Associated Forest Cover
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'Ohi'a lehua grows in association with many other trees in a
variety of forest types but not in any classified by the Society
of American Foresters. In rain forests, it is often associated
with koa (Acacia koa), and species of'olapa (Cheirodendron),
treefern (Cibotium), pilo (Coprosma), manono (Gouldia),
kawa'u (Ilex), kolea (Myrsine), 'alani (Pelea),
guava (Psidium), and kopiko (Psychotria)
(2,8,13). In drier habitats, 'ohi'a lehua is commonly
associated with lama (Diospyros), akoko (Dracaena),
wiliwili (Erythrina), naio (Myoporum), olopua (Osmanthus),
'ohe makai (Reynoldsia), mamane (Sophora), hame
(Antidesma), and maua (Xylosma). In many
habitats, 'ohi'a lehua is the most common tree.
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Climate
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Rainfall and associated cloud cover over the range of 'ohi'a lehua
vary considerably. Mean annual precipitation varies from 500 mm
(20 in) to greater than It 400 mm (450 in). Mean annual
temperatures range from 24° C (75° F) to 10° C (50°
F). Seasonal variation in mean monthly temperature probably does
not exceed 5° C (9° F). Frost and occasional ephemeral
snow occur at higher elevations. Relative humidity commonly
averages 70 to 80 percent in windward areas (exposed to northeast
trade winds) and 60 to 70 percent in leeward areas.
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Damaging Agents
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Many insects attack 'ohi'a lehua trees.
Among these, the endemic cerambycid borer Plagithmysus
bilineatus has the greatest potential impact. It may become
epidemic and fatal to weakened trees and is associated with
extensive canopy dieback. Environmental stresses are significant
in reducing 'ohi'a lehua vigor and predisposing the trees to
attack by R bilineatus (14). Other potentially damaging
borers are Ceresium Unicolor, Xyleborus saxesensi, and X.
simillimus. Defoliators and sapsucking insects also cause
minor injury to 'ohi'a lehua.
The root rots, Phytophthora cinnamomi (14) and Pythium
vexans, and the shoestring root rot, Armillaria mellea,
can be locally damaging and also are associated with canopy
dieback. Damping off caused by Rhizoctonia spp. also has
been reported (6).
Decline of 'ohi'a lehua canopy has been the subject of
considerable research since 1975 showing that the phenomenon is
most likely characteristic of the species. The loss of ability to
withstand environmental stresses, diseases, or insect attacks is
apparently synchronous among trees within populations. Entire
stands of approximately equal age trees may die back to a few
remanents (1,10,11).
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Flowering and Fruiting
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Flowering generally peaks in
spring or summer after vegetative flushing, but some varieties or
populations peak in fall or winter. Individual trees or branches
may produce flowers at any time during the year. The red, salmon,
pink, or yellow perfect flowers are arranged in a dense terminal
cymose corymb. The stamens are long and numerous and the flowers
are quite showy. Endemic Hawaiian birds (Drepanididae) and
insects are the most important pollinators of 'ohi'a lehua. The
inflorescence normally has 18 to 24 flowers in different
developmental stages. Fruit maturation takes 4 to 12 months (15).
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Genetics
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Taxonomists recognize 11 varieties of Metrosideros polymorpha
(16,17,18). Only M. polymorpha var. Prostrata
does not attain tree stature. Intrapopulation variability of
many morphological characters is large and some vegetative
characteristics vary clinally with altitude (6,7).
The distinction between races (ecotypes) and varieties in 'ohi'a
lehua is not clear. Altitudinal (7), edaphic, and successional
(13) ecotypes have been proposed in this variable species. Some
ecotypes or varieties appear to be pioneer plants in primary
succession. On Mauna Loa, an active volcano, the species is found
up to 2,500 m elevation, but on nearby Mauna Kea, a volcano
extinct since the Pleistocene, the upper limit is about 1,650 m.
On older high islands, the species seems to be limited to
continuously moist rain forest environments (12). Morphology of
the varieties also differs; those with pubescent leaves are
apparently pioneering forms of the species and glabrous leaf
varieties are found in later successional stages (19).
Intraspecific hybridization has been demonstrated in 'ohi'a lehua
but there is some evidence of partial incompatibility (6).
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Growth and Yield
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'Ohi'a lehua is a relatively
slow-growing tree. In one study (unpublished), saplings and
mature trees in stands rated as commercial 'ohi'a lehua forest
had a mean annual d.b.h. increment of 0.25 cm (0.10 in).
