Vahl Tabonuco Bursera family

Ariel E. Lugo and Frank H. Wadsworth

Tabonuco (Dacryodes excelsa), also known as gom- mier 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 com- pound 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.

Habitat

Native Range

Tabonuco (fig. 1) 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 as- sociation (1). From Puerto Rico, the native range of tabonuco extends into the Lesser Antilles on St. Kitts, Montserrat, Guadeloupe, Dominica, Martini- que, St. Lucia, St. Vincent, and Grenada, a gross forest area of about 2300 km* (888 mi*) (24). Other members of the genus are in South America and Africa (5,s).

Climate

Tabonuco in Puerto Rico is found within a mean Figure 1-A mature tabonuco showing spreading crown. 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 abun- fall, 3700 mm (146 in). Diurnal variations are dant except from February through April, when it smaller than some seasonal changes (16). may drop to about 75 mm (3 in) per month. At El Verde, on the northwestern slope of the Sierra de Soils and Topography Luquillo at an elevation of 420 m (1,380 ft), well within the tabonuco forest range, data collected over Tabonuco grows on deep, red, acid (pH 4.5 to 5.5), 3 years showed the following: mean temperature, clay soils (Ultisols such as Los Guineos and 22.6” C (73” F); mean absolute humidity, 18.7 g/m3 Humatas) derived from igneous rock. Typically these (0.02 oz/ft3); mean relative humidity, 91 percent; soils are stony, often with large boulders, and inter- mean daily insolation, 383 gcal/cm2 (383 ly) mean nal drainage is good. Large tabonuco trees tend to be daily pan evaporation, 1.8 mm (0.07 in); mean wind concentrated on upper slopes and ridges, where they velocity, 4.2 km/h (2.6 mi/h); and mean annual rain- may form nearly pure groups whose roots are grafted, thus forming a tree union or clumps of trees. The authors are Principal Ecologist and Research Forester, Presumably this reflects the better drainage of soils Southern Forest Experiment Station, New Orleans, LA. (Re- at such locations or the superior anchorage against search on this species was done in cooperation with the Univer- hurricane winds that the prominent boulders may sity of Puerto Rico, Rio Piedras.) offer. Significantly superior diameter growth rates of

