Myrcianthes fragrans (Sw) McVaugh
Myrtaceae GUAYABÓN
Rare evergreen canopy or emergent (25-35 m) tree found along the humid, more southern part of the Pacific slope. Nevertheless, this tree is locally common in Manuel Antonio National Park[s deep forest habitats. A combination of anatomical, botanical, and historical features make Guayabón one of the region’s most fascinating arboreal species. Of exceptional and unique morphology, Guayabón possesses smooth, orange-ish bark of an unlikely, at times even ghostly, hue. Thanks to its emergent stature and strange appearance, the tree was targeted early on as botanically important by both the area’s residents and herbarium specialists. 
The subsequent tale of its identification reads like a detective story and, for a time, Guayabón was thought to be a new species. After many years of anticipation and effort, its highly ephemeral flowers were collected for the first time in 1991. Based on this evidence, Guayabón was recognized as belonging to the previously identified and widespread species, Myrcianthes fragrans – a tree of consistently small, understory stature in nearly all of its range. Interestingly enough, the special set of climactic and edaphic conditions encountered in MANP (as well as in one small area of the West Indies) promote the development of Myrcianthes fragrans to a size several orders of magnitude beyond its usual phenotypical limits.
Description: Guayabón has, perhaps, the most unlikely trunk of any Manuel Antonio tree – though it varies considerably according to the age and girth of the specimen. Small trees (to 40 cm in diameter) are straight and mostly cylindrical. Their bark is reddish orange and so smooth as to appear polished. At roughly two-year intervals – but always in late May and June – the bark is completely shed in large but nearly-paper-thin sheets that pile up around the base of the tree. Afterwards, the newly exposed, more vividly orange bole nearly glows with its warm, remarkably un-forest-like hue. Large trees (to 1.5 m in diameter or more) have boles that are still mostly straight and cylindrical, though now their surfaces are dimpled and more undulating. Bark color is lighter and exfoliation less pronounced and complete. Low, rounded buttresses of moderate size – lacking in the smaller trees – are usually in evidence.
Branchless along most of its length, all Guayabón boles sport stout, upwardly angled limbs at canopy level. Subdividing profusely, these limbs terminate in tiny twigs that support a thick covering of small, rounded leaves. The finely-textured crown that results is single layered, moderately dense, and wide. (Small trees, still locked in the subcanopy or understory, possess much narrower crowns). The smooth, somewhat waxy leaves (5.5 by 3 cm) are simple and oppositely arranged. Widely elliptical to round in shape, they lack drip tips and possess only short (0.4 cm) petioles. A prominent mid-rib creases the think, stiff blade, while secondary veins remain inconspicuous. New foliage appears from branch tips, adding to the still intact canopy, from September through November and again in March and April. Guayabón flowers (2 cm) are extremely delicate, wispy, and short-lived. In late April, the first round buds appear from axillary and terminal leaf axils located in the tops of the trees. They grow rapidly as they are projected outwards – in groups of three (consisting of one nearly sessile bud flanked by two others on short, 1.5 cm pedestals) – by long stalks (5 cm). 
The buds open into fuzzy white blossoms, each with 4 green sepals; 4 translucent white-green petals (that fold back and form a box-like structure enclosing the pedestal and inferior ovary); a central pistil; and about 70, long, yellow-anthered, white stamens. Sweet and aromatic, the fragrance emitted by these flowers attracts many pollinators. Flowering seasons are fleeting, though usually intense and well synchronized. They begin in early May and last through the first week of June, though individual trees remain fertile for only about half of this short period. Flowering frequency in Guayabón is less well defined but probably occurs every other year. (Between 1984 and 1991 Guayabón showed fertility only during odd-numbered years. However in 1992 this trend was broken when the species flowered profusely. Flowering was again observed in 1993.) Fruits (2 cm) are ovoid berries with mostly smooth, semi-glossy skin. Aside from being brightly colored, their most conspicuous feature is the persistent, distally located, star-shaped flower calyx. Ripening high in the crown, the green fruits assume often mingled shades of yellow, orange, and wine-red. Each consists of a soft, pasty, outer layer of orange flesh surrounding a core of one to four green seeds (0.8 cm). Covered only by a thin, tissue-like skin, the seeds lack a strong, protective coat. Somewhat flattened and disk-shaped, they also appear coiled – resembling a stubby, curled grub. Harvests begin in mid-July and can last until early October, but are usually most intense during August. They are also quite variable in magnitude, with light crops more common than heavy ones. Germination of the bare seeds is immediate, and the first seedlings are visible on the forest floor in August – within a couple of weeks of fruitfall.
