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 Mediterranean lowland grassland during a relatively wet year (1991) and a dry year (1992); soil-water potentials were related to changes in soil water content. Species examined were the early-season C3grasses Poa bulbosa and Anthoxanthum odoratum, the late-season C4grasses Chrysopogon gryllus and Cynodon dactylon, and the mid-season C3forb Rumex acetosella. Species were also grown as monocultures in pots under two watering treatments (irrigation, no irrigation). The soil-water potential at the times of maximum aboveground 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 species in the field, except Cynodon, was lower in 1992 than in 1991. The early-season species Poa and Anthoxanthum showed the greater decrease in biomass in response to drought, and tended to exhibit an earlier maximum aboveground biomass. Similarly, these species exhibited an earlier maximum shoot biomass in the un-irrigated pots than in the irrigated pots. The mid- and late-season species tolerated lower soil matric potential. In comparison with the early-season species, they had higher leaf water potentials and greater values of relative water content when leaf water potential was lower than −1•5 MPa. These results indicate that the early-season species are more drought-sensitive than late-season species and that their productivity and phenology is influenced greatly by variation in soil water content between years.