PHYTOGEOGRAPHY OF MANOA VALLEY, HAWAIIAN ISLANDS 579 
8. THE TALUS ZONE 
Between the valley floor and the valley walls lies an intermediate 
physiographic and floristic zone, which may be designated as the 
talus zone. This zone comprises, as its name indicates, the talus 
deposits at the foot of the valley wall and resting upon the floor. It 
varies in width from approximately 100 to 1,000 "feet. The slope 
averages 10-15°, as contrasted, on the one hand, with the 40° slope 
of the wall, and, on the other, with the nearly flat floor. The soil of 
the talus zone varies considerably in nature in various parts of the 
valley — in some places it is fine-grained lava soil; in others, coarse 
volcanic ash and cinders; and in others the ground is littered with 
massive boulders that have been dislodged from the heights above. 
It is probable that the talus slope is not, in cross-section, wholly com- 
posed of talus; the surface layers are of debris, and below them are 
the ancient lava-sheets of the valley walls. This situation is revealed 
by the little streamways that are cut through the talus zone. 
The dominant plant of the Manoa talus zone is the guava, Psidium 
Guayava. Secondary species are: Lantana Camara, Paspalum con- 
jugatum, Andropogon contortus, Verbena Bo?iariensis, Psilotum nudum, 
Morinda citrijolia, Nephrolepis exaltata, Solamcm- sodomeum, Con- 
volvulus spp.. Cassia occidentalis, Opuntia megacantha, Waltheria Amer- 
icana, etc. The talus zone, like the valley floor upon which it rests, is 
covered almost exclusively with ruderal vegetation. Arborescent 
forms are infrequent; vigorous and drought-resistant herbaceous- 
woody shrubs are the prevailing types. 
In primitive times the talus zone of the upper valley was com- 
pletely clothed with native trees, the species being those of the lower 
forest zone. The forest retreated before the incursions of man and 
wild live-stock, and exposed the talus zone to the invasions of foreign 
vegetation. The hilo grass {Paspalum conjugatum) has been notably 
pernicious, as it forms a dense sod and efl'ectually prevents the native 
species from reseeding themselves. 
The talus zone of the lower valley probably has been always more 
or less xerophytic in character. Many of the indigenous or endemic 
xerophytes of Hawaii have become extinct or are now upon the verge 
of extinction. This condition is pronounced in several leeward 
localities on the various islands — Hawaii, Maui, Molokai, Kauai — • 
and undoubtedly obtained in Manoa. 
