Plant functional, biogeographical and phylogenetic diversity are related to native and non-native plant abundance in invaded Hawaiian forests

Numerous introduced species from cosmopolitan biogeographic origins have naturalized in the Hawaiian Islands and are spreading while native plant cover has declined. Phylogenetic, functional, and biogeographical diversity have been shown to influence invasion success, but relationships between these diversity measures and native plant abundance in invaded oceanic island forests have not been well established. Here we surveyed plant cover in 50 plots (400 m 2 ) on the island of Oʻahu, Hawaiʻi. We measured four performance-related functional traits (specific leaf area, stem specific density, seed dry mass, and maximum plant height), and determined the growth form of each plant species. We calculated the differences between native and non-native community weighted mean trait values and assessed whether these differences, as well as measures of diversity, were associated with native and non-native cover. We found that average trait values were significantly higher for maximum height in natives, and significantly higher for specific leaf area and stem specific density in non-natives. A larger difference in maximum height between natives and non-natives was correlated with reduced non-native cover. Functional divergence and phylogenetic diversity of natives were positively correlated with native cover. Functional divergence and richness of non-natives were positively correlated with non-native cover. Biogeographical diversity was not significantly correlated with non-native cover but had indirect effects via its significant correlation with non-native functional richness, which was positively correlated with non-native cover. These findings suggest that diversity and competition for light may be important determinants of native and non-native species cover in invaded Hawaiian forests. An increasing number of distinct non-native plant introductions is thus likely to be associated with continued declines in native plant cover.