Effect of exotic grass removal and native seed augmentation on re-establishment of common oak understory species on Santa Catalina Island, California

Background/Question/Methods

As in many oak ecosystems in California, there is limited recruitment of native oak trees on Catalina Island. In addition to the lack of recruitment, there are patches of dying Island scrub oak which are being replaced by exotic annual grassland. In one transforming oak stand on Catalina Island we tested the effect of two restoration techniques on the recruitment of common native oak understory species. The study was carried out as a two-factorial experimental design in which seeds of 8 understory species were added in October, 2008 and grass-specific herbicide was applied in January, 2009. We hypothesized that exotic annual grass removal and seed augmentation would increase native understory cover and native richness. In a corollary study, we hypothesized that two native woody species, Heteromeles arbutifolia and Rhus integrifolia, would promote greater emergence and vigor of Island scrub oak seedlings. Emergence and vigor of oak seedlings planted around the canopies of mature Heteromeles arbutifolia and Rhus integrifolia were compared to those planted in open grassland habitat. 

Results/Conclusions

Although there was seasonal fluctuation in the effectiveness of the restoration techniques, overall, these treatments tended to positively affect native species responses and adversely affect annual grass responses. Herbicide treatment had a stronger positive effect on native cover and richness during the wetter year of 2010; whereas, seed addition exhibited a stronger effect on these responses in the drier 2009. Herbicide treatment reduced exotic annual grass cover in both 2009 and 2010, perhaps facilitating the increased cover of native species during the wetter year of 2010. Although there was no difference in emergence between acorns planted in open grassland and those planted at the canopy edge of woody species, the canopy edge may reduce water loss during a prolonged drought period. These findings suggest that land managers faced with the encroachment of exotic annual grasses in areas previously inhabited by a scrub oak ecosystem can restore native understory community structure by the combined use of grass-specific herbicides and seed augmentation of native understory species. These combined treatments may also indirectly promote the recruitment of oak seedlings by reducing competition with exotic annual grasses and by increasing the cover of native species.