Costa Rica is located in Central American between the Caribbean Sea and the Pacific Ocean and bordered by Nicaragua to the North and Panama to the Southeast. It is a small country (one tenth the size of France) but vast in its biodiversity. Costa Rica is the perfect country to experience a tropical forest. Trails and suspension bridges through the canopy of trees allow nature lovers to discover a unique ecosystem, one of the world’s most diverse. Water is omnipresent in these rainforests: in the air, on the ground, and in the exuberant vegetation, which comprises multiple layers and whose height ranges from 98 to 200 feet. According to some estimates, tropical forests are home to 70% of the Earth’s species (excluding marine ecosystems). If, like us, you want to wander among the tree tops, buckle your bag, and let’s go together to discover the tropical forests of Costa Rica.

Located in the Cloud Forest near the Poás volcano, La Paz Waterfall drops nearly 120 feet before splashing into the pool below.


Wandering in the tropical forests: first steps in a « Selva » of Costa Rica

From sloths to tarantulas, toucans to venomous snakes, the tropical forest is either seen as green perdition – the perfect backdrop for any good adventure novel – or as a Garden of Eden with hummingbirds twirling and the restful sound of a waterfall. The rainforest is astonishing in its dense, tall, and lush vegetation. Some abuse language to call the tropical forest a jungle, while Spanish speakers prefer the term « Selva », and Anglo-Saxons « Rainforest ». The tropical forest formed the perfect setting for the English author Rudyard Kipling’s well-known 1894 collection of stories entitled « The Jungle Book », where Baloo sings, « Forget about your worries and your strife / I mean the bare necessities / Old Mother Nature’s recipes / That brings the bare necessities of life ».

« The richness and strangeness of the fauna and flora in the intertropical zone, the peculiarities of its climates and landscapes make it truly another world ».
(Francis Hallé – Un monde sans hiver)

In the tropical rainforests of Costa Rica, the vegetation is dense and tall. It appears dense and contiguous when viewed from the ground.

Trees can grow up to the size of a 20-story building. The trunk, usually cylindrical, sometimes has large flattened blades at its base that can be several feet high. This pyramidal base gives the tree a rocket-like appearance. In other cases, the trunk is lined with roots that plunge towards the ground ! These aerial roots are found in unstable environments such as in mangroves. Another oddity, some species start their lives as an epiphyte, a seed that germinates on a tree at 50 or 100 feet above the ground. From there the young plant develops small roots that seek moisture and food in cavities or cracks in the bark. Tropical trees, especially some fig trees, can develop nearly horizontal branches along which aerial roots. These aerial roots become as large as the original trunk… With these aerial roots a single tree can cover up to 1.2 acre.

The trunk of the large trees of the tropical forest is usually cylindrical. Sometimes it has large flattened blades at its base.

In the tropical forests, two seasons follow one another: the rainy season and…one when it rains. The natives will tell you, « If it doesn’t rain, it’s going to rain ». Forget the vision of trees playing the fall leaf-color game, turning orange, scarlet, and yellow. Fall does not exist. Even if the trees lose their leaves, the forest keeps its beautiful emerald color all year round: it is a deciduous forest « sempervirente » (permanently green.) The ideal cocktail of heat and humidity allows the proliferation of many species, including ferns, lianas, mosses, palms, and titanic trees such as the « pachira quinata», also known as pochote. This tree has its own special feature: pochotes bear large, stubby thorns on their trunk.

The young fronds of tree ferns emerge in coils that uncurl as they grow. The fiddlehead resembles the curled ornamentation on the end of a stringed instrument, such as a fiddle.

Did you know that in tropical climates, trees don’t have rings; the tree-ring grows homogeneously. The few rings present are the result of alternating dry and rainy periods and not of years. For tropical trees, dendrochronology or tree-ring dating is only possible in a limited number of cases and is, therefore, a delicate exercise. Dendrochronology derives from Ancient Greek, « Dendron » (δένδρον) meaning tree, « Khronos »  (χρόνος) meaning time and « logia » (-λογία) the study of. Thus, the oldest tropical trees are not necessarily the most imposing. Some small trees grow extremely slowly and can therefore reach a great age. Carbon-14 dating can be used to determine the age of a tree, but its application is limited by its high cost.


