“With time and with water, everything changes”
~ Leonardo da Vinci
The cool, clear water of the stream reflects a rapidly shifting mosaic of varying shades of green. The tallest trees rise two hundred feet above us, and a thousand intertwined layers of leaves and branches catch any sunlight passing beneath the highest canopy. We are walking along headwater streams in Costa Rica, at La Selva Biological Station. Thirteen feet of rain fall here each year on average, and this wealth of moisture supports dense masses of greenery. Plants do not just grow from the soil at La Selva; they also grow on each other, sometimes stealing nutrients from their host, sometimes sharing them, sometimes just squatting where there’s space. Many leaves of large trees are covered with a green rust of mosses and fungi. Lianas lace the forest together, and when a big giant comes down, it takes many of its neighbors with it.
The stream froths downward over a series of short, bouldery steps, but we can’t really hear the water for the louder background noise of the surrounding forest. Cicadas and other insects create a continual vibrating buzz that rises periodically to a crescendo. Parrots passing overhead with short wing beats scream as though they were being tortured. The gurgle of an oropendola is interrupted by the distant roar of a howler monkey or the sharp woof of a surprised peccary (a wild pig).
The foot-long leaves and seedpods of the tropical trees do not sift gently down to the forest floor; they fall with a crash that makes us flinch and look upward nervously. And then a small troop of capuchin monkeys moving through the trees overhead grows agitated at our presence and begins to throw branches down at us.
The rain forest is pre-eminently alive and changing, and when we pause even for a few moments, we can discern details like the fist-size spider camouflaged as a knobby white fungus, or the tiny stream of leafcutter ants moving relentlessly over fallen tree and mossy boulder, each ant carrying aloft a miniature white flower.
We are here to study the logs that fall into streams. In temperate environments, these logs last for years or even centuries before they decay. We think that tropical streams might be different. Rain does not trickle down here in a gentle mist; it falls in torrents as though an enormous bucket were being emptied from the sky. Flow in the streams rises quickly in response, but then falls again rapidly once the rain ceases. The frequent intense rains of the wet season produce stream flow that goes up and down like a yo-yo, and logs lying in the streambed are lifted and carried downstream by the swiftly moving waters.
The logs are equally vulnerable when they are stationary. There is no dormant period of freezing cold or exceptional dryness here, and microbes thrive. Rates of wood decay are much faster than in temperate climates, so the combined effects of flashy stream flows and rapid decay keep wood from remaining in the streams more than a few years, or in some cases days.
As we walk through the forest, we occasionally encounter a pocket of air sweetly scented by flowers. Mostly the air smells like the freshly turned earth of a garden – the scents of fallen plant parts decaying. Rain falls at least briefly nearly every day, and the humidity remains so high that the air forms a palpable entity rather than an absence.
This is a climate for frogs. We continually flush them as we walk along. Those well camouflaged in browns and tans are visible only as sudden leaps among the ferns on the forest floor. With their vivid scarlet and azure hues, miniscule poison-dart frogs appear like enameled miniatures in a jade-green setting.
This is also a climate for invertebrates. We constantly have to shift ourselves and our packs to avoid the truly remarkable variety of ants, spiders, and other insects that seek our flesh and blood. The rain forest is one of the few environments where I remember that invertebrates tremendously outnumber vertebrates and form a much greater proportion of the mass of living organisms in a landscape.
This is not a landscape for rocks. Relentless heat and humidity, combined with organic acids released by plants, accelerate the chemical reactions that change rock into soil, even as they ensure that these reactions proceed so thoroughly that all but the most resistant elements are leached from the soil, and carried away in the groundwater. Aluminium and iron oxides are about all that remain in the clay soils that slime our boots and send us slipping down the steep trails. The dense vegetation that so effectively obscures the original geological contours of lava flows underlying La Selva also fosters the decomposition of bedrock.
All streams change constantly. Flow rises and falls, earth moves from hill slopes into channels and downstream, fish migrate upstream, insects hatch and emerge into the air, and dissolved nutrients move from the subsurface into the stream. In this, the stream reflects the continual cycling of matter and energy that characterize the surrounding forest.
A floor of leaves crackle as we walk through it. Below it is pure clay containing very little organic matter. The thin skin of fallen leaves and branches is quickly decomposed by the countless insects, fungi, and microbes of the forest and reabsorbed as food by the living plants. The insects are eaten by frogs, lizards, and birds. Nutrients are stored in the tissues of living organisms rather than the soil humus in this most efficient of recycling systems. This landscape is alive in the truest sense.
This essay is an extract of Ellen Wold’s essay (with minor edits) published in SueEllen Campbell, The Face of the Earth, University of California Press, 2011.