Similar Species: Guayabón’s unusual bark color and texture make it one of the Pacific slope’s most easily identifiable trees. Only Surá (Terminalia oblonga) shares similarly colored, smooth, shedding cortex. Surá’s trunk, though, is paler in color – it lack’s Guayabón’s reddish-orange hue. Furthermore, Surá has larger, alternately-arranged foliage that is easily distinguishable from Guayabón’s small, paired, rounded leaves.
Natural History: Guayabón is perhaps the prototypical tree for illustrating an adaptive plant behavior known as bark shedding. The complete loss of exterior bark exhibited by this species in alternate years (an event that is apparently correlated with tree fertility) is an extremely effective way of eliminating parasitic and epiphytic plants that grow attached to tree surfaces. Such symbionts as the clearly parasitic “strangling” figs (Ficus sp. – see F. obtusifolia) and Matapalos (Clusia sp. – see C. rosea) frequently kill their hosts by out-growing and shading them, or by adding enough extra weight to cause them to topple. Other epiphytic plants like Bromeliads, though not as harmful, add weight to limbs and can cause significant damage and even precipitate treefall. When the outer layer of bark loosens and is shed, most of the plants that are attached to trunk and branch surfaces are lost as well. Of course, no defense is completely effective and it would be wrong to imply that parasitic plants are never found on Guayabón trees – nature is replete with exceptions (and, as Harvard biologist Stephen J. Gould says, in nature, everything happens once). But it does mean that far fewer parasites are successful at establishing such types of harmful symbioses and that Guayabón trees, on average, are better off because of their bark-shedding behavior. Guayabón flowers emit a strong, sweet scent, attracting the bees that are their principle pollinators. Fruits are avidly eaten by arboreal mammals such as White-faced monkeys and terrestrial mammals like Agoutis. Brown-hooded parrots have been observed to parasitize the green fruits by chewing into them and extracting the still-immature seeds. Adapted to Manuel Antonio’s drier sites, Myrcianthes is most commonly found growing in sandy, coastal soils and the well drained substrate of ridges.
Uses: Guayabón is a remarkable species that merits special attention and protection. A rare tree of small stature over most of its range, certain (unknown) circumstances induce it to attain the monstrous proportions witnessed Manuel Antonio. Guayabón’s trunk and bark features are extravagant and breathtaking in their beauty. They create an unexpected and enthralling contrast with the surrounding forest. When illuminated by the oblique rays of the setting sun, the russet bark seems to phosphoresce with an ethereal, golden warmth, creating an unparalleled esthetic event at the end of the day. Furthermore, Guayabón is an important forage tree for many forest animals. The wood is strong and hard and has been used traditionally as charcoal for cooking, where it is said to add a special flavor to food. Finally, the thin sheets of smooth, flat bark have been used as paper – in much the same way as that of the north-temperate paper birch tree.
Distribution: Myrciathes fragrans ranges from Mexico to northern South America and the Antilles. In MANP, Guayabón is sporadically distributed throughout the primary forest, where it occasionally appears to form small stands or groves. It is an especially common component of the beachside vegetation found along Playa Espadilla Sur. In Costa Rica, Guayabón trees of this stature are only known from Manuel Antonio and the La Leona sector of Corcovado National Park. Such a punctuated distribution, confined as it is to a couple of small nature reserves – themselves surrounded by vast areas of pasture and oil palm plantations – is just cause to marvel at the fortuitousness of this species’ salvation. Just as easily, these remaining fragments of forest could have been leveled with the rest, leaving no large Guayabón trees alive today. From this perspective, it is poignant to ponder what other rare, unique, or otherwise fascinating species, confined to similarly restricted regions, were lost forever when the vast forests of Costa Rica’s pacific coastal lowlands were decimated in the 1930’s. How many local populations of flora with one-of-a-kind characteristics and extremely punctuated or endemic distributions succumbed to the axes. I often wish that I could have been in this part of Costa Rica at the turn of the century – to have had the opportunity to see what must have been a fantastically diverse and towering jungle – superlative in every respect. The forested remnant that still remains in Manuel Antonio can only hint at the grandeur that once was.
Photos: Tree Trunk Trunk2 Trunk3 Trunk4 Bark Leaf Leaf2 Flower Flower2 Fruit
Trees of Costa Rica's Pacific Slope 
