The Flowering Plants of Hawaii
Part 31 Myrtaceae
Myrtaceae, the myrtle family, are home to about 5,500 species assorted into 131 genera (Mabberley, pp 568-569). Species occur in tropical and subtropical areas worldwide with a high representation in Australia. The most familiar member of the family to most readers is likely to be Eucalyptus, a genus of about 680 species, although, as pointed out by Mabberley (pp. 321-322), this number may rise as additional information about this complex group of trees becomes available. Eucalypts have been widely planted as forest trees in many parts of the world including the Hawaiian Islands where plantings numbered in the tens of thousands were not unheard of (Manual). Even higher numbers were achieved with such species as E. camaldulensis, the river or Murray red gum, 428,000 seedlings of which were planted in the period 1911-1941. The winner, if this were a competition, would clearly be swamp mahogany, E. robusta, of which nearly 5 million were planted before 1960. A mature specimen of swamp mahogany is featured here with flowers (see image) and fruits (image). Many of the introduced species obviously liked their new homes; there are 30 naturalized species listed in the Manual. One of the most attractive of the naturalized species is the rainbow, or Mindanao gum, E. deglupta (image). These specimens were photographed in the Keanae Arboretum on the road to Hana, Maui.
First-time visitors to the islands would not be expected to know `öhi`a, but it is so common that no one leaves without having seen this beautiful symbol of tropical Hawai`i almost everywhere. `Öhi`a is Metrosideros polymorpha whose red, brush-like flowers (see images) are a favorite for decorative plantings, and are the official flower of the Big Island. Young leaves (see image), liko in Hawaiian, are very colorful and are used to make lei. The wood of `ohi`a has also been used for caving ceremonial figures. `Öhi`a also occurs in a diminutive form in alpine bogs (see image), as seen in a photograph taken in the Pëpë`öpae Bog on Moloka`i. It is soon apparent why this species is described as the many-shaped one (polymorpha). The variation is so extensive (S. A. James et al., 2004), in fact, that eight varieties are recognized. Adding to the mix, a yellow color form is also known (see image). A physiological adaptation for life on volcanic islands is the capacity of this species to withstand sulfur dioxide, a major effluent gas of volcanic eruptions, up to concentrations of 100 ppm (parts per million) by closing its stomata (Mabberley, p. 541-542).
A relevant item appeared in the June 2008 issue of Environment Hawai`i concerning age structure in `öhi`a forests. Patrick Hart, of the Biology Department of the University of Hawai`i, Hilo, wanted to know if `öhi`a trees were being replaced as fast as, or slower than, their death through natural means. His prime interest lay with the continued availability of habitats for native birds, the `akepa (Loxops coccineus) in particular, whose nesting sites are under attack by logging and other commercial interests. While working in the Hakalau Forest National Wildlife Refuge, on the Big Island, he collected a cross-sectional slice of a downed tree and sent it to a dendrochronology (growth ring analysis) laboratory on the mainland. It was determined that the tree was approximately 600 years old, which he noted made it the oldest deciduous tree known in the United States. Correlating tree diameter with age he estimated that many of the trees in the refuge are in the order of 400 years old.
Four other species of Metrosideros are recognized, three of which are limited in occurrence to high elevation habitats on O`ahu, while the fifth occurs on high ridges on Kaua`i, Moloka`i, and Läna`i. Some folks think the rich, dark red flowers of M. waialealae (see image) are more attractive than the common `öhi`a. Two forms of this species are recognized, subsp. waialealae from Kaua`i, and subsp. fauriei from the other two islands. A comparison of the flower heads of the common species and the more darkly pigmented one can be seen in the illustration.
Metrosideros consists of about 50 species with a distribution that includes the Philippines, New Guinea, New Caledonia, New Zealand, high islands of the Pacific, and South Africa where a single outlier occurs. New Zealand, with 10 species is considered the center of variation of the genus and the likely origin of dispersal. Perhaps the most beautiful of the New Zealand species is M. excelsa (see image) which reaches its maximum showiness at Christmas, hence one of its common names is the New Zealand Christmas tree. In the Maori language this tree is known aspöhutukawa.
Based upon DNA sequence data, S. D. Wright and coworkers (2000, 2001) suggested that the present distribution of species could be accounted for by three radiations, one that gave rise to species in New Caledonia, with subsequent colonization of the Bonin Islands (south of Japan), Fiji, and the Solomon Islands; and two from which the other oceanic species arose. The older of these latter two events, which resulted in Metrosideros reaching Samoa, Fiji (a second colonization), and Vanuatu is thought to have occurred possibly as long ago as two million years. The third oceanic dispersal, resulting in the colonization of Rarotonga, Tahiti, Lord Howe Island, the Kermadec Islands, Pitcairn Island, the Marquesas, and the Hawaiian Islands occurred much more recently, possibly as recent as during the Pleistocene glaciation, that is within the past two million years.
