The Caribbean National Forest, better known as El Yunque, is the lushest forest-land in Puerto Rico, and the only tropical rain forest within the U.S. National Forest system. Thousands of plant and animal species, some extant only here, co-exist and interact beneath the forest canopy. Mountaintops can only rarely be seen, usually shrouded in cloud cover and rain, while only a few miles away on the beach, the days are entirely sunny, and cacti thrive in dry forest. Why should there be such diversity of climate and plant-life, in such close proximity?

The forest of El Yunque takes its name from one of the peaks that dominate this eastern end of the Puerto Rican Cordillera Central ("backbone") -- a chain of mountains that runs all the way across the island, east-to-west along the center of the island. El Yunque -- "The Anvil" in Spanish, is the group of mountains that seem to attract all of the clouds borne westward by the tradewinds across the Antilles. These clouds smash into the anvil and cannot cross without leaving behind a large proportion of their moisture; these mountaintops are usually buried in clouds. The Taino indians believed that their gods lived on the peaks of these mountains in the clouds.  The chief god, Yuquiyu, is most likely at the root of the name of El Yunque; arising from a Spanish bastardization of his name, as is the name of the town of Luquillo. 

Within the upper parts of the forest, tree-dwelling epiphytes -- orchids, bromeliads, lichens and moss -- spend entire lifetimes twenty, thirty, forty or more feet in the air nested in crooks of trees or in nooks along the bark, seemingly unaware that plants need good, firm soil in order to root and draw water and nourishment. Average rainfall on the south side of El Yunque is between 200 and 300 inches, while 15 miles away on Culebra, residents are lucky to receive 25 inches in a year. Summer storms -- not hurricanes, mind you -- can drop 8-12 inches of rain in 24 hours or less.

From the Equator to 30 degrees north latitude, eastern tradewinds blow steadily from the northeast, due to the earth's rotation. In the southern tropics, the eastern tradewinds blow from the southeast. The changing heat pattern of airmasses around this tropical belt, known as the intertropical convergence, causes seasonal rainfall.

If you're reading this, you're probably thinking about a vacation to Puerto Rico; and why not?  Lovely beaches, friendly people, English as one of the official languages, no need for a passport or shots, the same sunny, warm weather all year round . . . well, in the case of the latter, not necessarily.

Rainfall in the tropics varies by season. Here in Puerto Rico, the dry season runs from roughly mid-December to May. The rainy season begins haltingly in June, then picks up in mid-July and runs until mid-November. The following thirty days are a toss-up; it might rain every day, or nary a drop. During dry season less than an inch of rain might fall in a month. Lower in the foothills, foliage dries up and fires are common. None of this is to say that your early September vacation will get rained out; it will still be sunny most of the time, but the occasional showers will drop a lot more rain on their way through.

These seasonal changes are exceedingly important for many plant and animal species in El Yunque. Trees here tend to flower during the dry season, often timing their fruiting to coincide with the onset of the rainy season, for a number of reasons. Insect pollinators are much more active during the dry season, since showers are fewer and farther between; this allows for greater cross-pollination for plants and trees. Timing fruits to bear at the beginning of the rainy season also ensures that seeds will germinate at the exact time that will guarantee them the most available moisture. It is also during the wet season that the most treefalls occur, opening new gaps in the rainforest canopy. Young saplings are well-positioned to take advantage of gaps in the canopy created by the felling of an old canopy tree, which provides the valuable sunlight needed for these new canopy trees to thrive.

Rain forest animals, too, are susceptible to this change in seasons; some fruit-eating birds, dependent upon bountiful wild fruit crops, do not breed successfully if there is insufficient rainfall during the rainy season, a good predictor of wild fruit production in the dry season ahead. The yellow bananaquit, a tiny, nectar-feeding bird about the size of a hummingbird, is especially sensitive to variations in seasonal rainfall from year to year. Trees and plants that are not planning to fruit in a given year do not produce many nectar-producing flowers, thus severely limiting the nourishment available to a bananaquit. The bananaquit ("La Reinita," or little queen) reacts to the shortage by not mating, or by breeding unsuccessfully.

It is important to note that "rain forest" and "jungle" are not at all interchangeable terms. Mature rain forest has a canopy of tall, shade-producing trees which prevent the growth of sun-loving vines and bushes. The result is not entirely unlike the older forests in the US, where one may pass relatively easily even without the benefit of a trail.

Jungle, on the other hand, is choked with vines and shrubbery, and is next-to-impassable without a machete. In El Yunque, jungle is encountered in two types of landscape: at the borders of the forest, where there is plenty of sunlight available in the boundary between cleared pasture land and forest; and inside the forest in open spaces where large canopy trees have recently fallen. The opening in the canopy lets direct sunlight reach the ground, where vines can (if only temporarily) thrive. Fast-growing plants like vines and shrubs and thin-boled trees (such as yagrumo) own the patch for a few years until slower-growing, stronger, taller trees reach maturity. Once these "canopy trees" grow taller than the "pioneers," the pioneers die off for want of sunlight and do not reproduce. The former clearing returns to its usual stasis, a place where slow-growing, shade-tolerant, understory plants live for years, well-separated to soak up as much sunlight as possible.

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