Final Comprehensive Conservation Plan - U.S. Fish and Wildlife ...

Hakalau Forest National Wildlife Refuge
Comprehensive Conservation Plan
The koa specific longhorn beetle (Cerambicidae) has wood-boring larvae that require dying and dead
koa as part of their life-history (Goldsmith 2007). These larvae are the primary food source for the
„akiapōlā„au, an endangered Hawaiian honeycreeper that has evolved into a woodpecker-like niche
(Pejchar and Jeffrey 2006). Downed trees function as nurse logs, which are important for forest
regrowth. As these logs decay they provide microhabitat for forest seedling establishment (Leary et
al. 2004). Economically, koa is highly valuable and sold commercially for furniture and other crafts;
however, revenue can only be sustained when harvested selectively (Pejchar et al. 2005, TMA 2007).
Historically, the wood was used by ancient Hawaiians for canoes (Elevitch et al. 2006).
Koa is also considered a pioneer species during secondary succession due to its ability to quickly
regenerate on a disturbed site. Ecosystem disturbances, such as fire and soil removal, can stimulate
koa seeds to germinate. Koa is also able to regenerate from buried seed, root sprouts, or root
suckering. It can spread outward from parent trees at a rate of 1.5-8 ft per year (USFWS 1996b).
Similar to other pioneer species, koa is unable to tolerate shady environments. As a result, it usually
establishes in canopy gaps, replacing previously occurring koa trees (USFWS 1996b). An additional
characteristic typical of pioneer species is fast growth. During the first 5 years, koa can grow at rates
of 5 ft per year; however, growth is slower in less favorable environments (Elevitch et al. 2006). At
the HFU, growth varies depending on elevation (USFWS 1996b); however, studies by Goldsmith
(2007) suggest that koa grows equally well at the upper middle and highest elevation areas present on
the Refuge.
Koa has also been referred to as a “forest engineer” due to its ability to create favorable understory
conditions. Typical in legume species, koa develops nitrogen-fixing root nodules that enhance
nitrogen availability in the soil. The rate of nitrogen fixation declines as the trees age (Goldsmith
2007). The Draft Reforestation Management Plan for Hakalau (1996b) suggests that the nitrogenfixing
capacity of koa may prevent „ōhi„a dieback. Koa also adds organic matter to the soil and
acidifies the soil. By providing canopy, the tree moderates light, temperature, and moisture for
understory species (Scowcroft and Jeffrey 1999). Additionally, koa generates leaf litter that
suppresses weedy grasses and also traps moisture that collects as fog drip (Jeffrey, pers. comm.).
Threats to koa seedlings at Hakalau Forest NWR include grass competition, frost, drought, pig
rooting, and cattle browsing. Various seed predators (weevils and seed worms), fungi (Fusarium
spp.), moths (Scotorythra paludicola), and twig borers (Xylosandrus compactus and Xyloborus spp.)
also threaten koa populations. Koa wilt is a disease that causes rapid loss of canopy and death within
a few months. This disease is more prevalent in young trees below 2,500 ft (TMA 2007). In addition,
rats (Rattus spp.) have been implicated in stripping the bark off koa stands less than 6 years old. This
causes deformation and greater probability of infection (Scowcroft and Sakai 1984). Wild turkeys are
also known to forage on seedling leaves, and pull recently planted seedlings out of the ground
(USFWS unpub data).
Hundreds of thousands of native koa seedlings have been planted at the HFU to restore native forest
and habitat for endangered bird species. The first planting occurred in 1988 on Magnetic Hill at
6,500 ft and ongoing plantings are concentrated in the pasture areas of the Refuge (USFWS 1996b,
2002a). The Refuge, with the help of volunteers, germinates seeds picked from trees growing on or
adjacent to the Refuge, and plant koa seedlings in mauka to makai (uphill to downhill corridors). Koa
growth at HFU is slower and varies depending on elevation. After 10 years of growth, native forest
birds have been observed foraging within some planted stands. Pejchar et al. (2007) found that of
three habitat types studied, the koa plantation supported the highest density of the endangered
Chapter 4. Refuge Biology and Habitats 4-77