A few words about the ecology of the Costa Rica rainforest

Whatever our age and experience, tropical forests feed our imagination and passion. When we think of the jungle as described by Edgar Rice Burroughs in his famous Tarzan books with its creepers and lush vegetation, we imagine ourselves as an explorer crossing the rainforest with a machete. If this is the case, you are certainly not in a primary forest, i.e., unexploited by man. The trees are bushy, very close together, and their branches and leaves overlap, creating a forest canopy. The tree canopy strongly filters the sunlight, and vegetation struggles to develop: only about 1% to 2% of the sunlight reaches the floor of the forest. Only plants adapted to low light can grow there. Some plants have adapted to the low light by growing high up and others by growing over and covering other plants. The forest floor of a primary forest is relatively free of young trees and shrubs because of the low sunlight, and one can easily move around… it would be almost possible to ride a bike there! But if a giant falls, taking other trees with it, the resultant opening allows an abundant influx of sunlight. Pioneer species (those that need light to grow) will grow back very quickly. Other more shade-tolerant species will follow and may survive the intense competition that reigns in the undergrowth… Could this be the famous law of the jungle ?

Inconspicuous vines cling to the trunks of trees. The climbers grow vertically up the tree held fast by the triangular flattened leaves.

The tropical rainforest contains many species of plants. Climbing plants, vines, and lianas (woody) represent a large proportion of the vegetation. There are more than 2,500 species of climbing plants! These range from the small, inconspicuous vines that cling to the trunks of trees to giant lianas as thick as trees that seem to hang in the middle of the forest. Tarzan, raised in the jungle, might have used lianas to move around, but they are a real problem for trees. Trees have developed defense mechanisms to discourage their rapid growth. Palms or ferns drop their fronds, while some trees even lose branches to eliminate the vines. But some lianas are more malignant than others and have found a way out by rolling up and bending like springs to resist shocks better. Yet other species of trees produce toxins in their bark to discourage the growth of lianas… This is what we could call a development race in the strict sense of the word !

« The tree branches looked like immobile pythons. The lianas curled up like green snakes. A sense of danger and betrayal rose from the earth and descended from the foliage ».
(Renée Vivien – La dame à la louve)

Epiphytes, also called « air plants», characterize tropical forests. The term « epiphyte » (ἐπιφύτον) derives from the Ancient Greek epi- (επί) meaning ‘over’ and phytón (φυτόν) meaning ‘plant’: plants that grow on the surface of a host plant. Unlike parasites, epiphytes grow on other plants for physical support, taking nutrients and moisture from the air and rain and nourishment from the compost on tree branches. Many of these rainforest epiphytes are familiar to you because they make excellent houseplants. They include ferns, lichens, mosses, cacti, some bromeliads, and of course, orchids such as Vanilla planifolia. Orchids are the best-known epiphytic plants. About 70% of all orchids grow on trees. The next time you are captivated by the beauty of your indoor orchid, imagine that it could have reigned supreme in a parallel life, hanging from a tree trunk !

The spider orchid has large spindly flowers with long slender yellow-green sepals and petals that are barred and blotched with brown

I have a weakness for Brassia. This genus of orchids is characterized by long and spreading petals as long as 20 inches and tapered like a spider’s legs. It is also called the spider orchid.  If its appearance has given it its name, its relationship with spiders does not stop there… Yes, we must not stop at appearances! The pollinating insects of this orchid are « Spider-hunting wasps » of the genus Pepsis, exclusive predators of tarantulas. Beware, sensitive soul, go directly to the next chapter… The wasps that hunt tarantulas are solitary animals that use their venom to capture a spider and then offer it as food for their offspring. Its hunting technique is invincible: after luring the tarantula out of its lair by moving its web, the Pepsis wasp turns the spider over like a wrestler to expose its abdomen and sting it. It only takes a few seconds for the tarantulas to become paralyzed. When encountering the Brassia, the pepsis wasp, confused by the orchid’s mimicry of a spider, stings the orchid’s petal while trying to grasp its prey without success. By then flying to another Brassia flower, the wasp pollinates the plant. On Justin O Schmidt’s (American entomologist) pain index, which ranges from 0 to 4, a Pepsis wasp sting is rated a 4. The pain is described as « like a plugged-in hairdryer being thrown in your bath ». The wasp drags the spider to its nest and deposits a single egg in its abdomen, which is the beginning of the spiders’ slow agony. For a dozen eggs, it will take 12 tarantulas ! The spider must remain alive so that the young have fresh flesh to eat. Here again, the wasp is straight out of a horror movie ! The paralysis of the spider is only temporary; the wasp cuts the legs of the spider. When the spider regains consciousness, it can no longer move and is slowly eaten from the inside out… We no longer wonder where the authors of « Alien » got their ideas.

