What was Auwahi forest like when the first Hawaiians arrived? 

At first contact, diverse dryland forest's such as at Auwahi were probably amongst the richest and most diverse of Hawaiian ecosystems.   Based on small areas of more intact vegetation, the former vegetation of Auwahi forest at 3000-5000 feet (915-1525 meters) elevation appears to have been a dense full-canopied forest with trees ranging from 20 to 60 feet (6-18 meters) height and with well developed middle- and understories. The understory probably was dominated by ferns and forbs, such as pala palai (Microlepia sp.), laukahi (Dryopteris wallichiana), pōhole (Diplazium sandwichianum), and kā`ape`ape (Cyrtomium caryotideum), sedges (Mariscus hillebrandii, Carex wahuensis, Carex macloviana, Carex meyenii) and grasses (Panicum nephilophilum).  The middlestory was probably dominated by tangled shrubs, such as`a`ali`i (Dodonaea viscosa), `ūlei (Osteomeles anthyllidifolia), pilo (Coprosma foliosa), as well as vines such as pioi (Smilax) and maile (Alyxia).  The small leaved maile, maile lau li`i, (Alyxia oliviformis), with all leaves less than 2 cm long, was probably extremely common forming a tangling mat of vines through the understory and up to 25 feet (7.6 meters) high into dryland trees. Probably the greatest vegetation change that has occurred at Auwahi in the past

Relatively extensive exploration of lava tubes of the leeward slopes of Haleakala has yielded fossil evidence of an abundant and diverse avifauna which existed before the 1800s.  Two major conclusions can be drawn, 1) there were abundant flightless birds ranging from very small (golf-ball size) rails to very large goose-like birds dubbed moa-nalo, and 2) honeycreeper and honeyeater birds now restricted to high mountain areas of rain forest on northern Haleakala were once common in dryland forests down to low elevations, at least 2000 feet (610 meters) elevation (Olson and James 1982a; Olson and James 1991; James and Olson 1991) and probably down to sea level.  An obvious conclusion is that these large goose-like birds provided an easy and rich food supply for early Hawaiians.  It is also obvious that this stage of easy predation must have been short-lived as the populations of large, predator-naive geese quickly disappeared (Olson and James 1982b). 

References:

James, H.F. and S.L. Olson.  1991.  Descriptions of thirty-two new species of birds from the Hawaiian Islands: part II. Passeriformes.  Ornithological Monographs 46:1-88.
Olson, S.L. and H.F. James.  1982a.  Fossil birds from the Hawaiian Islands: evidence for wholesale extinction by man before Western contact.  Science 217:633-635.

Olson, S.L. and H.F. James.  1991.  Descriptions of thirty-two new species of birds from the Hawaiian Islands: part I. Non-Passeriformes.  Ornithological Monographs 45:1-88.