Interactions between the invasive Schinus molle (Peruvian pepper tree) with six plant species commonly found in Southern California nature reserves

Abstract Since Schinus molle (Peruvian pepper tree) was brought to Southern California, the dioecious tree has been widely adopted in urban forests and has become invasive. Prior to our study, it was unknown whether S. molle influences plant species in its understory through allelopathy as it does in other geographic areas. The present study sought to determine if S. molle has negative effects on three native and three non-native, invasive understory species. For two separate experiments seeds of six plant species were sowed in mulch (leaf litter) and soil collected from both genders of S. molle . Because a prior study showed that native succulents were most inhibited by males, we expected male mulch and soil would affect native and invasive species more than that of females; however, this prediction was not supported. Male leaf litter of S. molle stimulated the shoot biomass for Amsinckia intermedia , Bromus madritensis , Brassica nigra , and Stipa pulchra . Male soils reduced the shoot emergence of B. madritensis by 34%, however, both genders of soil did not influence the root and shoot biomass of any species. Additionally, field observations in three urban nature reserves recorded 20 native and 10 invasive plant species beneath S. molle canopies. The present study showed S. molle did not have the same influence on species as previously reported for native succulents.