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

Hakalau Forest National Wildlife Refuge
Comprehensive Conservation Plan
600 years or older and are considered the oldest angiosperm in the Northern hemisphere (P.J. Hart,
UH Hilo in press). Growth rates and height increase in areas with organic seedbeds, such as fallen
logs and stumps (USFWS 1996b).
An abundant amount of seeds are produced by „ōhi„a. These seeds are small, lightweight, and easily
dispersed by wind (Burton 1982, Stemmerman 1983, USFWS 1996b). „Ōhi„a seeds are able to
germinate quickly in diverse environmental conditions, even under low light levels. In general,
germination is exceptionally high when seeds were sown on the surface rather than under a layer of
soil. The optimal germination temperature for „ōhi„a is between 61-72°F. Different varieties exhibit
different germination characteristics. Seeds from pubescent plants are more successful at germinating
rapidly and in high temperatures; thus, pubescent „ōhi„a are considered more successful pioneers.
Due to the thin seed coat, seeds lose viability after several months, suggesting that there is not a
persistent soil seed bank (Drake 1993). Due to the abundant and continuous seed supply, „ōhi„a have
a competitive advantage over koa (USFWS 1996b).
„Ōhi„a is a polymorphic species, meaning it has distinct morphological varieties. Flowers can range
in color from red to yellow and leaf shape is highly inconsistent. Eight „ōhi„a varieties are
recognized – dieteri, glaberrima, incana, macrophylla, newellii, polymorpha, pseudorugosa, and
pumila (Wagner et al. 1999). These varieties occur in distinct or overlapping habitats, elevations, and
soil moisture regimes (Crawford et al. 2008). Pubescent varieties (var. polymorpha) occur in drier,
higher elevation sites with younger substrates compared to glabrous varieties (var. glaberrima)
(Stemmerman 1983, Drake 1993, Cordell et al. 1997, Wagner et al. 1999, Hoof et al. 2008). Leaf
pubescence may assist plants in stressful environments by reducing transpiration, freezing, and
herbivory rates (Hoof et al. 2008). Varieties with smaller leaf size, petiole length, and internode
length are generally observed in higher elevation areas. Thus, these measurements are inversely
related to elevation (Cordell et al. 1998). Stomata are larger and more dense on glabrous than
pubescent trees (Hoof et al. 2008). The fixed morphological attributes of „ōhi„a are likely the result
of consistent and strong selective pressures. Although the morphological variation of „ōhi„a is
partially based on environmental conditions (Burton 1982, Stemmerman 1986), these distinct
characteristics also appear to be genetically determined (Cordell et al. 1997).
At Hakalau Forest NWR, „ōhi„a trees play a vital role in ecosystem function. „Ōhi„a help sustain
forest bird populations and insect populations by providing nesting sites and cavities, harboring
insects, and producing nectar (USFWS 1996b). Flowers are present year-round at the HFU, with
peaks during the winter and spring (USFWS 1996b). The endangered „akiapōlā„au takes sap of
particular „ōhi„a trees by drilling holes in the trunks and branches of specific trees. These trees,
termed “aki trees,” have higher sap flow than other „ōhi„a. Functioning as an alternative food source
to nectar and insects, “aki trees” may play an important role in the diet of „akiapōlā„au during low
food periods (Pejchar and Jeffrey 2004).
During the mid 1960s, „ōhi„a forests experienced a dieback of the tree canopy, referred to as „ōhi„a
dieback. This loss of crown foliage across the entire landscape occurred in the montane areas of
Mauna Kea in both poorly drained and well-drained areas. Extensive research was conducted into the
cause of dieback (Jacobi et al. 1983, Mueller-Dombois 1983, Mueller-Dombois and Fosberg 1998)
and was determined to be a common, natural phenomenon resulting from aging and environmental
stress (USFWS 1996b). Regeneration of the canopy was found to occur following initial canopy loss
(Jacobi et al. 1983, Stemmerman 1983). Dieback has occurred in the wet „ōhi„a forest at the HFU and
a small portion of the wet koa/ „ōhi„a forest (USFWS 1996b).
Chapter 4. Refuge Biology and Habitats 4-79