Incredible plants: 'ohi'a lehua

'Ohi'a lehua (left) with Kilauea caldera and its
venting volcanic gases in the background.

The ‘ohi’a lehua, Metrosideros polymorpha, is the most common native tree species in Hawaii, distributed from near sea level to over 8000 ft elevation.

Metrosideros is a member of the Myrtaceae (myrtles), a family distributed throughout the world’s tropics. The most species-rich genus in the family, Eucalyptus, is likely familiar to many North Americans, since they have been planted throughout urban areas such as southern California (they are native to Australia).

The flowers of M. polymorpha look like small floral fireworks, because of long stamens that protrude from the flowers. The stamens are usually red, however, true to its species epithet (“polymorpha”), stamen color is variable, as are a number of other morphological features of the plant including tree size, and leaf hairiness.

The ‘ohi’a lehua is an early colonist of fresh lava flows and was present in the patchwork of recent lava flows that were visible along Chain of Craters Road in Hawaii Volcanoes National Park. It was also present in more densely forested areas of the park including the northern rim of the Kilauea Caldera and the Ola’a Forest tract – a small wilderness area in the National Park near the town of Volcano that is thick with tree ferns and bryophytes.

Close-up of leaves and flowers. The photo at left was taken near the steam vents in the Kilauea caldera. Note the relatively smooth leaves. The image at right was taken near the trailhead to Mauna Loa, at a higher elevation. the leaves are hairy and flower buds have not yet opened. 

Since the Hawaiian Islands have never been connected to any mainland, all terrestrial species present on the islands originally traveled over large expanses of ocean from other locations. In the case of M. polymorpha, DNA sequence data suggests that it may have colonized from the Marquesas Islands in the south Pacific. Other species in the genus Metrosideros are common in the southern hemisphere, and M. polymopha’s closest genetic relative is M. collina from the Marquesas. The ancestor of today’s M. polymorpha is estimated to have made the migration to Hawaii about 0.5 to 1.0 million years ago, about the time the Big Island was just emerging as the newest Hawaiian island from the vast Pacific.

Trees colonizing a lava field along Chain of Craters Road in Hawaii Volcanos
National Park.

Trees along the coastal slope, south side of the Big Island.

Interestingly, ‘ohi’a lehue trees on the Big Island have been in the news much recently because of concern over a disease outbreak termed “rapid ohia death”. The causative agent appears to be a fungus, Ceratocystis fimbriatus. The disease has apparently not yet spread to other islands in Hawaii, but it poses a significant threat to native forests if its spread continues. Also notable is the fact that rapid ohi’a death isn’t the first documented disease to threaten ohi’a trees over the decades. I located references to two other fungal diseases of these trees: ohi’a rust that affects seedlings in nurseries, and die-offs of mature trees in forests on the Big Island that may have been attributable to Armillaria. Of course, many organisms have evolved in concert with pathogens over their evolutionary history, but new invasions facilitated by human movement of pathogens could cause alarming ecological change to ecosystems unaccustomed to the presence of new diseases. Where did the rapid ohia death pathogen come from, and what will it mean for the future of Hawaii’s native forests?

References

Bohm BA. 2004. Hawai′i’s Native Plants. Mutual Publishing, Honolulu, HI.

Burgan RE, Nelson RE. 1972. Decline of ohia lehua forests in Hawaii. Pacific Southwest Forest and Range Experimental Station, Berkeley, CA. USDA Forest Service General Technical Report PSW-3.

Judd WS, Campbell CS, Kellogg EA, Stevens PF, Donaghue. 2008. Plant Systematics. A Phylogenetic Approach. Sinauer Associates, Sunderland, MA.

Lamoureux CH. 1996. Trailside plants of Hawai′i’s national parks. Hawai′i Natural History Association.