To better understand (and appreciate) the complexities and diversities of plants in Hawai'i, it is best to classify them based on location and climate. Lets start from the shoreline and work our way up the mountains. Or as said in Hawai`i, from makai (ocean) to mauka (mountain).
1. Coastal Habitats
Most of the plants growing along the coastal areas are very hardy as they have to endure the intense rays of the sun, tolerate high salt content, thrive in soil that is nutrient-poor, and deal with constant wind. For them to be successful, these plants have the ability to reproduce fast in large amounts and recolonize the same area frequently. This adds to gene flow and prevents speciation. It is not surprising that most of the native plants in this environment are widespread across the Pacific ocean and are indigenous rather than endemic.
2. Lowland Dry Forest and Shrublands
(<4,900 feet elevation, <1,300 mm rainfall annually)
Much of the dry lowland forests of Hawai'i have been decimated due to cattle ranching and fires. This section is the most impacted out of all other regions due to human and non-human activities. The forest is open without much canopy cover. Dominated by mostly Leucaena leucocephala or Acacia koa, a shrub-like tree that can grow very fast, covering large tracts of land. One can still find groves of wiliwili trees, an endemic tree that once covered the majority of this region but has now dwindled to a few groves here and there, scattered among ranching land.
The wiliwili trees along the dry landscape of Waikoloa, Big Island, are much larger than its Maui neighbors. Here the Waikoloa Dry Forest Initiative is supported by local residents who volunteer to rehabilitate the area. Wiliwili trees were almost wiped out by the Tanzanian gall wasp larvae. The state introduced another wasp to kill the gall wasp, and it seems to be pretty successful. Hopefully this will allow the wiliwili trees' return.
3. Mesic Seasonal Forest
(1,300 feet to 6,500 feet in elevation, 1,200 mm to 3,800 mm rainfall annually)
Primarily consisting of ohia or koa forests that reside on the windward slopes above and below wet forests, or on the leeward slopes above dry forests. It also contains the less dominant kopiko ula (Psychotria hawaiiensis) that grows along side the koa and ohia trees.
A short video of mesic forest consisting of koa, ohia and douglas fir, Hakalau Forest Reserve at `6000 feet.
4. Montane Wet Forests
(3,300 to 6,200 feet in elevation, 2,500 to 7,000 mm rainfall annually)
This forest is marked by the high content of moisture caused by the almost constant cloud cover, poor drainage, and muddy ground. It is often called a cloud forest which consists of many types of ferns, and mosses. The trees are usually small and stunted. This is also the home of Hawaii's rare endemic Lobelioids.
5. Bogs
(3,600 to 5,600 feet in elevation, maximum rainfall from trade wind inversion)
In general, bogs in Hawaii are relatively small in number and size. They occur primarily in areas of poor drainage in the montane wet forests. Alaka`i Swamp is the largest bog in Hawaii. Although this may be the case, it actually isn't a single large bog. It is instead a high elevation Metrosideros wet forest with a few small bogs scattered across the plateau. Bogs exists on all five islands and although generally found in between 3,600 to 5,600 feet, some are found at lower elevations. An interesting fact about the flora of Hawaiian bogs is that they resemble more closely to those in the southern hemisphere instead of in the north where Hawaii resides. Hence, the bog plants here in Hawaii are more similar to those in Southern Chile or New Zealand. The majority of the plants in Hawaii's bogs consists of grasses, herbs and shrubs. On the edges of the bog there are dwarf versions of trees that would normally grow larger if the conditions were different.
6. Subalpine
(10,000 to 11,500 feet in elevation , 500 mm to 1,300 mm rainfall annually)
The sub-alpine zone is separated from the cloud forest (montane) below where the trade winds winds reach an inversion layer that forms at around 6,000-6,500 feet. Rainfall also declines steadily as we move up to this zone. Above the subalpine zone, frost occurs on a regular basis especially during the night and the winter months, with subfreezing temperatures. In the lower subalpine zone, especially around Haleakala, one can find many Sophora chrysophylla or mamane, and scrub-like plants such as Leptecophylla tameiameiae or pukiawe. Huge swaths of Haleakala's basin is covered in a chapparal like grass called Deschampsia nubigena, a tussock type of grass that reminds one of the wild west. One can also come across false sandalwood or naio (Myoporum sandwincensis).
7. Alpine
(>10,236 feet, ~ 1,000 mm rainfall annually)
The best word to describe this zone is barren. Due to lack of soil, rough lava features, wide fluctuations in day-night temperatures, frost, snow and intense solar radiation, plant cover is almost non-existent. The surface resembles a lunar one, or martian if you prefer. It is perhaps not surprising that the characteristic of these alpine shrublands share the same resemblance of plants from North America rather than that in Southeast Asia or the western Pacific; due to the fact that the mountains in Hawaii are probably the highest in the world if measured from the sea floor. The summit section of Mauna Kea and Mauna Loa is devoid of vascular plants, except for around Lake Waiau where one can find the grass Trisetum glomeratus growing around the perimeter of the alpine lake. On Haleakala, we can find many stands of the endemic Argyroxiphium sandwicense also known more commonly as Silversword or Ahinahina.
