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Product Details
| ISBN-13: | 9781642830729 |
|---|---|
| Publisher: | Island Press |
| Publication date: | 09/19/2019 |
| Pages: | 456 |
| Sales rank: | 1,101,606 |
| Product dimensions: | 6.00(w) x 8.90(h) x 1.30(d) |
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CHAPTER 1
RAINFOREST — A CLUE IN THE NAME
How tropical rainforests make clouds and recycle water, sustaining farming far away from where they stand
If there is one factor that unites the diverse set of ecosystems we call tropical rainforests, then it is rain. This seems obvious. Rainforests take their name from the fact that a lot of rain falls upon them. But it is vital to know that they can also create it, and not only that, but help to move it far away from where the rainforests themselves are. The whole cycle is powered by photosynthesis, and how rainforests pump water into the air as part of the process of converting the sun's energy into chemical energy. In many rainforests it is possible to see them doing this on an almost daily basis.
I've observed this for myself a number of times, including on a trip to the Amazonian lowlands of Peru, sitting on a ridge looking toward the green expanse of a rainforest canopy close to the steep slope of the Andes. There I watched for a couple of hours the effects of the sun beating down on the sea of trees that stretched far into the distance. Birds flew back and forth, monkeys called and insects buzzed, and behind all of it a fundamental process took place. As solar radiation powered photosynthesis in the trillions of leaves held aloft on that canopy of trees, so they released more and more moisture until the sky above gradually turned from clear blue to hazy blue, and then grey. The moist air rose and condensed and the outline of trees on the distant higher slopes blurred, as cloud and forest seemed to become one. And this fusion between trees and atmospheric moisture was not just an impression. For when the clouds that evening discharged their loads of rain from beneath a dark crashing thunderstorm, the water would not only run down rivers in the rainforest, but also back up the stems of the trees, to emerge once more through those solar-powered leaves, and again flow into the atmosphere.
The first writer to suggest that there were connections between forests, the humidity of the atmosphere and climate was the German naturalist Alexander von Humboldt, who explored the Americas during the early 1800s. He saw rainforests while traveling along the Orinoco and the Casiquiare, the latter linking with the Rio Negro in the catchment of the Amazon, thereby connecting these two vast river systems.
At a time when knowledge was increasingly sought through more and more specific scientific disciplines, von Humboldt was a rare thinker who took the wider view, looking at the whole system as well as the individual species or components, such as trees or animals. He believed that the Earth was one great organism within which everything was connected and that the forest was a complex system, based not only on interactions between the many hundreds of thousands of distinct lifeforms that comprised it, but also with the atmosphere and water. He saw how forests released humidity into the air to form clouds, and then rain.
Despite von Humboldt's equatorial observations leading him to see the fundamental connections that exist between forests, clouds and rain, the phrase 'tropical rainforest' was first used only about a century later, in 1898, by a German botanist called Andreas Schimper. It took half a century more before Schimper's phrase (tropischer Regenwald in his native German) entered mainstream use, triggered by the publication of Paul Richards's 1952 book The Tropical Rain Forest. In the wake of Richards's work came a long-running debate among botanists, geographers and ecologists as to the definition of a rainforest. Generally speaking it was taken to be forest receiving more than 200 cm (80 inches) of rain or condensed mist per year, with that more or less spread throughout the year. Many humid tropical forests were included in the category, even though for some part of the year they had less rain or mist.
As time has gone by we've had more and more opportunities to gain a practical appreciation of how tight the connections between forests and rainfall often are. Take, for example, the 3,000-kilometer-long rabbit-proof fence built across the interior of Australia in 1907 to control rodents that had by then taken on plague proportions. On the side of the fence where the native vegetation remained, abundant cloud formed, whereas on the side with millions of rabbits, grazing animals and crops, the skies were clearer and there was less rain.
The recycling of water back to the air via vegetation is the result of a process called transpiration. On the underside of leaves are tiny pores that open to enable carbon dioxide to be extracted from the air. When plants do this, then through those same pores they lose water. As water is lost from leaves, so more is drawn up from the forest floor, through bundles of tiny tubes called xylem. These connect leaves all the way down to the microscopic hairs at the end of the roots.
