A brief look at Hawaiian Rutaceae

Major components of varied Hawaiian plant communities are the native members of the citrus family (Rutaceae). No tasty oranges here though. But the good thing, at least on O’ahu, is that they are an almost guaranteed sighting on most summit hikes. There are 3 main branches native to Hawai’i: Zanthoxylum, Melicope, and Platydesma.

A’e (Zanthoxylum oahuense) is an uncommon tree found only in the Ko’olaus. It is a pretty little tree with a neat diagnostic trait: it has an extra kink in the petiolule. We saw this individual on the ‘Aiea Ridge trail.

Little tangent. As much as I recommended the Hawai’i Loa Ridge trail to the average hiker, I would NOT do the same for the ‘Aiea Ridge trail. That trail was tough. You work so hard to gain elevation only to lose it again as you go up and down multiple hills. Great plants though.

Zanthoxylum is a fairly cosmopolitan genus. In the eastern U.S., Zanthoxylum americanum is more commonly known as the Prickly Ash. I don’t remember seeing that tree growing up in Chicago. Then again, I was too busy looking under rocks for Ring-necked and Garter snakes.

I talked about Platydesma cornuta before. Here is another picture showing more of the plant. P. cornuta is usually a plam-like shrub that is leafy toward the apex.

Alanis (Melicope spp.) are the Rutaceae that one is most likely to come across. Pictured above is Melicope peduncularis. Yeah, I had to choose one of the more oddly-looking ones. At about 47 species, Melicope is the most speciose of the different native Rutaceae. They are found in a wide variety of habitats but many are found on cloudswept slopes and summits.

One final note about Platydesma and Melicope. There are a number of differences between the genera that, for a long time, they were thought to have separate colonizations to Hawai’i. A recent paper shows otherwise. It seems that they shared a same ancestral colonizer to Hawai’i. Platydesma looks like it is the sister clade to the Hawaiian Melicopes (sect. Pelea). But it is still nested deeply within the broader Melicope genus itself. Will we lose the genus Platydesma? As Herbert Morrison might have said, “Oh the phylogeny!”

What is neat though is that Platydesma has perfect flowers. Most Melicope flowers are dioecious. This means it might be one of the few instances where dioecy has reversed to bisexuality. Dioecy in the Hawaiian flora is an interesting topic, but we’ll save that for another day. So many things to talk about! Sometimes I think this blog writes itself…


The Hawaiian Archipelago is a stepping stone for dispersal in the Pacific: an example from the plant genus Melicope Harbaugh, Danica T., Journal of Biogeography (2009) 36, 230-241

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3 Responses to A brief look at Hawaiian Rutaceae

  1. Nice work Sebastian. Thanks for bringing these insights to light.

  2. Dylan Armstrong says:

    Is that 47 not inclusive of the numerous mystery Melicopes in Dr. Lau’s flickr stream? The O’ahu plant inventory of course lists not even half of those 47.

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