Polyscias trees play an important role in Hawaii’s ecosystem

The native flora of ­Hawaii is full of examples that illustrate how diverse its trees and plants can be. Some have unique vegetation or a natural growth form that sets them apart from the rest; others are known for their dramatic blossoms.

Some herbs have become arborescent — resembling a tree in growth and appearance. Other species, such as the flowering tree Flueggea neowawreae are massive in stature.

Then there are the native members of the genus ­Polyscias. These trees vary in height from 8 to 30 feet, though some individuals can grow to around 98 feet! They can have coriaceous, or leathery-looking, leaves that can be smooth on both sides or covered in dense hairs on the bottom surface. Their trunks are smooth with silvery bark and can snake through the undergrowth or stand as a single stem; both terminate in tufts of leaves. Part of the Araliaceae family, they are related to ginseng and plants within the Schefflera genus. Like the octopus tree (Schefflera actinophylla) their crushed leaves smell strongly of carrots. It appears Native ­Hawaiians have had no ­documented uses for this tree.

While trees within the genus Polyscias are not useful to man; they do play an important role in the ecosystem. Many of the species have a symbiosis with insects. About 60 species of Drosophila — a genus of flies — use Polyscias trees as obligate hosts for their larvae, which feed on decomposing tissue. Many of these insects are specific to a single species of this tree. The most common species in Hawaii is Polys­cias ­oahuensis — known in ­Hawaiian as ohe mauka.

The first time any botanist observed an ohe mauka was during the United States Exploring Expedition, which spanned from 1838 to 1842. William Rich was the botanist on this voyage, but it was Asa Gray in 1854 who described the specimens that Rich collected in the field.

The type specimen — ­material used to describe a new species — Gray used to define what would become Polyscias oahuensis came from the back of Manoa Valley on the slopes of Konahuanui.

Other prominent botanists like Heinrich Wawra, Joseph Rock, Charles Noyes Forbes and Earl Edward Sherff would work on describing other new ­species of ­Polyscias. Surprisingly Forbes, Rock and Wawra were the only botanists to observe these species in the wild. Both Gray and Sherff worked from their offices at Harvard University and at Chicago’s Field Museum of Natural History, respectively. This would create a problem for future scientists studying this group. Without observing the diversity of these trees in the forests of Hawaii, one might use the slightest differences to denote new species.

Seven of the 11 species found in the Hawaiian Islands can be found on Kauai. This indicates Polyscias has a center of diversity on the Garden Isle, the oldest island in the archipelago. The other species can be found throughout the islands, with the exception of Kahoolawe. It is believed they all evolved from one common ancestor that arrived in ­Hawaii. The seeds of this ancestor could have been carried to the islands by birds. It is unsure when this happened, but records from Kaau Crater show that Polys­cias was present on Oahu at least 28,800 years ago. Also, DNA evidence demonstrates that the species found here are closely related to species in Tahiti and the Marquesas Islands.

You can view examples of Polyscias trees at the Lyon Arboretum on Oahu and the National Tropical Botanical Garden on Kauai; each have several species in their living collections. Also, some species can be seen by the road in Kokee State Park in Kauai.

Jesse Adams is a botanist at the Harold L. Lyon Arboretum, where he works to catalogue, propagate and conserve the plant diversity found there.