Restoration to a previous reference condition may not be feasible in all situations, due to lack of information, invasive species, or climate change. When site improvement is desirable, a valuable option is restoration based on functional traits, but the concept requires experimentation in order to understand the role of functional diversity at a given site. To meet the objectives of increased carbon storage, native biodiversity regeneration, and invasion resistance, we developed a restoration effort in a lowland wet forest in Hawai´i, employing both native and exotic (non-native but non-invasive) species with varying functional trait expression in experimental treatments. Core (n=4 per treatment) species were chosen based on expected C turnover rates (defined as slow or moderate) while trait (n=6 per treatment) species were chosen based on functional trait redundancy or complementarity to the core species. Sixteen plots with similar vegetation structure, light levels, and soil nutrients were cleared of all invasive species, while four additional ones were left uncleared as reference plots. The treatments consisted of different sets of ten planted species. The experimental restoration plots have been monitored for the last four years with a suite of parameters measured in order to evaluate the success of these hybrid communities.
Results/Conclusions
The restoration efforts at this site are being put to the test in the face of declining forest health with an 11% average annual rate of mortality being witnessed for the dominant canopy tree, Metrosideros polymorpha, due to disease caused by a Ceratocystis fungus. In addition, parallel trends of mortality have been recorded in two co-existing sub-canopy trees Myrsine lessertiana and Diospyros sandwicensis. At the beginning of 2018, four years post-planting, many of the parameters measuring success of the restoration technique suggest that the MOD RED (i.e., moderate C turnover core – redundant traits) treatment is prevailing with high rates of survival, growth, canopy closure, and outplant litterfall (with the slowest decomposition rates). These successes have led to greater invasion resistance, quantified with an invasive cover survey, and further indicated with lower plot maintenance efforts. In determining which of these treatments will be successful in the long-term we need not only consider the original goals of increased C storage, native biodiversity regeneration, and invasion resistance, but also which community assemblages will best mediate the impacts of a forest declining rapidly in terms of native canopy health.