Biodiversity loss is a global crisis, due in large part to the ubiquitous presence of nonnative invasive species. Invasive grasses are particularly problematic in many tropical ecosystems throughout the world. In addition to changing natural disturbance regimes (i.e. fires), they often possess physiological and competitive strategies that can inhibit natural regeneration and native plant community dynamics. Using a greenhouse experiment, we explored the ecophysiological basis for restoring native dryland ecosystems in Hawai‘i currently dominated by the nonnative invasive grass Urochloa maxima. Specifically, we examined competition between native species and U. maxima, and tested which individual native species and which combinations of species: (i) may suppress U. maxima growth; and (ii) survive and grow faster in the presence of the invasive grass. Native species tested included Myoporum sandwicense (canopy tree), Dodonaea viscosa (shrub), and Plumbago zeylanica (groundcover). We hypothesized that increased native functional biodiversity would result in greater restoration success (U. maxima suppression and increased native species growth), and that planting M. sandwicense individually or in combination with other native species would result in greater restoration success than planting P. zeylanica or D. viscosa alone or in combination. Leaf-level gas exchange and biomass growth were quantified as indices of plant productivity.
Results demonstrated that U. maxima had higher maximum photosynthetic rates (Amax in units of µmol m-2s-1; Mean=19.8) than the native species tested (Mean=10.2 and 14.2 for D. viscosa and M. sandwicense, respectively; P. zeylanica did not produce measurable leaves). Increased native biodiversity did not significantly affect U. maxima Amax (P=0.67). However, increased native biodiversity did positively affect D. viscosa Amax (P<0.01). Specifically, D. viscosa grown with U. maxima exhibited higher Amax when the planting combination included three native species vs. two native species vs. one native species (12.2, 10.0, and 7.4 µmol m-1s-1, respectively), all grown at the same overall plant density. Contrary to leaf-level measurements, biomass growth of U. maxima was significantly suppressed when grown with M. sandwicense in comparison to U. maxima monocultures (P<0.01). Recommendations for restoration of degraded grasslands in Hawai‘i include selection of native species that would be highly competitive with invasive grasses (i.e. those that have high leaf-level gas exchange and growth rates), and that have high functional biodiversity. However, gas exchange and growth rate information is unavailable for most native Hawaiian species, particularly those adapted to survive in dryland ecosystems that today are highly degraded and largely dominated by nonnative invasive grasses.