There is no heart or other pump to push the water up and it moves from the soil and up to the leaves, including those on the ends of twigs, 50 meters up, via suction. As water is pulled up to the top of the tree via what are the equivalent of very long narrow straws, more is brought up from below, carrying the dissolved nutrients that are needed in the solar-powered manufacture of the molecules necessary for growth and reproduction. Water pressure inside the bundles of tubes is up to about fifteen times that of the atmosphere. When water reaches the tiny pores on the underside of leaves, the sudden reduction in pressure means the water can become a gas — water vapor.
Alongside water vapor the trees discard another gas — oxygen — a by-product of plants using sunshine to split water so as to get hold of the hydrogen they need to make sugars. Of all the oxygen released on Earth through photosynthesis, the tropical rainforests contribute about 20 to 30 percent of the total; most of the rest comes from photosynthetic plankton in the oceans.
Photosynthesis goes faster in brighter light, causing leaves to open their pores wider to permit the entry of more carbon dioxide while at the same time letting more water vapor out. Water also flows out more quickly when it's warmer, and when it's windier. So long as there is a lot of water, as there generally is in a tropical rainforest, the plants can avoid the wilting effects that accompany water scarcity.
Those thin green leaves to which most of us pay hardly a second thought are thus not only manufacturing complex organic molecules using sunshine, water and carbon dioxide caught from the atmosphere, but also releasing life-giving oxygen and water that makes new rain. When it comes to that latter function, the most extreme example of the synergy between vegetation and moisture is seen across the vast block of tropical rainforests that lie in and around the Amazon basin.
Amazon: Earth's largest freshwater system
Stretching for thousands of kilometers from the Atlantic Ocean on the eastern side of South America to the slopes of the Andes in the west, the Amazon basin is the world's largest freshwater system. Rain falling on the Andean mountains on the western side of South America is recycled several times after it first fell on the far side of the continent, where clouds roll in with moisture evaporated from the distant Atlantic. Even though it is in the tropics, some of the water falling on the high Andean slopes does not do so as rain, but as snow.
From December to May there is an annual melt that usually coincides with torrential rains over the forests. When this happens the depth of the Amazon's main river channel increases by between about 9 and 14 meters. Huge tributaries back up andvast areas are inundated, creating a particular kind of flooded rainforest called várzea. During February and March, when the water is at its deepest, it is possible to glide in a boat through the forest canopy. The huge slow-flowing swamp that is the result is the habitat of a range of specialist animals, including huge fish. During this wetter season, an area of forests bigger than New Mexico (around 350,000 square kilometers) is under deep water, with the main channel swelling to some 40 kilometers across. Even during the dry season the main river in its middle reaches is still about 11 kilometers wide.
The várzea is but one component in a complex system, whereby the trees and water work in concert. At over 6,000 kilometers from source to sea, the Amazon basin river system drains nearly half of South America, a catchment of more than 7 million square kilometers. From streams arising on the slopes of the Andes to the sparkling rivulets flowing beneath trees in the forests, the capillaries drain into rivers that eventually aggregate into massive veins such as the Rios Negro, Madeira, Purus, Tapajos, Xingu, Caquetá and Putumayo, which in turn fill the great vena cava of the Amazon itself, that each day discharges some 15 billion tonnes of freshwater — all of which first fell as rain — to the Atlantic Ocean.
During the course of a year the Amazon system moves about one fifth of all the freshwater traveling in all of the Earth's rivers. Where this vast continental river reaches the sea, its mouth is over 325 kilometers wide — greater than the distance between London and Paris — projecting a vast plume of freshwater hundreds of kilometers out into the Atlantic. So huge is the river that ocean-going ships can navigate inland to Peru — around two thirds of the way up toward its source. The Amazon system is connected from end to end not only by water and shipping, but also wildlife, including a species of catfish that makes an annual migration in the headwaters on Andean slopes all the way to its mouth and then back again.
