Ohia trees are dying rapidly, Why?

[Midnight Rambler]

The thing about ohia is that although it has not formed distinct species (except for a few on the older islands, and even those may be able to hybridize), there are many varieties that are optimized for different habitats. That's a big part of what has allowed it to live in so many different places, from the wettest bogs and rain forests to the near-deserts on the leeward side and at 8000 feet on Mauna Loa. On the Big Island especially, since the conditions are constantly changing as the soil/rock ages, one variety gets succeeded by another. You can see this easily on the Saddle Road where there is a hairy-leaved variety on the newest lava flows and a hairless variety on older flows with developed soil. They grow close together and you can find some intermediate hybrids between them.

The nutrient balance, water level, soil permeability, etc. change over time; and as they do, the habitat can become unfavorable to the point where the ohia die. When this happens, you get a dieback event, affecting a large cohort of trees of the same variety and similar age. They then get replaced by another cohort, better adapted to the new conditions. The area affected by dieback on the windward side of Mauna Kea in the 1970s has regrown considerably, though the large number of dead trunks still visible make it look bad.

This, of course, is the natural cycle. There are a lot of human-caused disturbances that can alter it - pigs, introduced pathogens, climate change, etc.