DROUGHT RESPONSE OF A NATIVE AND INTRODUCED HAWAIIAN GRASS

The alien grass, Pennisetum setaceum, dominates many of the lowland ar id regions that once supported native Heteropogon contortus grassland on the island of Hawaii. Response to drought in a glasshouse was compa red between these C4 grasses to test if success as an invader is relat ed to drought tolerance or plasticity for traits that confer drought t olerance. Pennisetum produced 51 % more total biomass, allocated 49 % more biomass to leaves, and had higher net photosynthetic rates (P(n)) on a leaf area basis than Heteropogon. Plants of both species under d rought produced less total biomass and increased their allocation to r oots compared to well-watered plants, but there was no difference betw een the two species in the magnitude of these responses. The decline i n P(n) with decreasing leaf water potential (PSI1) was greater for Pen nisetum compared to Heteropogon. Plasticity in the response of P(n) to PSI1, osmotic potentials, and the water potentials at turgor loss in response to drought were not different between the two species. Stomat a were more responsive to DELTAw in Heteropogon than in Pennisetum and for well-watered plants compared to droughted plants. Plasticity for the stomatal response to DELTAw, however, was not different between th e species. There was no evidence that the alien, Pennisetum, had great er plasticity for traits related to drought tolerance compared to the native, Heteropogon. Higher P(n) and greater biomass allocation to lea ves resulted in greater growth for Pennisetum compared to Heteropogon and may explain the success of Pennisetum as an invader of lowland ari d zones on Hawaii.