Shade provided by riparian plants and biotic resistance by macrophytes reduce the establishment of an invasive Poaceae.
Riparian corridors are habitats that are prone to invasion by non-native plants, but the shade provided by trees that grow in riparian zones may limit the success of invasive species. In this investigation, we conducted three complementary experiments to test the effects of shade levels, along with sediment type and biotic resistance (and their interactions), on the establishment of a highly invasive grass (Urochloa arrecta (Hack. ex T. Durand & Schinz) Morrone & Zuloaga). This invasive grass presented higher growth rates in sediment collected under riparian vegetation than in sediment collected at the sites devoid of vegetation. Shade levels similar to those found underneath riparian vegetation suppressed the establishment of U. arrecta. In addition, when native macrophytes were planted a priori (i.e. before U. arrecta propagules arrived), the establishment of the invasive species was suppressed, even at lower levels of shade. Thus, biotic resistance was sufficient to prevent the establishment of the invasive grass at low shade levels. Therefore, our results indicated that although the sediment from riparian zones may favour the establishment of U. arrecta, shading by trees and the presence of native macrophytes suppress it. Synthesis and applications. Our results indicate that the shade provided by riparian trees and biotic resistance by macrophytes are likely the factors that negatively affect the establishment of the invasive macrophyte Urochloa arrecta. A recommendation for management arising from our results is that restoring riparian corridors to prevent invasion is most likely more efficient if native macrophytes are planted along with the trees. This would be an efficient strategy to prevent invasion by U. arrecta and other invasive Poaceae during the early stages of riparian vegetation regeneration.