Differential drought tolerance of five coexisting plant species in Mediterranean lowland grasslands

To investigate how soil water content affects plant productivity; phenology and changes in aboveground biomass of five species were recorded in a Medi terranean lowland grassland during a relatively wet year (1991) and a dry y ear (1992); soil-water potentials were related to changes in soil water con tent. Species examined were the early-season C-3 grasses Poa bulbosa and An thoxanthum odoratum, the late-season C-4 grasses Chrysopogon gryllus and Cy nodon dactylon, and the mid-season C-3 forb Rumex acetosella. Species were also grown as monocultures in pots under two watering treatments (irrigatio n, no irrigation). The soil-water potential at the times of maximum abovegr ound biomass for Poa and Anthoxanthum were similar in the un-irrigated pots and in the dry year (1992) in the field. Aboveground biomass of all specie s in the field, except Cynodon, was lower in 1992 than in 1991. The early-s eason species Poa and Anthoxanthum showed the greater decrease in biomass i n response to drought, and tended to exhibit an earlier maximum aboveground biomass. Similarly, these species exhibited an earlier maximum shoot bioma ss in the un-irrigated pots than in the irrigated pots. The mid- and late-s eason species tolerated lower soil matric potential. In comparison with the early-season species, they had higher leaf water potentials and greater va lues of relative water content when leaf water potential was lower than - 1 .5 MPa. These results indicate that the early-season species are more droug ht-sensitive than late-season species and that their productivity and pheno logy is influenced greatly by variation in soil water content between years . (C) 2001 Academic Press.