On good sites, 'ohi'a lehua commonly grows to 20 m (65 ft) and 45
cm (18 in) in diameter. Trees 30 m (100 ft) tall exceeding 120 cm
(48 in) in diameter have been found.
'Ohi'a lehua grows in both pure and mixed stands. Stand basal area
can exceed 40 m²/ha (175 ft²/acre) in pure stands on
good sites. Pure stands probably are the result of disturbance.
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Reaction to Competition
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Shade tolerance of 'ohi'a lehua
ranges from intolerant to intermediate, depending on varietal
differences (3,13).
A pioneer species on young volcanic substrates (17), 'ohi'a lehua
retains dominance on some relatively old soils. Acacia koa
is its primary competitor for canopy dominance in wet
forests. Cibotium spp. (treeferns) may displace 'ohi'a
lehua on those sites with optimal conditions for treefern growth
(2,13).
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Rooting Habit
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No quantitative studies are available
concerning the rooting habit of 'ohi'a lehua. Most roots
apparently are near the surface. On deep soil and broken lava
some deep woody roots may be formed.
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Seed Production and Dissemination
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Little is known about
the age at which the trees begin to bear seeds or the number of
seeds produced. Many small lightweight seeds may be produced per
capsule, but many of these are infertile (9). Seed germination is
best with newly produced seeds and varies from less than 1 to 78
percent (6). In a nonrandom sample of 142 trees more than 90
percent had germination rates less than 35 percent. In one study,
maximum germination of 'ohi'a lehua seed was obtained at 25°
C (77° F) and 4 to 15 percent full sunlight (2). Seeds
remain viable for as long as 9 months if stored at room
temperature (5).
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Seedling Development
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Germination is epigeal. Many seeds
germinate on downed or upright treeferns and downed moss-covered
trees. More than 70 percent of the seedlings in the rain forest
habitat grow on these substrates. In one 'ohi'a-treefern
community a late summer peak in the appearance of 'ohi'a lehua
germinants has been documented (2).
Seedling growth rates are relatively slow. In one study, seedling
height growth averaged less than 10 cm (4 in) per year. Survival
and growth of seedlings established in shade reached a maximum at
less than full sunlight after varying degrees of canopy removal
(2).
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Soils and Topography
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'Ohi'a lehua grows on many different soils and sites. It is
abundant on Histosols and Inceptisols over gently sloping recent
to Pleistocene lava flows on the geologically younger volcanoes.
It also is known to grow on soil associations within the soil
orders Histosols, Mollisols, Spodosols, Oxisols, Ultisols, and
Alfisols and on unclassified mountainous land on the geologically
older volcanoes of the Hawaiian archipelago.
The species develops best on relatively level welldrained sites.
On exposed ridges, steep slopes, or poorly drained sites,
however, 'ohi'a lehua does not reach large size and may be
reduced to dwarf shrub stature.
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Special Uses
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'Ohi'a lehua provides valuable watershed protection in Hawaii. It
is also an important source of nectar and insect prey of most
native birds. Among these birds are some endangered species, the
ãkepa(Loxops coccinea), the crested honeycreeper
(Palmeria dolei), and several species of Hemignathus.
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Vegetative Reproduction
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'Ohi'a lehua often reproduces
vegetatively from stem sprouts on fallen trees. The stem sprouts
can produce adventitious roots and eventually become independent.
Stem sprouts on a standing weakened or dying tree may outlive the
parent tree and grow to maturity. Planting stock can be produced
from at least 60 percent of new-growth cuttings within about 6
months compared with up to a year to produce seedlings (4).
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Distribution
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'Ohi'a lehua is a variable and unusual tree found from just above
sea level to 2600 in (8,500 ft) as a tree or small shrub on six
of the high islands of the State. 'Ohi'a lehua does not grow in
coastal areas with rainfall less than 500 mm (20 in). The species
reaches its maximum stand basal area on young volcanic substrates
in rain forest habitats on the Island of Hawaii.
-The native range of 'Ohi'a lehua.
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Brief Summary
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Myrtaceae -- Myrtle family
Ken Adee and C. Eugene Conrad
'Ohi'a lehua (Metrosideros polymorpha) is the most
abundant and widespread tree in Hawaii. This slow growing native
hardwood seeds freely and often starts as an epiphyte in fern
forests. It is the first tree to appear on new lava flows where
it offers watershed protection. The wood is of fine even texture
and takes a good polish. It is used for flooring, fenceposts, and
fuel. This tree provides important habitat to native birds,
several endangered.
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