Luehea seemannii Triana & Planch.
Tiliaceae
GUACIMO COLORADO
Occasional evergreen (or briefly deciduous) canopy (or emergent) tree (20-35 m) and a conspicuous component of the primary forest thanks to the unusual, tawny color of its foliage. Capable of growing to be quite large – it is perhaps the largest Tiliaceae species in Costa Rica – Guacimo Colorado’s thick crown never fails to stand out from the surrounding, arboreal vegetation.
Description: Guacimo Colorado has a very irregular, bending trunk that may reach diameters of 1.5 meters or more. Much of the bole’s surface is deeply contoured by many, thick, rounded folds that blend gradually into steep buttresses at the base of the tree. Slightly exfoliating, Guacimo Colorado is clad in tan or creme-colored bark. Profuse branching occurs along the upper half of the bole, and these, upwardly angled limbs form a relatively narrow (for a tree of this size) – but tall – crown. Leaves (24 cm by 11 cm), perhaps the most unique feature of this tree, are simple and alternate. As in other members of the Tiliaceae, they are arranged in two tiers, on opposite sides of the (slightly zig-zag) twig. The elliptical blades have serrate margins, notably straight bases, and poorly developed, blunt drip-tips. Leaf undersurfaces, petioles (1 cm) and to a lesser extent twigs, are densely covered by a finely textured layer of russet or tawny pubescence – giving these structures a color that contrasts strongly with the dark green of upper leaf surfaces. Furthermore, this pubescence imparts a hallmark, golden-brown color to the entire tree, causing it to stand out vividly from the surrounding vegetation. In May, all foliage is shed (leaving the trees briefly bare) and new growth ensues. By June, Guacimo canopies have been fully restored with fresh, bronzy leaves, and growth has temporarily stopped. Three months later (in September) Guacimo trees are growing again; this time continuing leaf production into December and adding some 40 cm to the lengths of their branches.
Flower buds appear at the ends of the elongating limbs by late November. Held in large, open, terminal panicles, the flowers (1.5 cm) are maintained well above the dense, Guacimo canopy. During blossoming, each flower’s five brown sepals roll back toward the pedestal and expose five, white, ribbon-like petals and – more conspicuously – a dense ball of stiff white stamens. The thick pistil, topped by an even wider stigma, is based on a five-part ovary. Flowering periods are annual, precisely timed, and well synchronized events that begin as early as late December and last until the end of January.
Young fruits begin to appear immediately, as the ovaries of the fertilized flowers expand. By February, many erect capsules (2.5 cm), each carrying five ridges and covered by a golden, velvety pubescence, can be seen above the Guacimo crown. In late March they begin to darken and dry. When a slit – present along the crest of each ridge – opens, myriad tiny (5 mm) winged seeds are released to the winds. Harvests – subtle and largely unnoticed – continue through April.
Similar Species: The shape and vein pattern of Tiliaceae leaves bare a superficial resemblance to those of the Melastomataceae. In a true Melastomataceae leaf, several palmate and unbranched main veins run the length of the blade and are interconnected by parallel sets of secondary veins. Tiliaceae rib arrangements – although palmate – are different because their main veins always show pinnate branching and subdivision. On another front, Guacimo Colorado shares its two-ranked leaf arrangement with most other Tiliaceaes. However, the distinct bronze color of Luehea foliage is enough to easily and positively distinguish it from any other tropical forest tree. In fact, Guacimo is one of the first species that observant hikers will recognize.
Natural History: The unusually colored, thick pubescence coating the undersides of Guacimo Colorado leaves is so unique that it has become the defining characteristic of this tree – a fact demonstrated by the common name chosen for this species. More importantly, this pubescence may offer Luehea some adaptive benefits. Sunlight, incident on a leaf from above, may be absorbed by chlorophyll containing chloroplasts on its way through the blade. If not, it will escape out of the leaf’s lower surface or be absorbed by it. In either of these latter two cases, the light is wasted – since it has gone unused by the leaf’s energy-storing chemicals. It has been suggested that the lower surfaces of certain opaque leaves reflect at least some of the sunlight reaching them, thus causing it to repeat its journey through the chloroplast-containing region of the blade. This would give the plant a second chance at capturing the sun’s energy and thus could enhance its chances of survival – especially in places where sunlight is not abundant. The thick, opaque pubescence found on Luehea leaves may serve this function, though it probably offers some protection against leaf-chewing parasites as well.
Guacimo Colorado flowers are visited by hummingbirds and bees during the day, while Croat (1978) indicates that beetles and moths are active pollinators at night. The tiny Luehea seeds are clearly wind-dispersed. Seed production is timed to coincide with the beginning of the wet season, giving the small seedlings (which develop from this particularly small seed) as much time as possible to grow an adequate root system before the onset of the next, life-threatening, dry spell.
Distribution: In Manuel Antonio National Park, Guacimo Colorado is found thinly distributed throughout all primary and some secondary forest habitats. Well represented in all of Costa Rica’s lowland forests, this species reaches superlative proportions in Corcovado national park, where emergent individuals (35 m high, or more) are not uncommon. Luehea ranges from Guatemala to Panama.
Images: Trunk Leaf Flower&Leaf Flower&Fruit Flower Flower2 Fruit
Trees of Costa Rica's Pacific Slope 
