Changes in growth and survival by three co-occurring grass species in response to mycorrhizae, fire, and drought

The goal of this study was to evaluate the effect of controlled burns, drought and the presence of arbuscular mycorrhizal fungi (AMF) on a dry coastal grassland in Hawai'i Volcanoes National Park. Two introduced African grasses, Hyparrhenia rufa thatching grass, and Melinis repens - Natal redtop, along with one indigenous grass Heteropogon contortus - pili grass composed most of the cover at the study sites. The response of the grasses to fire, AMF infection potential of the soil, and in situ seedling AMF infection were monitored in the field for three years from 1997 to 2000 at Keauhou, Ka'aha, and Kealakomo in Hawai'i Volcanoes National Park. A greenhouse experiment compared the competitive ability of the three grasses with or without AMP inoculation or water stress. The population dynamics of the three grasses were modeled based on their responses to fire, AMF infection, competition and water. At low fire intensities Heteropogon and Hyparrhenia had similar high survival rates while Melinis had a low survival rate. At higher fire intensities all species had low survival rates. The fire decreased the AMF infection potential of the soil at Kaaha, but in situ seedlings AMF infection levels remained high and not statically between the burned and unburned Kaaha sites. In the greenhouse portion of this study Heteropogon biomass increased in response to AMF infection while the other two species did not respond positively to infection. These results suggest that AM fungi increase the growth of the native species, thereby decreasing the impact of competition from two co-occurring alien grasses. When population dynamics were modeled to include the effects of fire, drought and AMF, Melinis and Hyparrhenia produced more biomass in the simulations than the native grass Heteropogon. Drought and AMF decreased the difference in biomass production between the species but did not reverse the competitive ranking of the species