Associated Forest Cover
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Tabonuco dominates a forest association known locally as the
tabonuco type (21). In the French West Indies, the association is
described as "forest hygrophytique" (20), and Beard
described it as lower montane rain forest, or Dacryodes-Sloanea
(1). These fall within the broader categories of Tropical or
Subtropical Wet Forest life zones (7).
In Puerto Rico this association averages about 50 tree species per
hectare (50/2.5 acres) larger than 10 cm (3.9 in) in d.b.h. In
the Luquillo Mountains close associates include motillo (Sloanea
berteriana), palma de sierra (Prestoea montana), yagrumo
hembra (Cecropia peltata), yagrumo macho (Didymopanax
morototoni), and caimitillo verde (Micropholis
garcinifolia) (21). The forest type has been described in
detail (2,15,21). Characteristics of tabonuco stands include a
rich mix of 170 tree species in primary and secondary stands (23)
with dominants of Sloanea berteriana, Guarea guidonia, and
Manilkara bidentata. The relative density of seedlings to
that of canopy trees approximates a ratio of 8 while that of
saplings is 0.4 (19). On the average, the stand contains 116
trees per hectare (47/acre) 10 to 15 cm (3.9 to 5.9 in) in
d.b.h., 100 to 150/ha (40 to 61/acre) larger than 30 cm (11.8 in)
in d.b.h., and 63/ha (25/acre) larger than 50 cm (19.7 in) in
d.b.h. with a total biomass of 424 t/ha (156 tons/acre) of which
33 percent is in tree boles (14,23). The basal area of the
average stand approximates 40 to 46 m² /ha (174 to 200 ft²/acre)
and the volume approximates 300 to 345 m³/ha (4,285 to 4,930
ft³/acre) (2). The diurnal gross primary production of a
stand with a leaf area index of 6 to 7 in Puerto Rico is reported
to average 16 grams of carbon per square meter of ground area
(0.052 oz/ft²) (14).
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Climate
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Tabonuco in Puerto Rico is found within a mean temperature range
of 21° to 25° C (70° to 77° F) and a mean
annual precipitation range from 2000 to 4000 mm (79 to 157 in).
Precipitation is generally abundant except from February through
April, when it may drop to about 75 mm. (3 in) per month. At El
Verde, on the northwestern slope of the Sierra de Luquillo at an
elevation of 420 m (1,380 ft), well within the tabonuco forest
range, data collected over 3 years showed the following: mean
temperature, 22.60 C (73° F); mean absolute humidity, 18.7
g/m² (0.02 oz/ft²); mean relative humidity, 91 percent;
mean daily insolation, 383 gcal/CM² (383 ly) mean daily pan
evaporation, 1.8 mm (0.67 in); mean wind velocity, 4.2 km/h (2.6
mi/h); and mean annual rain
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Damaging Agents
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The success of tabonuco in the mountains
of the West Indies speaks for its capability for dealing with
hurricanes. Many trees do not survive, and many of those that do,
suffer crown breakage and subsequent heart rot. Despite this,
most of the mature trees in Puerto Rico have sound butt logs. A
few trees exhibit bark swelling and cankers that provoke an
abundance of resin exudation. The pathogen is unknown. Abortion
of the fruits is a common and possibly an important phenomenon
whose causes are unknown. The significance of fertile seed
removal by parrots, other vertebrates, and invertebrates is not
known either.
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Flowering and Fruiting
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Male and female flowers appear on
different trees, making this a dioecious species. Flowers are
greenish, about 4 mm (0.16 in) across, and develop in lateral,
much-branched panicles. The fruit, a one-seeded oblong-ovoid
drupe about 19 mm (0.75 in) in diameter, has a corrugated surface
when dry (19). The seed is fleshy, with folded cotyledons (5,17).
Flowering peaks between May and November, with most fruit falling
from October to December. There is some flowering and fruiting at
other times and considerable annual variation. Empty fruits
commonly fall earlier than those that are fertile. At one period
during the autumn of 1963, two-thirds of the crop was composed of
empty fruits, although a second smaller crop of viable seeds
appeared later. Gamma radiation stress reportedly has led to
earlier and increased fruit fall (6). Fruits are generally found
in abundance beneath the crowns of the parent trees.
