PHYTOGEOGRAPHY OF MANOA VALLEY, HAWAIIAN ISLANDS 593 
i8. THE ZONE OF KOA AND LEHUA 
Directly above the kukui zone and commingling with it along 
its upper limits is the zone dominated by the koa, Acacia Koa, and the 
lehua, Metrosideros polymorpha. Originally the koa was much more 
abundant than it is at present; at this time practically all of the large 
koa has been cut or killed and the trees which remain are only of 
medium stature. The lehua is the most abundant tree in the Manoa 
forests, and in the forests of the archipelago as a whole. Both it and 
the koa attain their optimum development on the island of Hawaii, 
particularly in the region of Puna and Olaa. In these districts trees 
of 75-90 feet are not uncommon; in Manoa the average height is 
35 feet. 
The zone of koa and lehua does not have as sharp horizontal boun- 
daries as do some of the other plant zones. The koa thrives in Manoa 
at elevations as low as 50 feet and was at one time fairly plentiful in 
the valley floor, in districts from which it has been absent for the 
last fifty years. The upper limit of the koa is also somewhat indefinite, 
averaging 1,200 feet, but sometimes rising to nearly 1,800 feet. On 
the island of Hawaii the finest stands of koa occur at elevations of 
4,000 to 5,000 feet. The lehua occurs scattered throughout the Manoa 
rain-forest, particularly along the ridges, and ascends the highest peaks. 
On the island of Hawaii it rises to a height of 9,000 feet. 
I 19. DISTINCTIVE FEATURES OF THE MANOA RAIN-FOREST 
1. The forest flora is composed almost wholly of arborescent, 
shrubby, or woody species. Most of them are endemic and many are 
confined to the island of Oahu. There are no gymnosperms. 
2. The average stature of the trees is about 25 feet; many do not 
exceed 20 feet. The more stunted forms occur on the steep slopes 
and ridge crests; along the floors of the ravines the trees may rise to 
heights of 30 to 40 feet. 
3. Most of the shrubs are tall and semi-arborescent in character; 
it is difficult to discriminate between the two habits. 
4. The substratum is a thin layer of stiff, red soil, derived from the 
basaltic lavas which directly underlie it. This soil is continuously 
wet, and is exceedingly tenacious of its water. It contains very little 
organic matter, owing to the steepness of the slope and the rapidity 
of the erosion. 
