Resource competition of native and non-native species in Hawaiian lowland wet forests: Implications for restoration

Background/Question/Methods

The high degree of endemism and low species diversity enables Hawaii to serve as a paradigm for the impacts of invasions by non-native species. The lowland wet forests of Hawaii have been rapidly invaded and remain today only in patches on the islands of Hawaii and Kauai. Many Hawaiian species are subject to resource competition pressures by non-native species. In a lowland wet forest on the island of Hawaii (3280mm MAP), non-native species have invaded and now dominate the understory. Native tree species still found in the forest include Metrosideros polymorpha, Myrsine lessertiana, and Psychotria hawaiiensis; however only P. hawaiiensis is regenerating in the understory. Previous research has shown that during dry periods, soils are wetter when the non-native species were removed.  LAI also significantly decreases with non-native removal.  In combination with the well-drained ’a’a substrate and abundance of non-native species, light and water competition may be limiting the regeneration of native species, which ultimately may be influenced by a lack of plasticity found in native species.  To test the effects of light and water limitation, four native and four non-native common lowland wet forest species were grown for six months in a greenhouse under two light and water treatments.

Results/Conclusions

Light was a more important resource than water, as photosynthesis (A_max), light compensation point (LCP), light saturation point (LSP), stomatal conductance, water potential, leaf mass per area (LMA) and relative growth rate (RGR) were all greater under high light conditions, while water only significantly affected A_max.  In low light treatments, non-natives had a greater A_max and RGR than native species.  A plasticity index of each species was created based on maximum and minimum values of the physiological and morphological traits.  Species were individualistic in their plasticity indices, but there was no direct correlation between species origin and plasticity. Light was the main resource affecting species plasticity, especially influencing RGR, stomatal conductance, and LCP.  Our results confirmed that the lack of M. polymorpha regeneration is due to its extremely high mortality rate in low light conditions, but that water does not appear to be limiting native understory species. M. lessertiana and P. hawaiiensis responded similarly to non-native species under low light, and thus may be potential species to be considered for restoration of these rare Hawaiian forests.