April has been deemed Native Hawaiian Plant Month for some years now. While I’ve done posts in the past on various social media platforms, I don’t think I’ve ever done an overview. Indeed, if you do a google search, it is hard to find one.

I think it’s important, especially for people just getting into native plants. Mainly for the folks with an inkling of interest: what is all the fuss about? Why are there so many people out there who care so much about the flora that calls Hawai`i home? To me, it boils down to how distinctive the Hawaiian flora is compared to other regional plant communities. It just wonderfully stands apart.

I’ll break it down into the unique environment plants found in Hawai`i, where they came from, and what they did here.

Context

Let’s start with a little context both geologically and atmospherically. Both are important because there are some interesting conditions that arose because of them which influenced the plant life that evolved here.

Atmospherically, the Hawaiian archipelago sits at a latitude periodically dominated by fairly stationary high pressure systems that cause sinking air. Sinking air, natch, makes it difficult for moisture to rise and develop into rain. If you look at the antipode of Mililani, O`ahu, it’s a small town in Botswana. The exact opposite side of the globe from us is not a tropical paradise but an arid scrubland. But sinking air also compresses and heats up. Elsewhere in the world that compressed heated air goes all the way to the ground where one hears about the infamous heat domes that July in Northern Hemisphere countries are known for. Since here in Hawai`i the “ground layer” is mostly ocean, that warm air above meets cool ocean air below and you get a temperature inversion. If those cool surfaces breezes generated by the high pressure are forced up by, say an island, you can get precipitation at that inversion layer. And a lot of it. This is just a long way of me saying that Hawai`i has a pretty cool atmospheric paradox. It sits at a latitude dominated by deserts and lack of rainfall. But the very same forces that make for dry areas elsewhere in the world also give us the gift of steady, reliable precipitation where there shouldn’t be any.

Geology-wise, each of the high islands may have a life above water of about 10ish million years. A fair amount of time for plants to evolve. But because the Hawaiian hotspot has been active for so long, there has been an ability for plant life to evolve in isolation in the Central Pacific for much longer than a single island’s above surface lifespan. Plants just have to be able to keep hopping down the chain as new islands form. While a number of taxa seem to have arrived in Hawai`i after the formation of the current high islands beginning around 5.7 million years ago, some of the earliest known colonizers may be Asplenium ferns which have been here for possibly some 28 million years.

So there you go, a wet place in a dry latitude with let’s say 30 million years to play with. Who got here and what happened in that time?

The Migrants

Hawaiian plants came from all over the world on their own. Certainly some from Australasia (Leptecophylla, Scaevola glabra, Dianella). We also have some that are from Western North America (Hawaiian mints, the silversword alliance). Some from Temperate Asia (Nototrichium, Zanthoxylum, Hydrangea). Even some (Drosera sundews, Silene catchflies, Viola pamakani) that may have come from the cold Arctic via migratory birds. All those migrants began interacting and competing while facing similar, but also unique, pressures from the geology, atmosphere, and the fauna. It’s from these forces that the strikingly distinct Hawaiian flora developed.

For one, no gymnosperms ever seems to became established here before man. And oddly, considering their minute seeds, orchids are a very small component. Only 3 species of orchids are native to Hawai’i. In terms of species, the peas (Fabaceae) are not too diverse. Sure, koa and mamane can dominate their respective forests, but compared to continental floral communities, peas are a smaller component of Hawaiian flora.

What is a surprisingly rich contingent of the native flora here are the native lobeliads (Campanulaceae). No where else in the world have the lobeliads become such a large portion of a native plant community, filling niches here in Hawai`i they are normally not found in.

I would be remiss if I did not also single out the silversword alliance. Within this one lineage of native hawaiian asterids, these plants evolved into a multitude of forms. While many Asteraceae are herbaceous plants, the silversword alliance has pincushion yucca-analogues (Argyroxiphium), tree forms (Dubuatia spp.) and one (Dubautia latifolia) becoming a large liana.

Another large contingent of the Hawaiian flora are, well, non-flowering plants or ferns & bryophytes. This isn’t one closely related group; the archipelago has had repeated colonization from separate lineages. I just wanted to highlight them because they tend to get overlook even though they are some of the most characteristic elements in certain environments.

Hapu`u and `Ama`u tree ferns (Cibotium and Sadleria spp. respectively) are some of the more striking examples. They are a major component of mid & understories of wet forests here. The ground of these wet forests would have also been covered by a variety of colony forming ferns such palapalai (Microlepia), Ho`i`o (Diplazium), pala`a (Odontosoria), kupukupu (Nephrolepis), and of course uluhe (Dicranopteris). Old accounts talk of how pala (Marattia) was also a major element in the past. One element I find particularly interesting is waimakanui (Pseudophegopteris keraudreniana). It has a frond of indeterminate growth; basically the frond never stops unfurling and grows like a vine.

