Recovery of native species diversity and biomass following deforestation of wet forests on Hawaii Island: The hope of native ohia (Metrosideros polymorpha) populations and the curse of alien and/or “novel" forests

Background/Question/Methods

Conservation of native biodiversity is of paramount importance in Hawaii, where endemism exceeds 90%, and more endangered species are found per km² than anywhere else on earth. Ohia (Metrosideros polymorpha) is the foundation tree of Hawaii’s forests, occurring across the widest environmental conditions imaginable. Unfortunately, it is vulnerable to the influences of invasive species, many which were purposefully introduced as part of concerted and misguided efforts during the 1900’s to augment and/or replace Hawaii’s forests in the name of ecosystem services. Ohia forests in particular were characterized as slow growing and intolerant of disturbance. Here we report on the consequences of 27 years of secondary succession following deforestation (i.e., clear-cut) of intact, mature, ohia-dominated forest on Kilauea Volcano of Hawaii Island. In plots established within deforested areas and adjacent areas that escaped deforestation, we measured stem density and basal area of constituent species and estimated stand-level aboveground biomass and annual growth increment. We asked the following: Are ohia populations and associated native species able to recover following such catastrophic disturbances, and if so, what is the nature and rate of recovery? Are alien species present/abundant across the clear-cut, and if so how do they influence secondary succession?     

Results/Conclusions

Native species, particularly ohia, successfully re-colonized clear-cut areas and grew quickly throughout secondary succession. Aboveground forest biomass averaged 160 Mg/ha in the recovering clear-cut areas; more than half the biomass of adjacent intact native forests (292 Mg/ha). Ohia trees made up the vast majority (88%) of biomass in recovering forest, and themselves exhibited impressive stand-level growth rates of 6 Mg/ha/y. The impressive rates of recovery of ohia was attributable to a combination of factors; relatively well developed soils that provided a fertile substrate for widespread and rapid recruitment, the capacity to coppice from stumps, and abundant seed sources afforded by adjacent intact ohia-dominated forests. This last factor underscores the need to maintain as much native-dominated forest as possible to sustain Hawaii’s native biodiversity. In contrast, where localized stands of the alien invasive tree, albizia (Falcataria moluccana), had established, ohia was virtually absent, accounting for only 10% of the total biomass in albizia-dominated stands. Results confirm that, contrary to recent assertions regarding the potential benefits accrued by introduced alien species elsewhere, “novel” forest constituents such as albizia and other like alien species are instead in and of themselves serious impediments to the continued existence of Hawaii’s native forests.