The impact of the slow progressive rise in the atmospheric concentration of CO2 on the carbon sequestration capacity of stands of trees and other vegetation is difficult to measure. Some indications can be obtained, however, from the large number of recent experimental programs in which young trees have been exposed to double the current atmospheric CO2 concentration over periods of up to 6 years. When rooted in the ground with unconfined root systems in open-top chambers, open-side chambers, closed chambers, and Free Air Carbon Dioxide Enrichment (FACE) rings, there is similar enhancement (up to 60 percent) of aboveground and below-ground growth rate (NPP) and carbon accumulation (Saxe et al., 1998; Norby et al., 1999). Trees grown in double the atmospheric CO2 concentration translocate appreciably more carbon below ground than do trees grown in ambient CO2 concentration. For example, 4-year-old birch trees grown in elevated CO2 concentration translocated three times as much carbon below ground as trees grown in ambient CO2 (Wang et al., 1998). Much of this carbon ends up as fine roots, microbes, and mycorrhizae that contribute detritus to the pool of soil organic matter (Rey and Jarvis, 1997).
Meta-analysis of the results from several experiments in Europe on deciduous and coniferous species gave an average increase of 54 percent (Medlyn et al., 2000), with no significant differences between the functional types or between stressed and unstressed groups of experiments. Similar results were found through meta-analysis of similar experiments in the United States (Curtis and Wang, 1998; Peterson et al., 1999). Moderate lack of nutrients and water reduces growth but has little effect on the relative impact of the increase in CO2 concentration. Generally, the increase in CO2 concentration speeds up development, so trees get bigger more quickly but otherwise remain very similar in most respects to trees of the same size growing in current ambient conditions. The rate of photosynthetic uptake of CO2 is almost always higher in air with elevated CO2 concentration than in ambient air. Meta-analysis of data from experiments largely on temperate trees has shown that the key enzymatic parameters that define the capacity for photosynthetic CO2 as a result of growth in elevated CO2 uptake are down-regulated on average by no more than 12 percent, in approximate proportion to the reduction in foliar nitrogen concentration; stomatal conductance is down-regulated by approximately 15 percent, with consequent increased efficiency of use of both nitrogen and water (Curtis, 1996; Curtis and Wang, 1998; Peterson et al., 1999; Medlyn et al., 2000).
Most experimental studies to date have been limited to exposure of young trees to elevated CO2 concentrations for a few years of treatment. A key question that is yet to be resolved is the extent to which the responses of key parameters to increased CO2 concentration will change when a stand of young trees reaches canopy closure. It is uncertain whether the primary result of the increased rate of growth is that maturity is approached more quickly or whether more carbon will also be finally stored in the trees and soil as a forest stand matures. Some experimental and observational studies suggest that the initial positive effects of elevated CO2 on growth rate cited above are greatly reduced or disappear as a stand matures but that more carbon does remain sequestered (e.g., Hättenschwiler et al., 1997). These questions may be partially resolved through the long-term FACE experiments currently in progress on young stands (e.g., Ellsworth, 1999), but at present the long-term effect of CO2 fertilization on carbon sequestration by forest stands remains an open question.
The phenological development and growth rate of agricultural crops and grassland are accelerated in elevated atmospheric CO2 concentration. This leads to increases in crop productivity and increases in harvestable yield in some cases (e.g., Tubiello et al., 1999), though not in others (Wechsung et al., 1999). Tropical grasslands have very high NPP, especially below ground (House and Hall, 2000), but whether the amount of carbon stored in the soil is increasing is uncertain.
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