And lest we forget, mosses and liverworts! They may not get talked about much, but it is hard to imagine wet forests without mosses and liverworts covering most things. Leucobryum and Pyrrhobryum are common mosses in wet forests. While many folks may recognize the thallus liverwort Marchantia, they might be surprised that many of the “mosses” they see near the summits are actually leafy liverworts like Herbertus, Pleurozia, or Bazzania.

The Laboratory

Islands worldwide are often thought of as evolutionary laboratories as life was allowed the time and space to explore novel evolutionary trajectories. Here in Hawai`i, once the plants arrived (typically either by Wind, Wing, or Water) they encountered some unique conditions which put some distinct selective pressures. Not just geologic or atmospheric, but also the biologic. The colonizing flora began interacting with each other as well as the unique animals here, forming interesting relationships and specializations.

Arborescence

Plants with the ability for long distance dispersal over open ocean certainly had an advantage in colonizing Hawai`i, one of the most isolated landmasses on the planet. But it also meant that the dispersal process filtered out a lot of potential founding species. The plants that arrived here faced less, or at the very least, different competition from continental floristic communities. Perhaps because of this, there are lineages here that have become arborescent, or tree-like, where elsewhere in the world they have been relegated to smaller stature. Species in the amaranth family (Charpentiera), the lobeliads (Clermontia, Cyanea), the sunflowers (Dubautia, Hesperomannia), all have impressive larger forms compared to other relatives. I’ll even throw in our Laukahi kuahiwi (Plantago princeps). While not a tree, it has an upright stem compared to the basal rosettes of most other Plantago.

Shifting Defenses

One thing that was not putting pressure of our native flora was mammalian herbivory. In so many places of the world, plants have to survive and adapt to the pressures that animals like elephants, deer, goats, kangaroos, bison, cows, even down to the rabbits and rats are exerting.

Plants here did some interesting things once they were released from that type of pressure. Many conspicuous defenses were lost. Things like thorns (Hoi Kuahiwi Smilax, `Akala Rubus) and stinging hairs (Opuhe Touchardia glabra) have been lost in Hawaiian plants. Even plants like `akoko (Euphorbia) have a viscous latex, while fairly innocuous here in Hawai`i, harken back to their relative’s toxic sap.

Avian Influence

Above: Some birds of O`ahu by Julian Hume

Without mammals, many Hawaiian plants were reacting instead to the selective pressures and influences generated by native birds. Direct avian relationships via frugivory and pollination were some of the main ways birds were interacting with the native flora.

The decurved floral tubes of many of Hawaiian flowers speaks to how many different plant lineages have become adapted to bird pollination. Some examples include the Hibiscus family (Kokia, Hibiscadelphus), the peas (Erythrina, Sophora), all the lobeliads sans Brighamia, the myrtles (Metrosideros). Even a Ko`oko`olau (Bidens cosmoides) seems to have incipient specializations for bird pollination.

Another way birds were influencing plants was through frugivory. Obviously, multiple plant lineages developed fruit that are enticing to birds, but some plants did it in an interesting way. In Touchardia and Alsinidendron (Schiedea) a dry capsule develops but the calyx, instead of falling off with the rest of the perinath, becomes fleshy and grows over the capsule. Those persistent, fleshy calyces present a berry like structure similar to roselle Hibiscus.

Conclusion

This was just a quick look at some of the things I find interesting about Native Hawaiian plants. An assortment of migrants from across the globe found a unique, persisting area of high rainfall far from other landmasses. These long distance colonizers interacted with the land, the equally strange fauna, and each other over the course of some 30 million years to create their own unique evolutionary trajectory that was sheltered away on its own for so long. It is an utterly fascinating story for me. If people find this as interesting as I do, I’ll try to update the facts as we get new information and studies. This is by no means meant to be an exhaustive, definitive source. Though I do hope that this will help people get even more interested in the plants that we share this special place with and continue on their own journey of discovery.

Refs-

Price, J. P. & Clague D.A. 2002 How old is the Hawaiian Biota? Geology and Phylogeny suggest a recent divergence. Royal Publishing Society

Price, J.P. and Wagner, W.L. (2018), Origins of the Hawaiian flora: Phylogenies and biogeography reveal patterns of long-distance dispersal. Jnl of Sytematics Evolution, 56: 600-620. https://doi.org/10.1111/jse.12465