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tabonuco on ridges as compared to swales have been is some flowering and fruiting at other times and reported (22). considerable annual variation. Empty fruits common- ly fall earlier than those that are fertile. At one Associated Forest Cover period during the autumn of 1963, two-thirds of the crop was composed of empty fruits, although a Tabonuco dominates a forest association known lo- second smaller crop of viable seeds appeared later. cally as the tabonuco type (21). In the French West Gamma radiation stress reportedly has led to earlier Indies, the association is described as “forest and increased fruit fall (6). Fruits are generally hygrophytique” (20), and Beard described it as lower found in abundance beneath the crowns of the parent montane rain forest, or Dacryodes-Sloanea (1). These trees. fall within the broader categories of Tropical or Sub- tropical Wet Forest life zones (7). Seed Production and Dissemination-Of the In Puerto Rico this association averages about 50 fruits that fall, those dark in color were found to be tree species per hectare (5012.5 acres) larger than 10 heavier, with 60 fruits per kilogram (27/lb) compared cm (3.9 in) in d.b.h. In the Luquillo Mountains close to 73 per kilogram (33/lb), and more viable, up to 22 associates include motillo (Sloanea berteriana), percent compared to up to 5 percent, than green palma de sierra (Prestoea montana), yagrumo fruits. Completely developed fruits tend to sink when hembra (Cecropia peltata), yagrumo macho immersed in water. Of those that float, most are (Didymopanax morototoni), and caimitillo Verde hollow, although as many as 33 percent may still be (Micropholis garcinifolia) (21). The forest type has viable. The period of viability under natural condi- been described in detail (2,15,21). Characteristics of tions is short but not precisely known. A few seeds tabonuco stands include a rich mix of 170 tree are known to have survived 5 weeks storage exposed species in primary and secondary stands (23) with to the air. dominants of Sloanea berteriana, Guarea guidonia, Germination is epigeal and takes place within a and Manilkara bidentata. The relative density of few days under humid, shady conditions. Under seedlings to that of canopy trees approximates a ratio direct exposure to the sun, germination is less than of 8 while that of saplings is 0.4 (19). On the average, in the shade, 11 percent compared to 28 percent in the stand contains 116 trees per hectare (471acre) 10 one test. to 15 cm (3.9 to 5.9 in) in d.b.h., 100 to 150/ha (40 to 6l/acre) larger than 30 cm (11.8 in) in d.b.h., and Seedling Development-In nature the seedlings 63/ha (25/acre) larger than 50 cm (19.7 in) in d.b.h. grow best on steep slopes under low light intensities with a total biomass of 424 t/ha (156 tons/acre) of (0.07 to 0.05 gcal/cm2. min or 0.07 to 0.05 ly). They which 33 percent is in tree boles (14,23). The basal are fragile and root within the litter layer. At an age area of the average stand approximates 40 to 46 of 4 months they attain an average height of 16 cm m2/ha (174 to 200 ft?acre) and the volume ap- (6.3 in) and typically have six leaves. In some areas proximates 300 to 345 m31ha (4,285 to 4,930 ft3/acre) there may be as many as four seedlings per square 12). The diurnal gross primary production of a stand meter (0.4/ft2). Few seedlings grow beyond this stage, with a leaf area index of 6 to 7 in Puerto Rico is only half surviving beyond 8 months. Continued reported to average 16 grams of carbon per square growth of the remaining seedlings under inter- meter of ground area (0.052 oz/ft2) (14). mediate light intensities is slow. Net photosynthesis is low (60 mg of carbon per m2.h or 0.08 gr/ft2.h), and Life History the ratio of net photosynthesis to respiration is 0.78 C-12). Reproduction and Early Growth In the nursery it has been found that seedlings during their first month will not withstand direct Flowering and Fruiting-Male and female exposure to the sun. Under shady conditions seed- flowers appear on different trees, making this a dioe- lings attained 22 cm (8.7 in) in 6 months. Like many cious species. Flowers are greenish, about 4 mm (0.16 other large-seeded tree species, tabonuco does not in) across, and develop in lateral, much-branched survive bare root transplanting. In contrast, 85 per- panicles. The fruit, a one-seeded oblong-ovoid drupe cent survival has been attained by the use of con- about 19 mm (0.75 in) in diameter, has a corrugated tainers with an intact ball of earth about the roots. surface when dry (19). The seed is fleshy, with folded At 9 years, trees were 3.0 to 4.3 m (10 to 14 ft) in cotyledons (5,171. height and 3.8 to 5.0 cm (1.5 to 2.0 in) in d.b.h. (13). Flowering peaks between May and November, with Direct sowing in the forest has yielded germination most fruit falling from October to December. There as high as 33 percent but subsequent problems such

285 Dacryodes excelsa

as leaf fall, drought, and vines precluded successful posed to the intensive rays of the sun, due in part to establishment, so planting is recommended. scalding of previously shaded bark. Trees severely isolated in residual stands after logging have been Vegetative Reproduction-There is no evidence observed to exhibit crown deterioration and will be of vegetative reproduction. lost, within 10 years.

