Bogs, fern patches, and scrubby forest dominate this area, which is dissected by numerous deep gulches.

Further upslope, above 6,000 ft (1,800 m), rainfall decreases to 100 in (2,500 mm) or less and native forest merges into abandoned pastureland where alien grasses and weeds, introduced as forage for cattle, are the dominant vegetation.

[1] The Pua Akala Cabin, located within the refuge, was nominated to the National Register of Historic Places in August 2008.

Introduced animals such as mosquitoes, wasps, small Asian mongooses, cats, and rats have also harmed Hawaiian habitat and native species.

[1] Grazing pressure by cattle and pigs has resulted in the replacement of Hawaiian plants by more competitive alien grasses and shrubs within the upper portions of Hakalau Forest.

The replacement process may have been accelerated by efforts to create more pasture land through bulldozing and burning, and by logging mature trees for timber and fence posts.