Tarantulas grow to a length of 3 to 4 inches. They have adhesive ‘hair-brushes’ on the tip of their legs which allows them to climb vertically up.


The Costa Rican rainforest: mutual aid and competition

Is nature always the « law of the jungle», or is this idea wrong ? Suppose competition exists in the tropical forest, mostly in pursuit of light which plants need for photosynthesis. In that case, there are also numerous give-and-take relationships (or mutualism in biology). During one of our various hikes in the rainforest, we faced two types of mutualism. You have pins and needles in your legs, and you are impatient to discover these give and take relationships.

Leafcutter ants cut the leaves of trees surrounding their colony into small pieces with their powerfully adapted mandibles.

Leafcutter ants climb grasses and the leaves of trees surrounding their colony and cut them into small pieces with their powerfully adapted mandibles. These fragments are then hauled away to the colony to feed, not the other ants left in the nest, but rather a kind of mushroom ! These so-called fungus-growing ants belong to two genera, Atta and Acromyrmex. The Leafcutter ants and their fungi form a true ant-fungus mutualism, both benefiting from the relationship. The ants harvest leaves, which they can’t digest themselves. The fungi break down the indigestible plant cellulose and convert it into more edible proteins and sugars for the ants. As seasoned farmers, ants feed the fungus with freshly cut leaves. The fungus then produces edible filaments (gongylidia) used to feed the ant larvae, workers, soldiers, and queen. The fungus needs the ants to stay alive, and the larvae need the fungus to remain alive, so mutualism is obligatory. In addition to being farmers, these ants are also architects: the anthill’s structure must include

« The little ant never suffers from hunger. The lion, despite its fangs and sharp claws, does not always find food ».
(De Mocharrafoddin Saadi, Persian poet – The Bustan)

The Leafcutter ants have highly complex and organized societies. The colony can count up to 7 million individuals separated into different functional roles, called castes. The nest may have many chambers – some up to one foot and more in diameter – covering between 320 and 6,500 square feet. There are three castes. The smallest workers, called the minim (about 2-4 mm) or « gardener», care for the fungus garden. They prune it, protect it with antibiotics produced by their bodies and reseed the fungus with their droppings. The mediæ workers (5-10mm) cut and collect leaves from up to 50 different plant species within a radius of one mile and transport the fragments to the nest. Finally, the major workers protect the anthill in case of danger. The major may have body lengths up to 16mm. They are easily recognizable by their large head and strong mandibles. This symbiosis is incredibly advanced since ants cannot live without the fungus, which is their tool to digest the cellulose of leaves. And the fungus cannot survive more than a few days without the ants that feed and protect it.

Another plant from another time seen in the rainforest: it has the appearance of a green olive topped by a red umbrella.

Walking in a « Selva » is also crossing the path of plants that seem to come from another time. A primitive world of swamps where tree ferns, giant dragonflies, and the small Arlo de Pixar coexist… Gunnera Insignis or « Sombrilla del Pobre » is a plant endemic to Costa Rica, with giant leaves that can be used as a parasol… or umbrella. Its immense petiole barbed with thorns easily exceeds the size of a man. It is surmounted by a blade from 5 to 8 feet in diameter, lobed and crenelated with beautiful red veins. If the « poor man’s umbrella » has spectacular dimensions, it is undoubtedly thanks to its symbiotic association with a cyanobacterium of the genus Nostoc punctiforme.

Gunnera Insignis or "The Sombrilla del Pobre" is recognizable by its huge thorny petiole and its red spikes reaching up to the sky.

After water and light, it is the lack of nitrogen that limits plant growth. Nitrogen is essential for vital functions, yet plants don’t produce it on their own. They have to get it from their environment. Molecular nitrogen gas or N2 exists in almost unlimited quantities in the atmosphere, but most plants can only assimilate nitrogen in the form of ammonium or nitrates taken in through their roots. From point of view of the Gunnera insignis, associating with cyanobacteria of the genus Nostoc has an immense advantage since they can take atmospheric nitrogen and convert it to ammonia NH3 which is directly usable by the plant. Gunnera Insignis has glands filled with a low pH viscous mucilage that attracts Nostoc and stimulates its development. Once inside the glands, the cyanobacteria develop in the plant, eventually fusing with the gunnere cells. The Gunnera provides supplements of organic carbon compounds to the cyanobacteria and, in exchange, receives nitrogen in the form of ammonia. It is another excellent example of a give-and-take relationship.

