Monday, January 13, 2014

Ohia Decline on the Big Island of Hawaii


This is a phenomenon that has been observed as far back as 1906. In the late 1960’s 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

The decline is complex and not totally understood. We do know that older trees are not as vigorous as they once were and are more susceptible to stress. The stress may come as a prolonged period of rain or the opposite, drought; poor draining soils will compound the problem. Other factors may include vog (volcanic emissions), 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.   

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

What can be done?  Actually, not much.
·        If the property contains several older ohia trees, planting young ones will diversify the age group.
·         Keeping the trees healthy 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. 

Photos by Forest and Kim Starr