Lindroth, Richard L.
IMPACTS OF ELEVATED ATMOSPHERIC CO(2) AND O (3) ON FORESTS: PHYTOCHEMISTRY, TROPHIC INTERACTIONS, AND ECOSYSTEM DYNAMICS
Prominent among the many factors now affecting the sustainability of forest ecosystems are anthropogenically-generated carbon dioxide (CO(2)) and ozone (O(3)). CO(2) is the substrate for photosynthesis and thus can accelerate tree growth, whereas O(3) is a highly reactive oxygen species and interferes with basic physiological functions. This review summarizes the impacts of CO(2) and O(3) on tree chemical composition and highlights the consequences thereof for trophic interactions and ecosystem dynamics. CO(2) and O(3) influence phytochemical composition by altering substrate availability and biochemical/physiological processes such as photosynthesis and defense signaling pathways. Growth of trees under enriched CO(2) generally leads to an increase in the C/N ratio, due to a decline in foliar nitrogen and concomitant increases in carbohydrates and phenolics. Terpenoid levels generally are not affected by atmospheric CO(2) concentration. O(3) triggers up-regulation of antioxidant defense pathways, leading to the production of simple phenolics and flavonoids (more so in angiosperms than gymnosperms). Tannins levels generally are unaffected, while terpenoids exhibit variable responses. In combination, CO(2) and O(3) exert both additive and interactive effects on tree chemical composition. CO(2)-and O(3)-mediated changes in plant chemistry influence host selection, individual performance (development, growth, reproduction), and population densities of herbivores (primarily phytophagous insects) and soil invertebrates. These changes can effect shifts in the amount and temporal pattern of forest canopy damage and organic substrate deposition. Decomposition rates of leaf litter produced under elevated CO(2) and O(3) may or may not be altered, and can respond to both the independent and interactive effects of the pollutants. Overall, however, CO(2) and O(3) effects on decomposition will be influenced more by their impacts on the quantity, rather than quality, of litter produced. A prominent theme to emerge from this and related reviews is that the effects of elevated CO(2) and O(3) on plant chemistry and ecological interactions are highly context- and species-specific, thus frustrating attempts to identify general, global patterns. Many of the interactions that govern above- and below-ground community and ecosystem processes are chemically mediated, ultimately influencing terrestrial carbon sequestration and feeding back to influence atmospheric composition. Thus, the discipline of chemical ecology is fundamentally important for elucidating the impacts of humans on the health and sustainability of forest ecosystems. Future research should seek to increase the diversity of natural products, species, and biomes studied; incorporate long-term, multi-factor experiments; and employ a comprehensive "genes to ecosystems" perspective that couples genetic/genomic tools with the approaches of evolutionary and ecosystem ecology.
Pubmed ID: 20054619
Journal of chemical ecology, 2010; Vol