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Genetics
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Wood cutters in Puerto Rico have recognized two races of tabonuco
based on the degree of red color and other visible properties of
the wood. Other variations, such as the shape of the fruit, have
been observed. The genetic significance of these traits, if any,
is unknown. Dacryodes excelsa has a nuclear volume of
52.6 W (9) and shade leaves appear to contain less DNA than sun
leaves, 590 compared to 715 Mg/g (3).
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Growth and Yield
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Mature tabonucos in Puerto Rico may
reach 35 m (115 ft) in total height and 180 cm (71 in) in d.b.h.
The age of large trees is estimated at up to 400 years.
Unpublished preliminary volume tables exist for the species, and
a composite table for tabonuco type forest has been published
(2). Because of the mixed nature of the forests in which tabonuco
is found, growth and yield of tabonuco per unit of forest area
are somewhat meaningless.
Diameter growth of individual trees is most rapid on slopes and
unexposed ridges and for specimens that are large in size and
dominant in crown position (0.15 cm/yr, 0.06 in/yr, for dominant
crowns in mature stands vs 0.10 cm/yr, 0.04 in/yr, for suppressed
crowns). Growth rates are slow when compared to planted exotics,
but what the potential growth of tabonucos would be if planted
under comparable conditions is not known.
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Reaction to Competition
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Overall, tabonuco may be classed
as intermediate in tolerance to shade. Seedlings are very
tolerant. Only a tiny proportion of the seeds that fall produce
trees that reach maturity. Competitive factors include light,
moisture, mechanical damage resulting from the falling of litter
or other trees, and smothering by vines. For best survival,
seedlings need to be free of competition until they are at least
3 or 4 m (9.8 to 13 ft) tall.
Growth rates of established trees can be accelerated if the trees
are released from competition (i.e., from 0.15 cm/yr, 0.06 in/yr,
in mature stands to 0.66 cm/yr, 0.26 in/yr, in cutover stands).
However, even mature trees will suffer shock if severely exposed
to the intensive rays of the sun, due in part to scalding of
previously shaded bark. Trees severely isolated in residual
stands after logging have been observed to exhibit crown
deterioration and will be lost within 10 years.
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Rooting Habit
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The observed preference of tabonuco for
upper slopes and ridges and its obvious successful survival of
frequent hurricane winds that break crowns rather than uprooting
trees point to a strong and presumably deep root system. Root
grafts among trees of this species are common and can be seen in
stumps that remain alive and continue to develop callus over the
cut surface for decades. Anywhere from 10 to 20 individual trees
can form a union as a result of root grafts. Dense and deep
root-mats develop near the bases of some large trees, presumably
where unusually large amounts of litter are trapped on the
surface.
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Seed Production and Dissemination
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Of the fruits that
fall, those dark in color were found to be heavier, with 60
fruits per kilogram (27/lb) compared to 73 per kilogram (33/lb),
and more viable, up to 22 percent compared to up to 5 percent,
than green fruits. Completely developed fruits tend to sink when
immersed in water. Of those that float, most are hollow, although
as many as 33 percent may still be viable. The period of
viability under natural conditions is short but not precisely
known. A few seeds are known to have survived 5 weeks storage
exposed to the air.
Germination is epigeal and takes place within a few days under
humid, shady conditions. Under direct exposure to the sun,
germination is less than in the shade, 11 percent compared to 28
percent in one test.
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Seedling Development
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In nature the seedlings grow best on
steep slopes under low light intensities (0.07 to 0.05 gcal/cm².
min or 0.07 to 0.05 ly). They are fragile and root within the
litter layer. At an age of 4 months they attain an average height
of 16 cm (6.3 in) and typically have six leaves. In some areas
there may be as many as four seedlings per square meter (0.4/ft²).
