[Illustrations, footnotes and references available in PDF version]
The effect of elevated atmospheric CO2 concentrations on the water-use efficiencies of trees is clearly positive, having been documented in a number of different single-species studies of longleaf pine (Runion et al., 19991), red oak (Anderson and Tomlinson, 19982), scrub oak (Lodge et al., 20013), silver birch (Rey and Jarvis, 19984), beech (Bucher-Wallin et al., 20005; Egli et al., 19986), sweetgum (Gunderson et al., 20027; Wullschleger and Norby, 20018) and spruce (Roberntz and Stockfors, 19989). Likewise, in a multi-species study performed by Tjoelker et al. (1998)10, seedlings of quaking aspen, paper birch, tamarack, black spruce and jack pine, which were grown at 580 ppm CO2 for three months, displayed water-use efficiencies that were 40 to 80% larger than those exhibited by their respective controls grown at 370 ppm CO2.
Similar results have also been obtained when trees were exposed to different environmental stresses. In a study conducted by Centritto et al. (1999)11, for example, cherry seedlings grown at twice-ambient levels of atmospheric CO2 displayed water-use efficiencies that were 50% greater than their ambient controls, regardless of soil moisture status. And in the study of Wayne et al. (1998)12, yellow birch seedlings grown at 800 ppm CO2 had water-use efficiencies that were 52 and 94% greater than their respective controls, while simultaneously subjected to uncharacteristically low and high air temperature regimes, respectively.
In summary, it is clear that as the CO2 content of the air continues to rise, nearly all of earth's trees will respond favorably by exhibiting increases in water-use efficiency. And it is thus quite likely that as time progresses, earth's woody species will expand into areas where they previously could not exist due to limiting amounts of available moisture. Therefore, one can expect the earth to become a greener biospheric body with greater carbon sequestering capacity as the air's CO2 concentration continues to rise.