Ohia trees dying mysteriously


Thank you so much for your wonderful weekly article! Why do my ohia trees in Kalapana keep dying? Just here and there, in the surrounding forest, sometimes just a branch. They just turn brown and die. There are a lot of little brown beetles with long noses in the area.

This is a phenomenon that has been observed as far back as 1906. In the late 1960s, a major decline of ohias took place. In 1986, the USDA Forestry Service published a lengthy article on the subject http://www.fs.fed.us/psw/publications/documents/psw_gtr086/psw_gtr086.pdf

Recently, a number of calls have come into the CTAHR office reporting this same occurrence.

What you are seeing is not the result of a particular insect pest or fungal organism coming into the area and destroying the ohias. The decline is complex and not totally understood. Here is a possible scenario:

A catastrophic event occurs, a fire, lava flow, or perhaps a hurricane and wipes out a stand, a group, of ohias. New seeds are blown in from adjacent stands, and a new forest begins. The point here is that all the trees are of the same age. These trees grow up and become old. Old age itself does not kill the trees, but in this condition, they are not as vigorous as they once were and are more susceptible to stress factors.

Eventually, a stress will come such as a prolonged period of rain; poor draining soils will compound the problem. Another stress is long term drought conditions. Other factors may include vog, low soil nutrients, bulldozers and perhaps dense stands of invasive species. These stresses alone may cause some type of decline. But what usually happens in the next phase is the invasion of a root rotting fungus or perhaps a tree boring beetle. Fungal organisms are often found infecting the roots of declining trees but are not thought to be the primary cause.

Ohia trees do not tolerate waterlogged conditions. Dieback is especially common in high rainfall areas with underlying heavy clay soils or pahoehoe lava. Tree decline around home sites may come as a result of some type of mechanical tree injury, chiefly the bulldozer. Other than direct injury, bulldozers are a tremendous weight and will compact the soil, especially when it is wet.

Young ohia trees can grow well for many years on shallow soils. Eventually, decline may set in when they become large trees, and the shallow soil simply cannot support their growth, particularly under drier conditions.

Recently, the high rainfall received in the fall/winter generated a stressful environment, i.e. a saturated soil, which was favorable for the development of root rotting fungi. Then, with a diminished root capacity, the trees began the year under dry weather conditions — a double hit and one causing much decline.

One last note: younger, healthy ohia trees have been observed growing in areas where older trees are declining. This indicates that it is not a root rotting fungus alone that is killing the trees, but rather other factors, such as age and water logged conditions, are involved.

For more information on ohias and other native trees go to traditionaltree.org

What can be done?

Not much. If the property contains several older ohia trees, plant some young ones to diversify the age group. Keeping the trees health is important; fertilize if needed. It is impractical to water a stand of ohia trees during a drought, but watering a few trees around the house may be feasible.

Mahalo to Brian Bushe and JB Friday who are with UH CTAHR for the information they have provided.

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In regards to last week’s article about bananas, I may have misunderstood part of one question. With respect to the flesh of bananas being tough even when ripe, the writer of that question may have been referring to one of the cooking bananas called plantains. Their flesh remains bland with a starchy texture even though the peel has turned yellow. I will discuss plantains in a future article.

Hilo resident Nick Sakovich is a professor emeritus of the University of California. Email your questions to Sakovich at askthegardenguy@earthlink.net.