Few seedlings grow beyond this stage, only half surviving beyond
8 months. Continued growth of the remaining seedlings under
intermediate light intensities is slow. Net photosynthesis is low
(60 mg of carbon per m².h or 0.08 gr/ft².h), and the
ratio of net photosynthesis to respiration is 0.78 (12).
In the nursery it has been found that seedlings during their first
month will not withstand direct exposure to the sun. Under shady
conditions seedlings attained 22 cm (8.7 in) in 6 months. Like
many other large-seeded tree species, tabonuco does not survive
bare root transplanting. In contrast, 85 percent survival has
been attained by the use of containers with an intact ball of
earth about the roots. At 9 years, trees were 3.0 to 4.3 m (10 to
14 ft) in height and 3.8 to 5.0 cm (1.5 to 2.0 in) in d.b.h.
(13).
Direct sowing in the forest has yielded germination as high as 33
percent but subsequent problems such as leaf fall, drought, and
vines precluded successful establishment, so planting is
recommended.
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Soils and Topography
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Tabonuco grows on deep, red, acid (pH 4.5 to 5.5), clay soils
(Ultisols such as Los Guineos and Humatas) derived from igneous
rock. Typically these soils are stony, often with large boulders,
and internal drainage is good. Large tabonuco trees tend to be
concentrated on upper slopes and ridges, where they may form
nearly pure groups whose roots are grafted, thus forming a tree
union or clumps of trees. Presumably this reflects the better
drainage of soils at such locations or the superior anchorage
against hurricane winds that the prominent boulders may offer.
Significantly superior diameter growth rates of tabonuco on
ridges as compared to swales have been reported (22).
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Special Uses
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Tabonuco wood is used for all types of furniture, cabinet work,
interior trim, general construction, and carpentry. The wood is
also useful for crates, boxes, shingles, and small boats. It is a
substitute for mahogany in a variety of uses. The wood itself is
moderately heavy, with a specific gravity of 0.53 (19). It
air-seasons easily and satisfactorily, undergoes moderate and
uniform shrinkage during seasoning, and holds its place well
after manufacture. Tabonuco is a moderately good machining wood;
it cuts and saws easily but will dull saw teeth due to its high
silica content. Tabonuco lumber yields good surfaces when planed,
sanded, mortised, or shaped (turning and boring are more
difficult). It is easily glued, holds nails well, takes stain
well, and finishes beautifully with varnish or lacquer. The wood
is only slightly resistant to decay, lasting 3 years or less in
the ground, and is difficult to impregnate with preservatives by
either pressure or nonpressure methods (11,12,18). Tabonuco wood
compares favorably with mahogany and birch (24).
Early settlers used the resin of tabonuco for making candles and
torches, for caulking boats, for incense, and for medicinal
purposes. The endangered Puerto Rican parrot feeds on tabonuco
seeds.
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Vegetative Reproduction
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There is no evidence of
vegetative reproduction.
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Distribution
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Tabonuco is native to elevations from 200 to 900 m (660 to 2,800
ft) throughout Puerto Rico. On favorable sites, it may make up 35
percent of the basal area and 80 percent of the timber volume of
the forest, commonly termed Dacryodes-Sloanea association
(1). From Puerto Rico, the native range of tabonuco extends into
the Lesser Antilles on St. Kitts, Montserrat, Guadeloupe,
Dominica, Martinique, St. Lucia, St. Vincent, and Grenada, a
gross forest area of about 2300 km² (888 mi²) (24).
Other members of the genus are in South America and Africa (5,8).
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Brief Summary
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fornecido por Silvics of North America
Burseraceae -- Bursera family
Ariel E. Lugo and Frank H. Wadsworth
Tabonuco (Dacryodes excelsa), also known as gommier and
candlewood, is the dominant large tree of the native forests that
formerly covered the northern lower and middle slopes of the
mountains of Puerto Rico. It is distinguished by broad low
buttresses, a columnar bole, smooth gray bark, and pinnately
compound leaves with five to seven fragrant, dark-green leaflets.
When wounded, the tree exudes a clear, fragrant inflammable resin
that hardens and turns white on exposure.
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