Sapling and Pole Stages to Maturity Damaging Agents-The success of tabonuco in the mountains of the West Indies speaks for its capability for dealing with hurricanes. Many trees do Growth and Yield-Mature tabonucos in Puerto not survive, and many of those that do, suffer crown Rico may reach 35 m (115 ft) in total height and 180 breakage and subsequent heart rot,. Despite this, cm (71 in) in d.b.h. The age of large trees is es- most of the mature trees in Puerto Rico have sound timated at up to 400 years. Unpublished preliminary butt logs. A few trees exhibit bark swelling and volume tables exist for the species, and a composite cankers that provoke an abundance of resin exuda- table for tabonuco type forest has been published (2). tion. The pathogen is unknown. Abortion of the fruits Because of the mixed nature of the forests in which is a common and possibly an important phenomenon tabonuco is found, growth and yield of tabonuco per whose causes are unknown. The significance of fer- unit of forest area are somewhat meaningless. tile seed removal by parrots, other vertebrates, and Diameter growth of individual trees is most rapid invertebrates is not known either. on slopes and unexposed ridges and for specimens that are large in size and dominant in crown position (0.15 cmlyr, 0.06 in&, for dominant crowns in ma- Special Uses ture stands vs 0.10 cm&, 0.04 in&r, for suppressed crowns>. Growth rates are slow when compared to Tabonuco is used for all types of furniture, planted exotics, but what the potential growth of cabinet work, interior trim, general construction, and tabonucos would be if planted under comparable con- carpentry. The wood is also useful for crates, boxes, ditions is not known. shingles, and small boats. It is a substitute for mahogany in a variety of uses. The wood itself is Rooting Habit-The observed preference of moderately heavy, with a specific gravity of 0.53 (19). tabonuco for upper slopes and ridges and its obvious It air-seasons easily and satisfactorily, undergoes successful survival of frequent hurricane winds that moderate and uniform shrinkage during seasoning, break crowns rather than uprooting trees point to a and holds its place well after manufacture. Tabonuco strong and presumably deep root system. Root grafts is a moderately good machining wood; it cuts and among trees of this species are common and can be saws easily but will dull saw teeth due to its high seen in stumps that remain alive and continue to silica content. Tabonuco lumber yields good surfaces develop callus over the cut surface for decades. when planed, sanded, mortised, or shaped (turning Anywhere from 10 to 20 individual trees can form a and boring are more difficult). It, is easily glued, union as a result of root, grafts. Dense and deep holds nails well, takes stain well, and finishes beauti- root-mats develop near the bases of some large trees, fully with varnish or lacquer. The wood is only slight- presumably where unusually large amounts of litter ly resistant to decay, lasting 3 years or less in the are trapped on the surface. ground, and is difficult to impregnate with preserv- atives by either pressure or nonpressure methods Reaction to Competition-Overall, tabonuco (11,12,18). Tabonuco wood compares favorably with may be classed as intermediate in tolerance to shade. mahogany and birch (24). Seedlings are very tolerant. Only a tiny proportion Early settlers used the resin of tabonuco for of the seeds that fall produce trees that reach making candles and torches, for caulking boats, for maturity. Competitive factors include light, moisture, incense, and for medicinal purposes. The endangered mechanical damage resulting from the falling of lit- Puerto Rican parrot feeds on tabonuco seeds. ter 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 ftJ tall. Genetics Growth rates of established trees can be ac- celerated if the trees are released from competition Wood cutters in Puerto Rico have recognized two (i.e., from 0.15 cm&, 0.06 in/yr, in mature stands to races of tabonuco based on the degree of red color 0.66 cm/yr, 0.26 in&, in cutover stands). However, and other visible properties of the wood. Other varia- even mature trees will suffer shock if severely ex- tions, such as the shape of the fruit, have been ob-

