The vegetation cover of the islands and restoration efforts

In this section we cover the flora of the islands and what the visitor might want to especially be aware of prior to planning your trip to the islands.  Some of the references and links should be especially useful, since we are not by any means experts on the Hawaiian flora.

Overview of the island’s flora

We describe aspects of the flora of the Hawaiian Islands that the average naturalist will find most apparent – or that which contrasts with other locations the visitor might have traveled to.  We are not specialists of the Hawaiian flora, but did have access to some valuable references about the flora.  These can be found here.

Much of the Hawaiian flora can be traced to plants found either south or west of Hawaii – that is from Polynesia (see this link to a nice figure that shows the geographical difference between Melanesia, Micronesia and Polynesia).  Relatively few can be traced to the Americas – except for those brought in during the past few hundred years.  Many of the common and useful plants  found near sea level in the islands were brought by the first Polynesians arriving from the direction of Tahiti approximately 1000 years ago.  But even the non-human introduced plants in the Hawaiian islands have an affinity to the Polynesian flora – despite the surface trade winds being from the opposite (northeast) direction much of the year.  The distance to Hawaii from California and from Tahiti is about the same – 2500 miles.

Figure showing schematic colonization of Polynesia with times before present (BP) indicated. From Thorsby 2016.

Some visitors remark after landing that Hawaii was much drier than they thought.  They’d seen the photos of waterfalls, palm trees and lots of greenery, but what they saw upon landing was not this (especially landing at the Kona airport on the Big Island or the Maui airport and their subsequent drive to resorts on Maui’s west coast).  So what’s going on?

The climate section helps to explain the rainfall patterns that produce the very wet and very dry regions of the various islands and we won’t repeat those here.  Rather, lets look at what stands out to the first-time visitor.  These include the following.

1) There are large east-west vegetation gradients across the islands and also with altitude.  This is most obvious driving the Saddle Road between Hilo and Kona and the Big Island and (less so) driving to the top of Haleakala on Maui.

2) Much of the original vegetation cover appears to have been strongly modified by humans.  There are many invasive and non-native species present in areas modified by human activity.

3) Recent volcanism strongly affects the vegetation cover on the younger islands (especially Hawaii and Maui) and less so on the older ones.

4) Most visitors will not appreciate the extent of environmental degradation on the Hawaiian Islands because they cannot recognize what flora is native and what is not.

We will treat the last point first.  The resorts where most visitors stay are on the western, low-rainfall, mostly sunny sides of most of the islands (Kauai is a bit of a partial exception).  Tourists come to Hawaii primarily for the beaches, sun and warmth.  All with an American infrastructure (which can be both good and bad).  The sunny and less rainy sides of the islands, also with weaker winds, lie in the lee of the high terrain on each island.  The original vegetation of these places was anything but forested with tropical plants like palms and leafy trees, together with orchids and tree ferns.  To try to bring a “tropical” appearance to the resorts, the landscape has been extensively modified with introduced (non-native) plants from other parts of the world.  These survive largely due to irrigation from water sources elsewhere on the islands (either high up or on the windward side of the islands).  Thus, tourists are surrounded by lush vegetation in their resorts, many unaware that most of the plants are not from Hawaii.

Once tourists leave their resorts they are exposed to agricultural fields (tropical fruits, former sugar cane fields etc) or large cattle ranches with introduced grasses.  More frequent fires in these landscapes (fire was relatively rare in pre-human Hawaii) has altered the dominant vegetation in favor of certain grasses.  And, like roadsides almost anywhere in the tropics, the flora is mostly disturbed by human activities.

The least disturbed landscapes with the greatest percentage of the original flora is found on the higher peaks of Hawaii and Maui – protected in substantial part by National Parks.  Here an active effort is made to eliminate non-native species wherever feasible – though it is currently impossible to eliminate some invasive species that are too widespread.

Interestingly, there are some large areas on the Big Island and Maui that have nearly-undisturbed wet forests.  But these tracts, in State of Hawaii Forest reserves, National Parks,  or Nature Conservancy lands, are largely inaccessible to the public.  There are some biological reasons for this; the new fungus that kills Ohia trees is spread by soil on boots or tires, or the spreading of additional invasive species.  But the main reason may be that there is little motivation to put expensive roads into such semi-pristine places –  relatively few tourists travel to Hawaii to see them and there are no commercial interests within them.

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Key aspects of Hawaii’s flora

For those with some botanical background the unique aspects of Hawaii’s flora can be summarized in the following points.

  • Just over 1000 species of flowering plants native to Hawaiian Islands, 90% of them endemic to the islands.
  • Large fern diversity (~160 species, 80% endemic to the islands).
  • Large diversity of Lobelias and other members of the Campanulaceae family (~140 species). These were apparently derived from one original colonizer, meaning that about 12% of the entire Hawaiian flora came from this single colonizing event!
  • Relatively few plant families have a large percentage of the Hawaiian flora – about 40% of the species in the islands come from 5 plant families.

