When the air's CO2 content is increased, most plants exhibit increased rates of net photosynthesis and biomass production. Moreover, on a per-unit-leaf-area basis, they often lose less water via transpiration, as they tend to display lower stomatal conductances. Hence, the amount of carbon gained per unit of water lost per unit leaf area - or water-use efficiency - should increase significantly as the air's CO2 content rises. In this review, we summarize some recent experimental results pertaining to this phenomenon in trees.
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., 1999), red oak (Anderson and Tomlinson, 1998), scrub oak (Lodge et al., 2001), silver birch (Rey and Jarvis, 1998), beech (Bucher-Wallin et al., 2000; Egli et al., 1998), sweetgum (Gunderson et al., 2002; Wullschleger and Norby, 2001) and spruce (Roberntz and Stockfors, 1998). Likewise, in a multi-species study performed by Tjoelker et al. (1998), 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 are also obtained when trees are exposed to different environmental stresses. In a study conducted by Centritto et al. (1999), 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), 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.
In some parts of the world, perennial woody species have been exposed to elevated atmospheric CO2 concentrations for decades, due to their proximity to CO2-emitting springs and vents in the earth, allowing scientists to assess the long-term effects of this phenomenon. In Venezuela, for example, the water-use efficiency of a common tree exposed to a lifetime atmospheric CO2 concentration of approximately 1,000 ppm rose 2-fold and 19-fold during the local wet and dry seasons, respectively (Fernandez et al., 1998). Similarly, Bartak et al. (1999) reported that 30-year-old Arbutus unedo trees growing in central Italy at a lifetime atmospheric CO2 concentration around 465 ppm exhibited water-use efficiencies that were 100% greater than control trees growing at a lifetime CO2 concentration of 355 ppm. In addition, two species of oaks in central Italy that had been growing for 15 to 25 years at an atmospheric CO2 concentration ranging from 500 to 1000 ppm displayed "such marked increases in water-use efficiency under elevated CO2," in the words of the scientists who studied them, that this phenomenon "might be of great importance in Mediterranean environments in the perspective of global climate change" (Blaschke et al., 2001; Tognetti et al., 1998). Thus, the long-term effects of elevated CO2 concentrations on water-use efficiency are likely to persist and increase with increasing atmospheric CO2 concentrations.
In some cases, scientists have looked to the past and determined the positive impact the historic rise in the air's CO2 content has already had on plant water-use efficiency. Duquesnay et al. (1998), for example, used tree-ring data derived from beech trees to determine that over the past century the water-use efficiency of such trees in north-eastern France increased by approximately 33%. Similarly, Feng (1999) used tree-ring chronologies derived from a number of trees in western North America to calculate a 10 to 25% increase in tree water-use efficiency from 1750 to 1970, during which time the atmospheric CO2 concentration rose by approximately 16%. In another study, Knapp et al. (2001) developed tree-ring chronologies from western juniper stands located in Oregon, USA, for the past century, determining that growth recovery from drought was much greater in the latter third of their chronologies (1964-1998) than it was in the first third (1896-1930). In this case, the authors suggested that the greater atmospheric CO2 concentrations of the latter period allowed the trees to more quickly recover from water stress. Finally, Beerling et al. (1998) grew Gingko saplings at 350 and 650 ppm CO2 for three years, finding that elevated atmospheric CO2 concentrations reduced leaf stomatal densities to values comparable to those measured on fossilized Gingko leaves dating back to the Triassic and Jurassic periods, implying greater water-use efficiencies for those times too.
On another note, Prince et al. (1998) demonstrated that rain-use efficiency, which is similar to water-use efficiency, slowly increased in the African Sahel from 1982 to 1990, while Nicholson et al. (1998) observed neither an increase nor a decrease in this parameter from 1980 to 1995 for the central and western Sahel.
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. It is thus 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 atmospheric CO2 concentration continues to rise.
References
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