286 Dacryodes excelsa

served. The genetic significance of these traits, if any, 12. Lugo, Ariel. 1970. Photosynthetic studies on four species of is unknown. Dacryodes excelsa has a nuclear volume rain forest seedlings. In A tropical rain forest. p. I-81-102. U.S. Atomic Energy Commission, Washington, DC. of 52.6 p3 (9) and shade leaves appear to contain less 13. Marrero, Jose. 1948. Forest planting in the Caribbean DNA than sun leaves, 590 compared to 715 Mg/g (3). National Forest: past experience as a guide for the future. Caribbean Forester 9:85-146. Literature Cited 14. Odum, Howard T. 1970. Summary, an emerging view of the ecological system at El Verde. In A tropical rain forest. p. 1. Beard, J. S. 1949. The natural vegetation of the Windward I-191-289. U.S. Atomic Energy Commission, Washington, and Leeward Islands. Oxford Forestry Memoirs No. 21. DC. Clarendon Press, Oxford, England. 192 p. 15. Odum, Howard T., and Robert F. Pigeon, eds. 1970. A tropical 2. Briscoe, C. B., and F. H. Wadsworth. 1970. Stand structure rain forest. U.S. Atomic Energy Commission, Washington, and yield in the tabonuco forest of Puerto Rico. In A tropical DC. (Available as TID-24270, National Technical Infor- rain forest. p. B-79-90. H. T. Odum and R. F. Pigeon, eds. mation Service, Springfield, VA 22161.) U.S. Atomic Energy Commission, Washington, DC. 16. Odum, Howard T., George Drewry, and J. R. Kline. 1970. 3. Canoy, Michael J. 1970. Deoxyribonucleic acid in rain forest Climate at El Verde, 1963-1966. In A tropical rain forest. p. leaves. In A tropical rain forest. p. G-69-79. U.S. Atomic B-347-418. U.S. Atomic Energy Commission, Washington, Energy Commission, Washington, DC. DC. 4. Crow, T. R., and P. L. Weaver. 1977. Tree growth in a moist 17. Record, Samuel J., and Robert W. Hess. 1943. Timbers of the tropical forest of Puerto Rico. USDA Forest Service, Research New World. Yale University Press, New Haven, CT. 640 p. Paper ITF-22. Institute of Tropical Forestry, Rio Piedras, PR. 18. Reid, David. 1942. Creosote penetration in tabonuco wood as 17 p. affected by preliminary boiling treatments in organic 5. Cuatrecasas, Jose. 1957. The American species of Dacryodes. solvents. Caribbean Forester 4(1):23-34. Tropical 106:46-65. 19. Smith, Robert Ford. 1970. The vegetation structure of a 6. Estrada Pinto, Alejo. 1970. Phenological studies of trees at El Puerto Rican rain forest before and after short-term gamma Verde. In A tropical rain forest. p. D-237-270. U.S. Atomic irradiation. In A tropical rain forest. p. D-103-140. U.S. Energy Commission, Washington, DC. Atomic Energy Commission, Washington, DC. 7. Holdridge, L. R. 1967. Life zone ecology. Tropical Science 20. Stehle, H. 1946. Les types forestiers des Iles Caraibes. Center, San Jose, Costa Rica. 206 p. Caribbean Forester 7 (Supplement): 337-709. 8. Kalkman, C. 1954. Revision of the Burseraceae of the 21. Wadsworth, Frank H. 1949. The development of the forest Malaysian area in the wider sense. Blumea 7(3):498-552. land resources of the Luquillo Mountains, Puerto Rico. Thesis 9. Koo, F. K. S., and Edith R. de Irizarry. 1970. Nuclear volume (Ph.D.), University of Michigan, Ann Arbor. and radio-sensitivity of plant species at El Verde. In A 22. Wadsworth, Frank H. 1953. New observations of tree growth tropical rain forest. p. G-15-20. U.S. Atomic Energy in tabonuco forest. Caribbean Forester 14(3-4):106-111. Commission, Washington, DC. 23. Wadsworth, F. H. 1954. Tropical rain forest. In General 10. Longwood, Franklin R. 1961. Puerto Rican woods: their Papers, Fourth World Forestry Congress. p. 119-129. Dehra machining, seasoning, and related characteristics. U.S. Dun, India. Department of Agriculture, Agriculture Handbook 205. 24. Wellwood, R. W. 1946. The physical-mechanical properties of Washington, DC. 98 p. certain West Indian timbers. Caribbean Forester 7(2):151-173. 11. Longwood, Franklin R. 1962. Present and potential commercial timbers of the Caribbean with special reference to the West Indies, the Guianas, and British Honduras. U.S. Department of Agriculture, Agriculture Handbook 207. Washington, DC. 167 p.

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