Facilitation by shrubs is known to increase the density of beneficiary plant species under the shrub canopy. Previous reforestation projects have shown that shrubs can increase tree establishment within their canopy, and this positive interaction would likely benefit restoration projects in other ecosystems such as arid systems where shrubs are prevalent. In California, many arid ecosystems are heavily invaded by Mediterranean annual grasses which have reduced native plant diversity. In this study we examined the capacity for shrubs to increase the establishment of native annuals to a heavily invaded arid shrub land by planting six native annual plant species in a full factorial design within shrub or open microsites, with and without non-native removals, and with and without herbivore exclosures. We aim to determine if i) the benefits of shrub facilitation can increase native establishment despite non-native competition and ii) if herbivore exclosures will have a greater relative effect in the open versus the shrub as the shrub is likely providing some protection from herbivory. Seeds of six native species were sown into plots prior to seasonal rains in October 2015, and half of all plots were sieved to remove non-native seed, and any non-natives that appeared in these plots were removed. Half of all shrub-open paired plots had herbivore exclosures. We monitored species survival, abundance, biomass, percent cover and reproductive output.
Results/Conclusions
Shrubs had a consistent positive effect on the height and biomass of the planted species, however, there was no difference in abundance between shrub and open plots. Exclosures also had a significant positive effect on plant abudance and height suggesting that herbivory may be an important limitation to native species establishment. Interestingly, these planted species could successfully co-exist and potentially compete with the non-natives. The results of this study support previous work showing the potential benefits of incorporating nurse plants into restoration and expand upon this previous research by extending its use to annual species in an arid environment.