Wiliwili

Wiliwili (Erythrina sandwicensis) is a species of flowering tree in the pea family, Fabaceae that is endemic to the Hawaiian Islands. It is the only species of Erythrina that naturally occurs there. It is typically found in dry forests on leeward island slopes up to an elevation of 600 m (2,000 ft).

Wiliwili is occasionally seen in cultivation in Hawaiʻi. It is easily propagated from cuttings. Seven other species of Erythrina were under general cultivation in the Hawaiian Islands, but have been mostly extirpated by the alien gall wasp (see Threats, below). They were popular street trees in dry areas and windbreaks on fields. At least 80 others have been known in botanical gardens there. No species of Erythrina is known to be naturalized in Hawaiʻi.

The wiliwili is distinguished from the other seven cultivated species by a pod with only one to three red or yellow-orange seeds, which sink in water; non-native Erythrina have pods with larger numbers of brown seeds, which float in water. Wiliwili trees grow to a height of 4.5–9 m (15–30 ft) with a gnarled and stout trunk that reaches 0.3–0.9 m (0.98–3.0 ft) in diameter. The bark is smooth, slightly fissured, and covered in gray or black spines up to 1 cm (0.39 in) in length. The bark on the main trunk of mature trees has a distinct orange cast, which is caused by a terrestrial alga.

The wiliwili is summer (dry season) drought deciduous. The dry season usually begins in late April or in May, and trees in the wild typically lose all of their leaves before they bloom. Trees in cultivation may retain much of their foliage through blooming time. The flowers appear in the first half of the dry season, from April through July. They form on horizontal or nearly horizontal racemes that are 7.5–15 cm (3.0–5.9 in) long. The flower color may be orange, yellow, salmon, greenish or whitish. Sometimes all of these colors occur in a single population. The standard petal is erect, not enclosing the other petals. Like all of the erythrinas, the wiliwili is pollinated by birds. The horizontal raceme and the erect standard are adaptations to pollination by passerine birds. Many other erythrinas are pollinated by hummingbirds.

Pods develop and persist on the tree, with the seeds remaining attached long after the pods have opened. The seeds are dislodged by heavy downpours that generally start around November in the islands. Many seeds germinate quickly, and a well-established seedling can grow to 4 ft (1.2 m) in height before the start of the next dry season.

The wiliwili is an unusual spine-bearer because this is a species that has evolved in the isolated Hawaiian Islands without the presence ungulates or other large herbivores. It is thought to be closely related to E. tahitensis, a tree endemic to the Tahitian Archipelago, and E. velutina, a widespread species found in tropical South America and the Caribbean.

Wiliwili means "repeatedly twisted" in the Hawaiian language and refers to the seedpods, which dehisce, or twist open, to reveal the seeds.