tabular results, it can be calculated that the 360-ppm CO₂ increase employed in their study boosted the simple phenolics concentrations of the aspen, maple and willow leaves by 16, 30 and 22%, respectively, while it boosted their condensed tannin concentrations by 60, 85 and 26%, respectively.

Because of the fact that both foliar phenolics and condensed tannins often enhance plant resistance to herbivore and pathogen attack, plus the fact that ruminants browsing on foliage containing condensed tannins may have a tendency to expel less methane (an important greenhouse gas) to the atmosphere, the increased concentrations of these substances in the leaves of trees grown in CO₂-enriched air bodes well for the health of the trees themselves and for people concerned about CO₂- and methane-induced global warming.