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Flowering Plants‎ > ‎
Kauila (Colubrina oppositifolia) – between a rock and a hard place
At the 2011 Hawai'i Conservation Conference, I had the good fortune to sit down with Talbert Takahama (DOFAW) between sessions and talk about one of his and my favorite trees, kauila or Colubrina oppositifolia. Talbert knows more about the small remaining O'ahu population (about 80 trees) of these once-common-now-endangered trees than anyone. He has visited, mapped, and collected seeds and cuttings (for propagation at the State's Rare Plant Facility at Pahole) from nearly every remaining wild tree. During our talk, he shared with me a recent and remarkable discovery he made regarding the trees' distribution in the Wai'anae Mountains. Using GPS, Talbert has mapped the locations of all the kauila he's visited over the years. To his surprise, he discovered that over the approximately two miles of the population present range the kauila plot out on a straight line at an elevation of 1,400 feet. Talbert described this two-mile elevation band as the lama (Diospyros) belt, a steep, rocky area difficult to access by even accomplished hikers. Above and below the lama belt are "rubbish forests" composed of Christmasberry, Java plum, silk oak, toona, guava and other invasive alien trees. I asked Talbert why he thought O'ahu's remaining kauila were today wedged (and still alive) between these two alien forests. The answer, he believes, is the inaccessibility of the lama belt. In the past, both feral cattle and loggers were able to easily reach and eliminate the native forests above and below the belt by using the gradually ascending ridges or gulches as highways. Fortunately, the steep rocky terrain and difficult access to the lama belt spared this native forest and the kauila living in it. So, what is the future for O`ahu's remaining wild kauila? Well, the very thing that saved them is also the thing that will make their preservation and management difficult. Erecting and maintaining field fences to keep out destructive pigs (that have no problem accessing the lama belt) will not be easy on such difficult terrain. Then, there is the constant rain of alien seeds from the surrounding forests. For right now, Talbert and other State botanists are planting out propagated kauila inside existing State enclosures as well as at public gardens. And, even though most of these sites are above or sometimes way below the magical 1,400 feet, most of these new kauila are doing quite well thank you.
(Top to Bottom) Ripe Colubrina fruits and flowers. Colubrina seedlings. A cluster of three 20-year-old Colubrina.
Habitat & Appearance: Colubrina is a dry to mesic forest tree with a maximum height of about 40 feet. Today, they are only found on O'ahu (Wai'anae Mountains), Maui (two trees, I'm told, in West Maui), and leeward Hawai'i. Colubrina is an attractive tree with its distinctive gray, puzzle-piece bark. Young leaves are glossy with pinkish veins and petiole; mature leaves are duller with yellow veins. The leaves have pimple-like glands on their lower surface. The small, light green, star-shaped flowers are clustered at the stem-tips. Colubrina fruits are described below.
Kauila in Hawaiian Culture: You would not mistake the leaf from a Colubrina for the leaf of an Alphitonia. And, if I gave you a mixed bag of Colubrina and Alphitonia fruits, you could easily sort them into separate piles. Why, then, did Hawaiians give these two distinct trees the same name? It was, of course, because of the wood. The wood of both Alphitonia and Colubrina is incredibly dense and heavy. So much so that a piece of kauila wood sinks in water. And, it was kauila wood, not the fruits or leaves, Hawaiians prized. From it, they made their finest tools and weapons, kāhili, game pieces, and figurines. Sometimes, hale beams were made of kauila. And, kauila pegs were favored for repairing wooden bowls (Krauss 1993). Even kauila splinters were not discarded but fashioned into hairpins (Lamb 1981). Lastly, both medicine and a dye were made from the bark and leaves (Krauss 1993).
Collecting Seeds: Colubrina fruits ripen from late spring through early fall. However, older fruits are often visible on the tree into the winter. And, if you're willing to spend some time, you can find good seeds littering the ground beneath a tree. The ripe fruits are greenish-brown to black, dry, hard, and about ½ inch in diameter. If the fruits are greenish-brown, place them in a paper bag or similar container for about one month to further ripen. Crack the ripe fruits open with pliers; the fruit normally separates into three capsules, each containing one brownish-black seed. Wait a day for the capsules to dry further since this will make it much easier to remove the seed from its chamber. Depending upon the source, nearly all to almost none of the seeds will contain viable embryos. Therefore, you should test the seeds prior to sowing or storage by placing them in water; seeds with well-developed embryos will sink, those without will float. Colubrina seed remains viable for at least five years if stored in the refrigerator.
Growing from Seed: Soak Colubrina seeds 1-2 days prior to scarring with sandpaper or a nailclipper. While soaking, some of the seeds will swell to about twice their dry-size. Directly sow these without scarring. Using Method One, the seeds take 1-4 weeks to germinate. Occasionally, seeds will swell, the seedcoat will split open, but the embryo does not germinate (i.e., there is no root growth). After 1-2 weeks these embryos become putrid. Why this happens is not clear. Perhaps, these embryos are genetically defective, the result of inbreeding. Or, it could be a disease that only attacks/afflicts some of the seeds in the germination tray. Very young seedlings (prior to true leaf development) are also susceptible to a damping-off fungus that attacks the young stem causing it to blacken, shrink, and fail (to support the cotyledons). If detected early, you can save the seedling by painting the stem with a sulfur paste; a cotton swap works well as a paintbrush. Painting seedling stems with sulfur as a preventative to this disease is an option since the sulfur doesn't seem to harm normal, healthy seedlings. Seedling growth is good (six inches in about three months), particularly, if you use a liquid fertilizer once or twice a week. Colubrina seedlings are susceptible to broad mites; leaf loss is a common symptom. Eliminate the mites, invisible to the naked eye, with repeated treatments of sulfur (see Enemies in the Garden). I normally plant out Colubrina seedlings when they reach 8-10 inches in height.
Growing from Cuttings: I have occasionally grown Colubrina from cuttings using Method Two (mist system) but the resulting plants sometimes do not grow as straight as those grown from seed and are prone to falling over. The cuttings take 2-4 months to root.
Growth in the Garden: Planted in full sun, Colubrina grow at a moderate rate of about one foot a year. Young trees (less than ten years old) are shrub-like with many side branches while older Colubrina develop into proper-looking trees with one or two main trunks. I've often been tempted to prune away the side branches of a young Colubrina to discover if this would hasten vertical growth and a widening of the main truck – but I've always lacked the courage. Colubrina begin flowering and fruiting after only a couple of years (i.e., two to three feet in height). However, these early fruits often contain nonviable seeds.
Diseases & Pests: The greatest threat to Colubrina in the garden is the black twigborer. This pest can be fatal to young plants of two years or less. After this, the borer cannot deliver a fatal hit to the main truck but rather slows the growth of the tree by killing its numerous thinner branches; this damage also makes the tree unsightly. Several factors regulate the amount of damage caused by the black twigborer to Colubrina. First there is the plant's location. Expect the most damage in damp, shady, windless sites, the least in dry, sunny, windy locations. Second is the health of the tree. Occasionally watering your Colubrina to avoid drought-stress will reduce or eliminate twigborer attacks. However, do not overwater your plant or you're likely to face new problems from unfriendly soil microbes. Third is the prompt removal of infested branches. Twigborer larvae within the dead branch take time to mature and then spread to new healthy branches. If you prune away and dispose of the infested branches quickly you can break the cycle of infestation. Disposing of the branches means taking them far offsite – don't just toss them onto your compost pile.