Effects of fire and defoliation on the life history of native and invader C-4 grasses in a Neotropical savanna

African grasses, introduced into Neotropical savannas to improve forage qua lity, have spread successfully and displaced native plants. To understand t heir competitive relationships, we compared biomass production and allocati on, plant architecture and phenology, net photosynthesis (Pn), water relati ons, and nutrient content under fire and simulated herbivory between two C- 4 grasses, the native Trachypogon plumosus and the introduced Hyparrhenia r ufa from a seasonal savanna in Venezuela. All variables were strongly influ enced by the rainfall regime. Huparrhenia,pan produced bigger plants (in ma ss and size) with a large proportion of mass (>75%) allocated to leaves and culms. Its biomass production was more affected by fire than by defoliatio n. In contrast, Trachypogon was more affected by defoliation than by fire w hich promoted a flush of leaf growth even in the dry season. Fire caused up to 85% mortality in Hyparrhenia but none in Trachypogon, where it increase d inflorescence production. However, fire promoted abundant seed germinatio n and fast seedling growth in Hyparrhenia, enabling it to colonize new area s. During the growing season Trachypogon,pogon had higher Pn and lower leaf water potential (Psi) than Hyparrhenia but differences among treatments we re not significant for either grass. Pn of Trachypogon ceased at a lower Ps i(-3.0 MPa) than in Hyparrhenia (-2.0 MPa), indicating its higher tolerance to water stress. During the dry season, Trachypogon,pogon leaves remained alive and retained low Pn. Leaf nutrient content was higher during the rain y season in both species. Differences in Pn could not explain the higher se asonal biomass production of Hyparrhenia. However, its water stress evasion strategy, larger biomass allocated to leaves, abundant germination and fas t seedling growth appeared to be responsible for the success of Hyparrhenia as an invader of Neotropical savannas.