To begin with, there are four distinct species of ohia tree that are endemic to Hawaii; there are also eight different varieties. But in addition to genetics, there are environmental factors which will cause physical variations in the trees. Elevation, rain, and temperature all have an effect upon these trees.
With trees, in general, the shapes of leaves from trees growing in hot, dry deserts are often small in order to conserve water loss through the leaf. Contrast this to the large-leafed plants of the tropics. Ohias occur in a wide range of habitats, from just above sea level to 8,200 feet; from dry forests with less than 16 in annual rainfall to wet forests with more than 33 feet of annual rainfall.
This along with its genetic propensity for variation will produce trees of diverse appearance. The species name, polymorpha actually means many forms. Thus tree height and form, leaf shape and flower color are highly variable.
Concerning their care, ohia trees can be damaged and even killed from a lack of water. During periods of drought especially, supplemental watering will be needed. Trees growing in the pahoehoe lava (exhibiting a smooth, billowy, ropy surface) may do well when young and water requirements are low. Nevertheless, as trees increase in size, if adequate water is not supplied, they can dry out and even die. Mulch will help, but of course, not piled against the trunk.
Since ohias are adapted to grow in low nutrient soils, beware of over-fertilizing. Slow-release fertilizers or natural compost is best.