Forest regeneration following ungulate removal in a montane Hawaiian wet forest

Most Hawaiian forests lack resiliency following disturbance due to the presence of non-native and invasive plant and animal species. The montane wet forest within Hakalau Forest National Wildlife Refuge on Hawai'i island has a long history of ungulate disturbance but portions of the refuge were fenced and most ungulates excluded by the early 1990s. We examined patterns of regeneration within two 100 ha study sites in this forest following the removal of ungulates and in the absence of invasive woody tree species to determine, in part, if passive restoration techniques can be successful under these conditions. We characterized growth, mortality, and basal area (BA) changes for approximately 7,100 marked individuals of all native tree species present in two surveys over a 17-18-year period within two hundred 30 m diameter forest plots. Considerable recruitment within plots of new trees of all species significantly changed size class distributions and erased deficits in small-sized trees observed during the first survey, particularly for the codominant canopy tree, koa (Acacia koa). Overall, growth of established dominant 'ohi'a trees (Metrosideros polymorpha) and recruitment of mid-canopy trees contributed to increases in BA while high levels of mortality for large A. koa trees contributed to decreased BA. This resulted in a slight increase in BA between the two surveys (+1.9%). This study demonstrates that fencing and ungulate removal may have rescued the A. koa population by facilitating the first real pulse in recruitment in over a century, and that passive restoration can be a successful management strategy in this forest.