13.3.1 Atmospheric [CO2]
Most forage species on rangelands have either the C3 or C4 photosynthetic pathway. Over 95% of the world's plant species, including most woody plants, utilize the C3 pathway. Photosynthesis in C3 plants is not CO2-saturated at the present atmospheric concentration, so increasing [CO2] is predicted to stimulate carbon gain and productivity in these species (Drake et al., 1997). Plants with C4 photosynthetic pathways comprise fewer than 5% of the world's species, but are an important component of tropical and subtropical grasslands (Ehleringer et al., 1997). The final steps of photosynthesis in C4 plants occur in bundle sheath cells, where a highly efficient biochemical pump maintains CO2 at concentrations that nearly saturate photosynthesis when atmospheric [CO2] is near the current 360 |mmol mol-1 (Bowes, 1993). The C4 metabolism does not, however, preclude photosynthetic and growth responses to CO2 enrichment (Ghannoum et al., 1997; LeCain and Morgan, 1998). Wand and Midgley (1998, unpublished results), for instance, measured a growth enhancement of 15% in C4 grasses compared with 23% in C3 grasses on doubling [CO2] over the present level.
The photosynthetic pathway partially explains growth responses to CO2, but CO2 effects on transpiration and plant water-use efficiency (WUE; biomass produced per unit of transpiration) will be at least as important as photosynthetic metabolism in the future productivity of rangelands. Stomata of most plant species partially close as [CO2] increases (Field et al., 1995; Drake et al., 1997). As this partial closure tends to reduce transpiration more than photosynthesis, leaf-level WUE (photosynthesis/transpiration) rises with [CO2] (Polley et al., 1996a). Reduced water loss and enhanced WUE can also be realized at the canopy level (Kirkham et al., 1991; Nie et al., 1992; Ham et al., 1995), improving plant and soil water relations (Knapp et al., 1994; Morgan et al., 1994; Wilsey et al., 1997), increasing plant production under water limitation (Owensby et al., 1993a) and lengthening the growing season (Chiariello and Field, 1996). Water relation benefits of CO2 enrichment largely explained the greater growth enhancement of C4 than C3 grasses in tallgrass prairie during dry years and the similar growth responses of C3 and C4 grasses under typically water-limiting conditions in shortgrass steppe (Owensby et al.,
1993a, 1997; Hunt et al., 1996; Coughenour and Chen, 1997). Water relation benefits also apply to annual C3 grasslands, where effects of elevated [CO2] on plant production are more evident in drier years (Jackson et al., 1995).
The ability to recover from defoliation is a major determinant of plant productivity and persistence on grazing lands. Recovery of grasses following grazing is controlled initially by re-mobilization of reserves, followed by photosynthetic gains (Caldwell et al., 1981). To the extent that CO2 enrichment increases photosynthesis and storage of reserves, it should enhance recovery from grazing. Plant response to defoliation depends on complicated interactions between grazing history and the environment (Milchunas et al., 1988) and so effects of CO2 enrichment are not likely to be simple. In a controlled environment, CO2 enrichment had little effect on regrowth of grasses from three distinctly different grasslands (Wilsey et al., 1997).
Carbon dioxide enrichment and global warming are predicted to increase net primary production on most rangelands (Baker et al., 1993; Parton et al., 1995; Coughenour and Chen, 1997; Neilson et al., 1998). Because of severe cold-temperature restrictions on growth rate and duration, warmer temperatures alone should enhance production in high- and mid-latitude and high-altitude rangelands (Baker et al., 1993; Körner et al., 1996; Rounsevell et al., 1996). Warmer temperatures should also enhance the growth response of most C3-dominated grasslands to rising [CO2] (Long, 1991; Jones and Jongen, 1996; Coughenour and Chen, 1997; Drake et al., 1997). This positive effect of warmer temperatures on production may be lessened, however, by an accompanying increase in evapotranspiration (ET) rate in drier systems such as the arid and semi-arid rangelands of Central and South America, Africa, the Middle East, Asia and Australia.
Current models yield widely varying estimates of future patterns in precipitation (Giorgi et al., 1998), making it difficult to predict consequences of altered hydrological cycles for rangelands. Productivity on most rangelands is limited by water (Campbell et al., 1997); therefore changes in the amount of precipitation will significantly impact these systems. Arid and semi-arid lands will be most sensitive to changes in precipitation, while usually wet mountain meadows will be minimally affected. Shifts in seasonal patterns of precipitation and predicted increases in storm intensity will probably have a greater impact on rangelands than shifts in precipitation amounts (Giorgi et al., 1998). It is widely agreed that storm intensity will increase, resulting in greater runoff and concentration of water in smaller portions of the landscape. Such changes could reduce productivity or increase its heterogeneity (Campbell et al., 1997; but see Williams et al., 1998). The proportion of annual precipitation that falls during winter months is predicted to increase at high to mid-latitudes (Giorgi et al., 1998). Such a change in seasonality of precipitation, combined with warming (predicted to be greatest at high northern latitudes) and increased runoff resulting from more severe storms, could increase the incidence and severity of summer droughts in semi-arid grasslands of North America and Asia. Conversely, any increase in rainfall during the growing season will help to mitigate the desiccating effects of warmer temperatures.
Long-term responses of rangelands to global change ultimately depend on the soil and its ability to supply nutrients, as well as water. Carbon dioxide enrichment appears to improve the efficiency with which plants utilize nutrients for growth (Stock and Midgley, 1995; Drake et al., 1997). Interactions between [CO2] and nutrients are complicated, however, and plant responses to CO2 enrichment may be constrained by low fertility, especially in relatively mesic environments (Sage, 1994; Stock and Midgley, 1995).
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