The Flowering Plants of Hawaii
Part 40 Rhamnaceae - Rhizophoraceae
Rhamnaceae, the buckthorn family, are represented in the islands by seven species, five of which are endemics, one is indigenous, and one is naturalized and has the potential to become troublesome. The endemic species belong to three genera, Alphitonia, Colubrina, and Gouania. Alphitonia ponderosa (image), kauila, kauwila, or o`a on Maui, is the sole member of the genus on the islands; the others, a dozen or so, occur in eastern Australia, Philippines, Borneo, and other Pacific islands. The wood of kauila, which is very hard, strong, and heavy (it sinks in water), was an important resource being used for house building, carving, the manufacture of kapa-pounding mallets and digging sticks (ö`ö), and, at one time, javelins. Dyes were made from bark and roots of kauila. This tree grows on all of the main islands, except Ni`ihau and Kaho`olawe, but is common only on Kaua`i.
Colubrina oppositifolia (see image), kauila or kauwila, is a rare species known from the Wai`nae Mountains on O`ahu and leeward Hawai`i. Like Alphitonia, the wood of this species is very hard and was used in many of the same applications. Most of the 33 species of Colubrina occur in warm and tropical regions of North America and South America, and in the West Indies, but with others scattered in East Africa, islands in the Indian Ocean, and in Malesia (Mabberley, p. 204).
Our third example of endemics from the family is Gouania whose 50 species enjoy a pantropical distribution with a center of diversity in the Americas. Three species occur in the Hawaiian Islands all of which are marked as rare in the Manual. Gouania hillebrandii (Gouania.jpg), occurred at one time in dry forests on Läna`i, Moloka`i, Maui, and Kaho`olawe, but is known at the present time only from two sites on Kaua`i. Threats to this species include browsing by livestock, loss of habitat caused by animals and associated loss of normal vegetation, and attack by insects including a leaf-cutter (Manual).
The only naturalized member of the family on the islands is Rhamnus californica from western North America. This tree, commonly called coffee berry, was planted in the islands to provide food for game birds. Authors of the Manual report that this species is “…rapidly spreading.” This is never good news. How much of a problem this tree will become remains to be seen.
The mangrove family, Rhizophoraceae, consists of 135 species in 15 genera primarily of tropical areas. The family is probably best known for the mangroves, species with stilt roots growing in tidal or inundated areas. There are two genera in the islands, each with one naturalized species. Bruguiera gymnorrhiza occurs in salt water marshes at He`eia, O`ahu, where it was introduced in 1922 in order to stabilize the marsh area. The much more common and widespread Rhizophora mangle), the American or red mangrove occurs in marshes and tidal waters on all of the main islands, except Ni`ihau, Kaho`olawe, and, presumably, Maui.
The accompanying images show a mature plant at water’s edge (see image); details of the root system (see image); a mass of roots-to-be (see image); mature flowers (see image); a single plant becoming established in the rock wall of a fish pond (see image); young plants in shallow water in a fish pond (see image); and young plants among rocks at the same site (see image). The fish ponds in these images are on the southern coast of Moloka`i, east of Kaunakakai, but similar situations occur throughout the islands.
The history of mangroves on the islands is well documented. Several species have been introduced over the years, most of which did not survive, with R. mangle the only one that has had a serious impact on the environment. The Oriental mangrove, Bruguiera gymnorrhiza, kukunaokalä in Hawaiian, much appreciated for its use in lei making, has never proved troublesome. Red mangrove was planted by the American Sugar Company in mud flats on the southern coast of Moloka`i—west of Kaunakakai—in 1902 as a soil stabilizer. Propagation of red mangrove occurs when the fruits (see image), which grow out of the fertilized flower, drop into the water or onto muddy soil. The pendulous fruits can become imbedded in the mud or can be carried away by tidal movement or ocean currents. Since the fruits remain viable in seawater for considerable periods of time, it is possible for them to be carried significant distances before encountering a new and suitable habitat.
An early indication that mangroves were moving away from Moloka`i came in the early 1920s when plants were found growing in Käne`ohe Bay on the east coast of O`ahu. That mangrove migration doesn’t require line of sight routes to new sites is evident by its becoming established on the coasts of islands to the east, counter to the direction of the predominant trade winds and currents. The arrival of mangrove fruits on the north coast of the Big Island and their impact on fish ponds there will be met below. Mangrove had also spread to beaches on the eastern coast of Moloka`i, where it can easily be seen at the present time (see images).
