Water deficits

The yield of perennial ryegrass under moderate drought stress increased by an average of 19 and 14% in low- and high-N swards, respectively, at elevated [CO2] (Table 12.1) (Casella et al., 1996). In summer, when the water deficit was greater, elevated CO2 increased yield by 48%. In spring and autumn, the yield increased by only 6%. However, the effects of water deficit and temperature were confounded. At elevated CO2, a temperature increase of 3°C increased the water deficit in summer, due to increased transpiration; thus, grass yield was reduced. Compared with ambient temperature and [CO2], the yield at elevated [CO2] and temperature did not change during periods of water deficit. The yield of tallgrass-prairie swards that are dominated by C4 grasses increased under elevated [CO2] only in dry years, due to an improved plant water status (Owensby et al., 1996).

Root biomass of ryegrass increased during periods of drought; the increase was much larger at elevated [CO2] (Soussana et al., 1996). The CO2 effect on root mass was twice as strong in one year with severe drought compared with years with lower water deficits (Table 12.1) (Hebeisen et al., 1997).

During a period of severe water deficit, white clover yield decreased much less than did perennial ryegrass yield. The soil water content decreased by the same amount under both species. The proportion of white clover in the yield of ryegrass/white clover mixtures increased by up to 80%. The different responses of the species may indicate that growth of ryegrass was reduced during drought because the availability of mineral N was limited, due to a reduced mineralization. Because white clover does not depend on mineral N, the water deficit affected the growth and carbohydrate allocation of this species much less.

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