Forest regeneration following ungulate removal in a montane Hawaiian wet forest

Abstract

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 'ōhi'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.

Spatial patterns of tree recruitment in a montane Hawaiian wet forest after cattle removal and pig population control

“…Indeed, at this time of the first survey, i.e., only 4–6 years after cattle removal (and about 2–4 years before feral pig population control), most species, and more particularly Acacia koa , exhibited a deficit in small trees. This deficit has been reversed since the removal of cattle (e.g., the number of Acacia koa trees within the 5–10 cm dbh classes increased from less than 10 to almost 100 trees between the first and second surveys), which supports the assumption that cattle grazing maintained very low recruitment rates (Hart, Ibanez, Uehana, & Pang‐Ching, ).…”

“…This deficit has been reversed since the removal of cattle (e.g., the number of Acacia koa trees within the 5-10 cm dbh classes increased from less than 10 to almost 100 trees between the first and second surveys), which supports the assumption that cattle grazing maintained very low recruitment rates (Hart, Ibanez, Uehana, & Pang-Ching, 2020). TA B L E 2 Best generalized linear mixed-effect models (ΔAICc < 2) explaining the number of wind-or bird-dispersed tree recruits across plots.…”

Landscape configuration alters movement behavior and space‐use of a Hawaiian forest bird community

“…Hakalau Forest NWR (19°51′N, 155°18′W) which is ~ 18 km from the kīpuka system lies on the eastern slopes of Mauna Kea Volcano and is one of the largest continuous forest tracts on Hawaiʻi Island (13 240 ha) (Scott et al 1986). Like the Upper Waiākea FR, the habitat at Hakalau consists of native old‐growth forest ( > 6000 years old) comprised mainly of ʻōhiʻa and koa trees with an understory of introduced grasses and native plants (Hart et al 2020) which extends to the tree line at ~ 2000 m a.s.l. There was a long history of cattle ranching at Hakalau, and while there has been significant regeneration over the last 30 years after the NWR was established, some areas of the NWR higher in elevation and adjacent to our netting locations include recovering ranch lands that still have open canopy forest (Yelenick et al 2022).…”

Activity patterns of Hawaiian forest birds in a fragmented and continuous landscape

“…The 13 240 ha Hakalau Forest National Wildlife Refuge (hereafter Hakalau; 19°51′N, 155°18′W) lies on the eastern slopes of Mauna Kea Volcano and is one of the largest continuous forest tracts on the Island of Hawaiʻi (Scott et al 1986). Like the Upper Waiākea Forest Reserve, the habitat at Hakalau consists of native old growth forest (> 6000 years old) comprised mainly of ʻōhiʻa and koa trees, with an understory of introduced grasses and native plants (Pejchar et al 2018, Hart et al 2020). The primary predator of our focal species, the Hawaiian Hawk, or ʻio Buteo solitarius , occurs at both sites (Gorresen et al 2008).…”