The canopy appears dense and contiguous when viewed from the ground, but in reality, the treetops do not touch each other: it’s called crown shyness.


The canopy: exploring the roof of the Costa Rican rainforest

Like a house or rather a building, the vegetation of tropical forests is organized into several layers: emergent trees, the canopy, the under-story, the shrub layer, and the forest floor. At each stratum or layer, the forest offers a different facet.

« The tropical forest canopy is the most alive environment in the world; plants are more numerous at the upper part than at the bottom, because of the epiphytes (…) which form hanging gardens covering the high branches ».
(Luc Jacquet et Francis Hallé – Il était une forêt)

The canopy is still an unknown and strange place. It consists of intertwined branches and leaves of closely spaced trees. It appears dense and contiguous when viewed from the ground, but in reality, the treetops do not touch each other and remain several tens of inches apart, forming a canopy with channel-like gaps, called crown shyness. Several hypotheses exist as to why crown shyness is an adaptive behavior: it might be a protective margin in case of an epidemic (genus Covid19), a technique to limit the spread of invading insects, or a simple phenomenon allowing sunlight to penetrate the canopy. To date, none of the hypotheses has been scientifically proven, which means that “crown shyness” remains a mystery! This phenomenon has also been observed at the root level: trees do not touch each other, either at the top or underground…

In the Costa Rican rainforests, many trails include suspension bridges, sometimes perched several dozen feet above the ground.

More than 75% of the biodiversity of the forest is in the upper part of the canopy. For some, the canopy is an ecosystem of its own within the tropical forest. But how do we observe the flora and fauna from about 160 feet above the ground ? Observing from above, like a toucan, we venture onto impressive suspension bridges to look at the forest. Gaining height and rubbing shoulders with the canopy’s treetop has become possible all over Costa Rica thanks to an integrated network of suspension bridges. Sometimes perched several dozen feet above the ground, these bridges, mostly made of steel, are integrated into trails and offer unique views of the canopy and greater contact with nature.

Heliconius hewitsoni is a butterfly which lives in rainforests. It has black wings with two white bands with red at the base of the hind wings.

Between 70 to 90% of canopy tree species depend on wildlife for pollination and seed dispersal. This is why some species have adapted in terms of color or shape, odor, or nectar to better attract pollinators. Butterfly-attracting flowers, such as this Heliconius hewitsoni, have a sweet smell and are characterized by long petals painted orange or red, like the beautiful scarlet passionflower or Passiflora miniata. Butterflies are one of the few insects with good color vision. Hummingbirds only pollinate tubular-shaped flowers that adapt to the shape of their beak and offer a high content of nectar. And then we have the Stachytarpheta, a flowering plant of the Verbenaceae family that irresistibly attracts this rufous-tailed hummingbird (Amazilia tzacatl) The flowers of the Arum are real flying insect traps. Insects are attracted to the bottom of their receptacle by the heat and a putrid smell. Finally, some species are very specific in choosing their pollinators, such as Gongora claviodora, an orchid with intense red flowers and a clove-like scent (hence its name). These are only pollinated by solitary male bees of the Euglossinae family. The bees are attracted to the strong clove scent, and pollination occurs when the bee reaches the source of the scent within the flower.

The Rufous-tailed hummingbird has a bronze-green plumage with a red tail. It comes to consume the nectar of flowers of the family Verbenaceae.

To visit a tropical forest in Costa Rica is to discover a more complex and fascinating structure than you might think at first sight. One of the great mysteries of tropical diversity lies in the number of plant species capable of coexisting. Such diversity goes against the niche theory, which argues that each species must have its own niche. Normally, one would expect that one or a few tree species better adapted to the local climate and soils would take over and dominate the vegetation of the tropical forest… The green lung of mother earth and cradle of extraordinary biodiversity, these rainforests, lush, teeming with life and full of secrets yet to be discovered, still captivate our imagination and hopefully will do so for a long time to come.

« When I wandered through the eternal greenery, I had the impression that I was reading the universe and the forest was for me the most beautiful library ».
(De Gonzague Saint-Bris –Da Vinci’s Child)

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