The good with the bad: when ecological restoration facilitates native and non-native species

Organisms interact with each other along a spectrum ranging from competition to facilitation. A theme in restoration ecology is tipping the balance of these interactions to favor desired species and site conditions, exemplified by restoring fertile islands and their nurse plant effects to encourage plant recruitment. We tested the effectiveness of outplanting nursery-grown native perennials and vertical mulching (placing dead plant material upright in soil) for stimulating annual plant recruitment in a disturbed Mojave Desert shrubland in Joshua Tree National Park, California, U.S.A. Over 9 years, differences in annual species richness and cover between interspaces and below outplants and vertical mulch varied among years, potentially via inter-annual fluctuations in precipitation or maturation of restoration sites. In the ninth year, which was the wettest, both native and non-native cover averaged 3x higher below outplants than in interspaces. Overall among years at the microsite scale, non-native annual plants more consistently exploited environments provided by outplants and vertical mulch structures than did native annuals. However, these restoration structures were important for native annual diversity. At the 40-m(2) plot scale, disturbed plots that received outplanting supported greater richness of native annual species than disturbed unrestored plots. By facilitating both non-native and native plants, reestablishing fertile islands to restore dryland ecosystems is a conundrum for restoration. Treatments reducing non-native plants may need to accompany fertile island restoration to tip the balance of facilitative plant interactions in favor of native species.