Volume 7, Number 36: 8 September 2004
In addition to boosting plant photosynthetic rates and biomass production, as well as enhancing light-, nutrient- and water-use efficiencies, atmospheric CO2 enrichment often alters the chemical composition of plant foliage in ways that may ultimately lead to reductions in atmospheric greenhouse gas concentrations. Perhaps the best known of these changes is the CO2-induced increase (Penuelas et al., 1997) in the phenolic concentration of living leaves -- by the time leaves senesce these changes typically no longer exist [see our Summary dealing with Decomposition (Processes and Properties)] -- and especially important in this regard is the CO2-induced increase in a subclass of phenolics called tannins (Goverde et al., 1999, 2002).
The climatic significance of this phenomenon may be deduced from a 1 May 2002 press release of New Zealand's AgResearch Grasslands institute (see our Editorial of 7 Aug 2002), which states that the institute's scientists have discovered that the condensed tannins found in certain pasture plants significantly reduce emissions of methane from grazing ruminants such as sheep and cattle. With plants in a CO2-enriched world thus producing tannin-enriched foliage for the planet's wild and domesticated ruminants to dine upon (see Tannins in our Subject Index), it can be appreciated that the resulting reduction in ruminant methane emissions will reduce the strength of the atmosphere's greenhouse effect below what it would be in the absence of the foliar chemical changes produced by the rise in CO2.
So how widespread are CO2-induced increases in foliar phenolics and tannins? ? and what are the magnitudes of the increases? Almost all of our relevant knowledge comes from studies of temperate species; and in temperate-region trees, leaf phenolic concentrations have been shown to rise by 20-60% in response to a doubling of the air's CO2 content (Koricheva et al., 1998; Peņuelas and Estiarte, 1998; McDonald et al., 1999; Agrell et al., 2000; Hartley et al., 2000). This knowledge base was vastly enhanced not too long ago, however, by the experiment of Coley et al. (2002), which focused on nine different species of tropical trees. A further notable aspect of this study was the fact that the trees were rooted in the ground and grown in their natural environment (near the Smithsonian Tropical Research Institute's experiment site in central Panama), rather than being planted in pots and grown in greenhouses. This point is especially important, since the latter protocols lead to dramatically different conditions from those in which trees are found in nature, and they can produce results that are substantially different from those obtained with open-top chambers or FACE facilities constructed around trees that are growing out-of-doors in the absence of artificial root restrictions (O'Neil and Norby, 1996).
Coley et al.'s six-month open-top chamber experiment produced some impressive results. Eight of the nine species studied exhibited positive leaf phenolic/tannin responses to a doubling of the air's CO2 content, the largest of which was a concentration increase of 119%. The singular negative response was a 27% decline, while the mean response of all nine species was an increase of 48%. These results are comparable to those obtained for temperate-region trees, and they provide the basis for Coley et al.'s primary conclusion, i.e., that although "both temperate and tropical trees show large interspecific variation in the extent of their response to CO2 ? the overwhelming pattern is for an increase in phenolics by approximately 50%."
In light of this suite of observations, it would appear that one of the biological-based negative climate feedbacks about which we have written in the past (reduced global warming potential due to reductions in methane emissions from ruminants as a consequence of CO2-induced changes in the chemical composition of their daily diet) is both significant and ubiquitous.
| Sherwood, Keith and Craig Idso |
References
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