NOTES 
Adventitious Roots of Eucalyptus robusta in Hawaii 
Ronald M. Lanner 1 
Adventitious rooting on the trunks and 
limbs of Eucalyptus robusta Sm., a species 
widely planted in Hawaii, is a remarkable 
example of how a new environment can change 
a plant’s growth habit. E. robusta is native to 
swampy areas on the coast of southeastern 
Australia, where annual rainfall ranges from 40 
to 60 inches (Forestry and Timber Bureau, 
1957). Layering of prostrate stems sometimes 
occurs there, but adventitious roots on standing 
trees seem to be unknown. 2 
ORIGIN AND GROWTH OF THE ROOTS 
Newly initiated roots may grow through the 
thick fibrous bark into the open air, or they 
may grow downward while remaining within 
the bark. LeBarron (1962:18) reported that 
these roots can extend to the ground separate 
from the trunk. In all cases I have observed, 
roots that grew into the open died back when 
only a few inches long. I tried to trace several 
of these roots by dissecting trunk sections at 
the point where a root appears, but in all cases 
the root traces disappeared at a point outside 
of the trunk pith. 
Typically, the young roots remain inside the 
bark, branching freely. Eventually they enter 
the soil at the base of the tree without having 
had their growing points exposed to the air, 
except for brief periods when they grew out 
of a ridge of bark, across a fissure, and into an 
adjacent ridge. 
Growing roots thicken and eventually burst 
through the bark. This process, aided by gradual 
sloughing of the outer bark, places the adventi- 
tious roots outside the trunk (Fig. 1), but by 
1 Pacific Southwest Forest and Range Experiment 
Station, Forest Service, U. S. Department of Agricul- 
ture, Hilo, Hawaii. Present address: School of For- 
estry, University of Minnesota, St. Paul, Minnesota. 
2 Personal correspondence with M. R. Jacobs, For- 
estry and Timber Bureau, Canberra, Australia, Sept. 
23 , 1963 . 
then the roots are protected from dessication by 
their own bark. In extreme cases, large roots 
may completely conceal the trunk. 
OCCURRENCE OF ADVENTITIOUS ROOTS 
Stands in which trees commonly bear adven- 
titious roots are generally in wet climates. Thus, 
these roots are common in Hilo, Piihonua, and 
Mountain View, where median annual rainfall 
is 147, 210, and 195 inches, respectively (Talia- 
ferro, 1959), and comparatively uncommon at 
Waikii, Honokaa, and Laupahoehoe (m. a. r. 
23, 88, and 100 inches, respectively). At 
Waimea and Puu Kapu (m. a. r. 39 and 48 
inches) frequent fogs compensate for low rain- 
fall and permit adventitious roots to persist. 
Within a closed stand, large adventitious 
roots are found mainly on "wolf” trees and 
border trees. Open-grown trees with massive 
spreading limbs generally have the largest ad- 
ventitious roots (Fig. 1). In contrast, small- 
Fig. 1. Strongly developed adventitious roots 
emerging from crotches of open-grown E. robusta 
near Kurtistown, Hawaii. 
379 