A somewhat surprising outcome of the 2001 study was that Metrosideros collina from the Marquesas and M. polymorpha are more closely related to each other than to any other species. (Historically, it was thought that Hawaiian plants, now considered as M. polymorpha, were only a variety of M collina; this view was subsequently rejected.) The differences in sequences between the Marquesan and Hawaiian specimens were small enough to lead to the conclusion that these two species have been separated by only half a million to one million years. This is in line with the findings of subfossil remains of Metrosideros on the Hawaiian Islands that were dated at about 350,000 years. Differences between Marquesan Metrosideros and species from farther west suggest a residence time on the Marquesas of two million years before the jump to the Hawaiian Islands occurred.
Recent study of different chloroplast genes by Diana Percy and coworkers (2008) led to a different conclusion with regard to the time of the colonization event. Using 10 non-coding regions in the chloroplast genome, thus increasing the likelihood of capturing variation patterns that might have been missed in the earlier study, the latter workers suggested that the colonization of the Hawaiian Islands could have occurred much earlier, possibly as long ago as 3.9-6.3 million years. Kaua`i was identified as the site of the ancestral population with colonization and subsequent diversification of populations (and species) on the younger islands occurring fairly rapidly. A study of nuclear microsatellite differences by workers in the same laboratory (D. T. Harbaugh et al. 2009) revealed significant genetic structure within M. polymorpha possibly requiring rethinking of the taxonomic situation. For example, they suggest that varieties of M. polymorpha and M. waialealae may constitute one “hypervariable” species. A comparison of the flower heads of representatives of these two species was shown above.
Regardless of when colonization occurred, there are questions about the journey from the Marquesas to the Hawaiian Islands that need to be addressed. A problem with the Marquesas as the source is they lie south, and a little east, of the Hawaiian Islands at a distance of nearly 1,900 miles (ca. 3,000 km) with no intervening high islands that could serve as stepping stones. There is another barrier, the inter-tropical convergence zone (ITCZ), which lies between 5-10N latitude and marks the discontinuity between the hemispheres. This zone separates the northern and southern hemisphere air circulation patterns providing a formidable barrier to airborne seeds. The problem is that winds in the southern hemisphere blow in the wrong direction for them to be invoked as away to get seeds carried more or less directly to the Hawaiian Islands. From time to time, however, the ITCZ forms below the equator between 5-10S latitude, touching the Marquesas but not islands farther to the south such as Tahiti. Seeds lofted from the Marquesas could be carried northward by high altitude air flow, which descends in the vicinity of 25N latitude. Seeds could then be swept in the Direction of the Hawaiian Islands by the trade winds and delivered to the islands from the northeast. As Wright and his coworkers (2001) pointed out, it would be of interest to see if any other Hawaiian species might have come from Marquesan ancestors by this route.
There are other requirements for successful colonization to occur. In the first place, the seeds must be light enough to become airborne. According to Carlquist (1980) M. polymorpha is one of a relatively few species whose seeds are small and light enough to meet this requirement: a gram of `öhi`a seeds would contain about 1,750 seeds; which would be the equivalent, roughly, of 50,000 per ounce (Friday and Herbert, 2005). In addition to size, seeds must be able to withstand the low temperatures of high elevation, and, should they not make land on their first try, be tolerant of salt water. Experiments have shown that `öhi`a seeds are viable after at least six hours at -30C, and at least a month in sea water.
Psidium, the genus of the guava, consists of about 70 species of tropical American origin. The fruit of some species are highly prized for the preparation of jellies, jams, chutney, and juice, either alone or in mixture with other tropical fruits, e.g. passion fruit, various citrus fruits. The principal species of commerce is P. guajava (see image). Guava has been cultivated in the Hawaiian Islands since the early 1800s, but never became a major crop compared to sugar. Nonetheless, it has been produced in substantial amounts; for example, at the height of production in 1990 about 25 million pounds of guava were harvested. Yields have declined more or less steadily since, with the yield down to about five million pounds in 2007 (numbers from the National Agricultural Statistics Service, June 2008). Despite the attractiveness of guava, it does tend to become a serious pest at lower elevations. Its fruit is attractive to birds and animals, especially feral pigs, all of whom serve as very efficient distributors of seeds.
The real culprit, as far as invasive species are concerned, however, is the strawberry guava, P. cattleianum (see images), which is considered one of the most aggressive of alien plants. This plant, in addition to maturing quickly as seen in the illustration, is capable of forming extremely dense thickets, a growth form that some refer to as a ‘dog-hair’ forest (see images). Native plants don’t have a chance of survival in this environment, one reason for which is the blockage of sunlight; another is the fact that these plants poison the soil—the phenomenon is known technically as allelopathy—preventing germination of other seeds. Other examples of this phenomenon that may be more familiar to some readers include the 'dead zone' beneath walnut trees, and 'old orchard disease' which refers to the poor germination and growth of plants in old apple orchards caused by toxins released by apple roots.
In addition to the unsightliness of strawberry guava, and its negative affects on native plants, the plant has a very serious economic impact on commercial fruit growing on the islands. Strawberry guava is a critical wild host for fruit flies; of particular concern are the Oriental fruit fly (Bactrocera dorsala) and the Mediterranean fruit fly (medfly, Ceratitus capitata). Loss of crops to these insects in the Hawaiian Islands is reckoned in the millions of dollars annually. As well, the potential contamination of exported fruits and vegetables with these flies, to markets in the United States and Japan is a serious concern. Thus, efforts to rid the islands of strawberry guava have substantial support.