Much of the above statistics are common to most oceanic islands that have resulted from volcanism.  Remote islands have relatively few successful colonizing events and the successful colonizers often diversify to fill the large number of available environmental niches.  Thus, such islands tend to have many species of a particular genus or family.

Botanically inclined naturalists (most of you presumably) can benefit greatly by reading Hawaiian Plant Life before you travel to Hawaii and indeed bringing it with you to help with plant identification.  The first three chapters are essential reading.

Links to the Hawaiian Natural Area Reserves

Dominant plants on the islands

Although there are over 1000 plant species on the islands, some are dominant.  Among the trees, much of the biomass is comprised of two species in particular.  The Ohia (Metrosideros polymorpha) colonizes recent lava flows and ranges from small shrubs to 50 ft or higher trees on mature soils.  It is found on all of the main islands and is the most abundant tree by far in the islands.  It is a key nectar source for some Hawaiian Honeycreepers and other native birds.

Another widespread tree species is Acacia koa, whose leaves closely resemble those of Eucalyptus to those not familiar with the endemic Hawaiian Koa.  Eucalyptus are widely planted in Hawaii and we were at first unsure of whether Koa was just another Eucalyptus – we suspect most visitors will not be able to make the distinction.

Restoration efforts

Large parts of the islands are dedicated to cattle ranching and have been for a long time.  Once non-native grasses come to dominant an environment they can be periodically burned or grazed to prevent the return of the native dry (or wet) forest originally present.  In fact fires in the Hawaiian cattle lands are much more common now than in the native forests of the past.

The figure above shows some restoration efforts on the Big Island and on Maui.  Step through the images in sequence and read the text.  Some text and symbols have been added for clarity.

Some efforts are being made to restore the original vegetation cover to parts of the Hawaiian Islands.  This is most apparent, at least from historical satellite imagery,  on Maui, on the south side of Haleakala.  The area can be seen here in Google Maps.  These efforts are mostly supported to attempt to save certain birds from extinction, though restoring the natural flora is also important.  One description of these efforts on Maui can be found here.  This sites first (home) page has an excellent changing display of images showing the restored landscapes compared with the cattle-grazed lands next to it.

Kipukas and succession on lava flows

One of the most apparent aspects of the natural vegetation cover for a naturalist is the effect of the lava flows on moist forests.  A fresh lava flow is the ultimate “disturbance”, completely covering over the underlying vegetation and making inaccessible to wind-borne seeds any soil that might have developed since the last lava flow.  High rainfall areas that have not been covered by lava flows in hundreds of years have time to develop substantial forests with tall trees and these often isolated patches of forest among otherwise barren landscapes have the Hawaiian term “kipuka”.  One self-guided nature trail close to the visitor’s center in Hawaii Volcanoes National Park goes through an old Kipuka with many tall trees.  Other kipukas are evident on the Mauna Ulu trail and elsewhere.  A sampling of kipukas is shown in this powerpoint that we prepared from screen captures of Google Earth imagery.

The first plants to colonize very recent lava flows are usually ferns (lichens come even earlier).  These are followed by some Lycopodiums (Club mosses) and then Ohia trees.   The Ohia can flower while still small shrubs, yet they can eventually grow to trees of 50 ft height or more on older kipukas with developed soils.

Dicranopteris linearis, perhaps the most common fern in the Hawaiian islands.

One fern, abundant on all of the islands, is the Dicranopteris linearis.  It has an interesting dichotomous branching pattern and is a pioneer species and spreads across disturbed landscapes and can even grow vertically against trees.  After it dies it retains the basic network of vascular tissue and traps organic materials to help further soil development.  Other ferns that are very prominent in Hawaii include the tree ferns of the genus Cibotium and Sadleria.  The latter has an interesting growth character in that young fronds are reddish, only turning green with age.  There are a handful of different species in each of these genera but the non-specialist probably will have difficulty identifying them.

Fern of the genus Sadleria showing color difference between young (red) and older (green) leaves. 
A tall Sadleria fern in the open.
The first colonizers on recent lava flows are lichens.  This can occur in a few years, here along the side of the Saddle Road.
Ferns follow the lichens; here is Polypodium pellucidum.
Polypodium pellucidum emerging on fresh pahoehoe flow. 
Polypodium pellucidum
Closer view of Polypodium pellucidum.
A large clump of Polypodium pellucidum.
A Cibotium fern on the side of a trail in Hawaii Volcanoes National Park.

Wet montane bogs

We did not visit some of the wettest environments on the islands – the wet montane bogs that are found near 4000-5000 ft elevation on the older islands that reach this altitude.  These bogs are very limited areas where the land is relatively flat and thus the water doesn’t run off rapidly.  Ponds and permanently saturate bogs are found here, and with them some rare plants found nowhere else.  The Alakai Swamp Trail goes through these bogs on Kauai, but somewhat similar bog environments on the west Maui mountains are essentially inaccessible.  The Big Island does not have such bogs, since volcanism is relatively recent and there are no comparable flat areas where water cannot readily drain from.  Bogs are also almost non existent on Oahu since the mountains do not reach this elevation range.




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