The relationship between Batis (see Part 6 of this series) and mangroves has been described in four stages: (1) pre-Hawaiian phase with neither Batis nor mangroves; (2) Batis appears sometime prior to 1859 followed by expansion to form meadows; (3) appearance of mangroves—first planted on Moloka`i—and their eventual invasion of Batis meadows; (4) Batis eventually replaced completely by mangroves (Allen,1998). Mangroves have invaded sensitive areas such as fish ponds and other archeologically important beach sites, as well as naturally occurring anchialine pools. Since mangrove is a terminal successional species, once it becomes established only major natural events pose any threats to it. Although capable of overwhelming its target sites, mangrove does not cover extensive areas in the islands. When it does occur in sensitive areas, however, it poses daunting challenges for removal. We will look at two examples.
The first example, described by Rauzon and Drigor (2002), involves preservation of an important water-bird habitat on the windward coast of O`ahu. The Nu`upia Ponds are a 195 hectare (ca. 482 acres) wildlife management area and historic Hawaiian fishpond located on the U.S. Marine Corps Base near Käne`ohe. The ponds are home to about 10% of the Hawaiian endemic and endangered black-necked stilt (Himantopus mexicanus knudsenii) population, as well as an assortment of native fish species. In an effort that took 20 years, the work of many volunteers, plus more than 2.5 million dollars in contracted labor, more than 20 acres of mangroves were removed. Twenty years may seem like a long time for such a project but work had to be suspended during nesting seasons. Infestations of Batis were repeatedly disrupted during training exercises using heavy military vehicles. Stilts quickly reclaimed areas once infested with mangroves after the offending trees had been removed.
The second example takes us to the lee side of the Big Island a few miles south of the Kona International Airport, and north of the Kailua-Kona area which, as many readers will recognize, is one of the most frequently visited tourist areas on the island. The particular place of interest is the Kaloko-Honoköhau National Historical Park. Well before the arrival of Europeans, the Kaloko-Honoköhau coastal wetlands had been converted into fishponds, which were of great significance in the Hawaiian community. It has been speculated by Rizal Fronda and coworkers (2008), who were closely involved in restoration work at the site, that this may have been a prototype fishpond: the word kaloko in the pond’s name can be translated as ka loko, ‘the pond.’ If not a prototype pond, it certainly was a significant one; the seawall is the most massive of the walls in the islands. I have not seen the pond at Kaloko, but the photograph (see image) taken at the Kaloko`eli fishpond on Moloka`i, illustrates the size of the lava rocks routinely used.
As Fronda and coworkers point out, the native vegetation of the area, inland from the pond, would have consisted of naupaka (Scaevola), tree heliotrope (Messerschmidia), naio (Myoporum), kiawe (Prosopis, not native), milo (Thespesia), and kou (Cordia, Polynesian introduction). This normal strand-border vegetation existed until the 1970s (no mangroves in 1972), but when mangrove began to move in it did so very quickly becoming the dominant tree by 1990. Removal efforts were begun in 1988 during which year over 70,000 square feet of cover had been removed. By the end of 1994 over 207,000 square feet (4.75 acres) had been cleared. Since this is an extremely sensitive archeologically important area, much of the work had to be done by hand with bundles of cut trees lifted by helicopter. During fiscal year 1990 about half of the pond had been cleared at a cost of $53,000; in fiscal 1991 the total area cleared amounted to about three quarters of the pond. The cost in fiscal 1991 was $59,000 of which nearly $20,000 was the cost of helicopter rental time. There has been little resprouting, but annual monitoring is necessary to remove any mangrove propagules that may have been newly brought into the pond through wave action.
Reference was made above to black-neck stilts, but these are not the only island birds that have been affected by changes in intertidal vegetation. Other endemic Hawaiian water foul are the duck (Anas wyvilliana), the coot (Fulica alai), and the moorhen (Gallinula chloropus sandvicensis), none of which nest or forage in mangroves; their nesting places have been compromised to greater or lesser extent by infestation of mangroves. As Allen (1998) pointed out, mangroves alone are not responsible for decline in water-bird numbers—hunting, predation by alien animals, disease, and hybridization with invaders have all had an impact—but they may well have made recovery more difficult. Added to this is the fact that two alien birds, black-crowned night herons (Nycticorax nycticorax) and cattle egrets (Bubulcus ibis), both of which prey on water-bird chicks also live in some mangrove colonies.