As in all other aggressive aliens species of any significant size, mechanical removal of strawberry guava, other than in very local situations, is labor intensive and prohibitively expensive. Herbicides have shown some promise but, again, application over large areas is expensive and potentially damaging to other plants, native and commercial. Seed load in the soil is not a serious problem since seeds remain viable for only a matter of months. A search for a biological control agent was undertaken with surprisingly positive results. The Brazilian scale insect Tectococcus ovatus is highly specific for host plants, being active on only three species of Psidium, P. cattleianum, and two species that do not occur on the Hawaiian Islands. Equally important is that this insect does not parasitize P. guajava, the commercial crop. Infestation does not kill the plants outright but rather weakens them and reduces their capacity to spread. In due course the plants will die.
There has been a low level of local complaint about this release project. Some people grow a few strawberry guava plants for fruit, and the wood is a favorite fuel for barbequing. It is possible to protect home-grown plants by spraying them with an oily mixture (horticultural or summer oil) commonly used to control scale on fruit trees, and there’s a lot of guava for the cutting of firewood (maybe 60,000 acres or so), as was pointed out to one of the concerned citizens at a local meeting.
Eugenia is a primarily tropical genus consisting of about 950 species. Three species occur in the Hawaiian Islands, the very rare O`ahu endemic E. koolauensis, which I have never seen; the indigenous E. reinwardtiana that occurs on Kaua`i, O`ahu, Moloka`i, and Maui, which I have never seen in flower; and the naturalized E. uniflora (see image), known as the Surinam cherry or pitanga. Pitanga is a native of Brazil and has been cultivated on the Hawaiian Islands since the 1870s. It can be found on Midway Atoll and all of the main islands except Ni`ihau and Kaho`olawe.
Three naturalized species of Leptospermum occur in the islands. The genus is largely Australian where 82 of the 85 known species occur, 80 of them endemic to the continent. Our example is the widely planted Leptospermum scoparium (image), called manuka (from the Mäori mänuka) in its native New Zealand and Tasmania. In English it is known as the tea tree or New Zealand tea tree. Leaves can be used for making a tea, but it is more likely known for its honey which is much prized for its medicinal value. The photograph was taken along the Munro Trail on Läna`i where plants were first introduced by the New Zealand naturalist and bird authority G. Munro in the 1880s. Several other species of Leptospermum are cultivated in the islands, one of which is the very attractive round-leaf tea tree L. rotundifolum (see image).
Leptospermum has several other species with peculiar, or at least interesting features. Having spent some time in Australia one feature in particular caught my eye. Leptospermum liversidgei has earned the name “mozzie blocker” owing to its capacity to release citronellal into the air, thus deterring mosquitoes (Mabberley, p. 483). Citronellal is a ten-carbon (terpenoid) compound that readers would recognize as the aromatic component of lemon grass.
Another visitor from Down Under, although visitor is hardly the proper term since nearly two million trees have been planted in the islands, is the paper bark, Melaleuca quinquenervia (see images). I have seen plantations of this tree in many places on the islands; it is hard to miss. The genus consists of about 250 species with a high concentration of them in Australia: 215 species occur in Australia, 210 of which are endemic. This is an extremely fast growing, and spreading, species that can form nearly pure stands easily out-competing native vegetation. It is easy to see how this tree gained paper bark as its common name (image).
Rhodomyrtus tomentosa (image), the downy or rose myrtle, was introduced to the islands as a decorative plant on Kaua`i, but has become naturalized on several islands, usually in disturbed wet areas. I have seen this species beside several hiking trails on Kaua`i. Rhodomyrtus is a genus of 10 species, seven of which are Australian endemics.
The genus Lophostemon comprises four species native to eastern and northern Australia and southern New Guinea. Lophostemon confertus (see images), known commonly as the vinegar tree, brush box or Brisbane box, has become naturalized on the Hawaiian Islands. Its attractive foliage, unusual flower, drought resistance, and low susceptibility to disease make it a choice tree for planting where minimal upkeep is available. It is commonly seen as a street tree in southern California.
Our last examples of Myrtaceae come from the genus Syzygium. Four of the estimated 1,200 species in the genus occur in the Hawaiian Islands, two naturalized, one endemic, and one that was brought to the islands by Polynesian colonists. Syzygium malaccense (see images) was among the plants taken on the colonists’ ocean journeys. The wood of this small to medium sized tree was used for house construction and for carving, the fruit was eaten, and the bark served as a source of medicine for sore throat. Hawaiians have many names for this plant among which are `öhi`a `ai (edible `öhi`a), `öhi`a, `öhi`a `ai ke`oke`o (ke`oke`o is white), and `öhi`a `ula (`ula is red).
The endemic species is Syzygium sandwicensis (image), `öhi`a hä or just hä in Hawaiian. This species grows on all of the main islands except Ni`ihau, Kaho`olawe, and Hawai`i. Wood was used for construction as well as for fuel, while the bark was used to prepare a dye for kapa.