Do nonnative species arriving after native seedling establishment affect restoration outcomes?

Species that arrive first influence community assembly. This “priority effect” has been extensively applied in ecological restoration through the removal of invasive plants and subsequent introduction of native species, assuming that recolonization by invaders will be hindered by introduced natives. Nonetheless, often the target community is very prone to reinvasion, which might be partially due to poor early growth of natives. This study examines this phenomenon and potential underlying mechanisms in a woodland restoration area in southwestern Australia, considering both, the response and effect of sown native species in relation to a nonnative community re-emerging relatively late due to its initial removal. We aimed to elucidate the effects of a range of factors including phylogenetic relatedness of the seed mixes to annual grasses on nonnative community, seed mass, soil moisture and other soil variables linked to it. We sowed three natives seed mixes in an old field following removal of the nonnative annual grass community broadly dominated by annual grasses. We examined the direction and intensity of the interaction between these and the nonnative community. The seed mixes included (1) monocultures of the native grass Rytidosperma caespitosa, (2) monocultures of six woody species and (3) cultures mixing the native grass with each woody species. Overall, nonnative species dominated by annual grasses emerging late did not affect woody natives but suppressed native grass Rytidosperma, both as a monoculture and mixed with woody species. When growing alongside late nonnatives, the survival of woody natives was favored by woody species seed mass and soil moisture, while the depth of the sandy layer on the soil surface limited the response of Rytidosperma. These results suggest that phylogenetic distance or relatedness might be key to optimizing priority response and effects when restoring ecosystems. The other variables found to enhance native species’ competitive abilities (seed mass, seeding density of Rytidosperma and possibly soil moisture) indicate the importance of considering a variety of factors to optimize restoration based on priority effects.