Multiple ecological strategies explain the distribution of exotic and native C_(4) grasses in heterogeneous early successional sites in Hawai’i

Aims Biotic homogenization results in novel communities contain-ing multiple exotic species.Many attributes that contribute to the dominance of invaders and increase invasibility of communities have been identified,but rarely have factors governing the domi-nance of co-occurring invaders been considered.Here we assess the presence and performance of five exotic and one native C_(4) grass species across local(microsite)and more broad-scale variation in fire history,soil age,altitude and precipitation.We ask whether species show preferences for the same conditions,whether all are constrained by poor soil development,and how their performance changes when alone versus co-occurring.In addition to providing information useful to managers who want to control some of these species,such information can help to predict how distributions might change with climate change Methods We measured percent cover and plant height of each grass species in 64 plots in each of 15 sites,located in young volcanic substrates across environmental gradients in Hawai’i Volcanoes National Park,Hawai’i,USA.Sites varied in fire history and soil age,across altitu-dinal and precipitation gradients.We estimated microsite heteroge-neity by measuring rock cover and soil depth,where each species was present and in plots as a whole.We analyzed effects of pos-sible controlling variables using chi-square tests and Generalized Additive Mixed Models.Important Findings Two species(Schizachyrium condensatum and Hyparrhenia rufa)were absent in shallow soil,restricted to more mesic sites and showed increasing performance with soil depth.By contrast,two other species(Andropogon virginicus and Melinis repens)tended to occur in shal-low soil with high rock cover across a wide range of sites.One addi-tional species(Melinis minutiflora)was weakly affected by soil depth and was the dominant at higher elevation(more mesic)and on burned sites,especially in old soil.This species was largely absent from lower elevation sites,where the more widespread and opportunistic conge-ner,M.repens,and a drought tolerant native grass,Heteropogon con-tortus,were more abundant.The latter was confined to this low hot zone.Introduced C_(4) grasses in this region of Hawai’i are distributed according to an interaction between soil requirements,climate(cor-related with elevation),competitive ability and fire response.No one strategy can explain the distributions,which will likely persist unless climate changes toward warmer and drier conditions or new fires occur.The latter would promote dominance of M.minutiflora in more mesic sites,while the former would promote either native(H.contor-tus)or other exotic grasses(M.repens,A.virginicus).