The rivers will of course only maintain flow so long as the rain continues to fall from clouds. That in turn depends on a supply of new water getting into the air and to the slopes of the Andean mountains. As might be expected, the proportion of water entering the air from leaves, rather than that evaporated from the surface of the ocean, increases with distance from the Atlantic, and with increasing distance from the sea, so the importance of the forest in maintaining rainfall goes up.
Why the rainforest is like a green ocean
In the tropical rainforests, where the organic green engine of photosynthesis hums at high speed, a single large tree can pump up over a thousand liters of water per day from the soil to the air, and in so doing dramatically changes the humidity above the forest where it grows. The vapor is lighter than the rest of the air and this triggers convection currents that take it skyward. At higher altitudes the air is cooler and this causes the vapor to turn back to tiny liquid droplets via condensation. The return to the liquid phase requires not only lower temperature, however, but also a non-gas surface upon which the return to liquid can be precipitated. You can see this taking place on a cool window in a steamy kitchen, or on grass at the dawn of a new day that follows a warm but cloudless summer evening. There are no windows or grass stems high above the rainforests, though, so other surfaces facilitate the return to liquid.
Whatever gets up to the cooler altitudes, where condensation can most readily take place, will of course be very small, and also very abundant. The materials that do this take the form of tiny floating non-gas particles called aerosols. These are the microscopic nuclei upon which water vapor condenses tocreate the tiny water droplets that come together in clouds. There are many different sources of these cloud-seeding materials, including dust kicked up from storms over deserts, salt grains from sea spray and various organic materials. Among the latter are compounds released through those pores on the leaves themselves, including terpene and isoprene.
At millionths or even billionths of a meter across, numberless trillions of these minuscule particles drift up from the forest in the humid air and in the presence of sunshine unite with oxygen to form very fine dust particles that have an affinity for water. These are among the tiny surfaces upon which condensation occurs. This treegenerated cloud fuel is augmented by billions of trillions of pollen grains rising into the air every day from the vast number of flowers, as well as fungal and bacterial spores. The result is a vast bank of cloud-seeding material being released by the rainforest itself, magic dust feeding into rising moist air to generate cloud, and then rain — lots of it.
So it is that the Sun's energy is harnessed by plants to lift water into the atmosphere, uniting with microscopic particles to create the rain that replenishes the headwaters of rivers, refills the aquifers that give rise to springs and moisten the soils that enable plant growth, and on a colossal scale. Each day some 20 billion tonnes of water vapor is emitted from the Amazon basin rainforests — more even than the huge quantity of liquid water that flows each day along the Amazon river itself. That vapor is a globally significant source of freshwater and a major proportion of the 90 percent of all the water that reaches the atmosphere via plant transpiration on land each day. The fact that only about 10 percent of the water arriving in the air from land is via simple evaporation (in other words isn't mediated by plants, and instead is coming directly from soils or concrete) underlines the vital roles played by vegetation in recycling freshwater.
Having said this, it is important to remember that most of the water vapor that goes into the air is from the surface of lakes and oceans (which are of course pure water). The oceans are more extensive than the land, but surfaces covered with forests can evaporate at least as much water as an equivalent area of lake or ocean. Dense stands of trees with understories of shrubs and covered in epiphytes present multiple layers of vapor, emitting surfaces, whereas the sea or a lake has just one.
Recent research suggests that strong parallels can be drawn between how rainforests and oceans work. One study using images generated by cameras borne on satellites and aircraft found that Amazonian clouds have a striking resemblance to maritime clouds, leading some researchers to describe the rainforest as a 'green ocean.' The parallel is seen in the expansive forest surface covering millions of square kilometers, stretched out beneath the atmosphere, vast, wet and exposed to the winds that move moisture, very much like an actual ocean.
How the rainforests pump water around the planet
Significant areas of the green oceans of rainforests that once covered so much of the humid tropics have, however, been drained, and (as we'll see later) with increasingly serious implications for humans. This is because the rainforests not only replenish cloud and thus freshwater, but, it seems, also help drive large-scale air movements, effectively pumping moisture-laden air inland. This is the result of the condensation of the vapor released by the forest, which causes dense clouds to form above the sea of trees. The condensation leads to a drop in air pressure, causing warm moist air to be sucked in from over the ocean. This brings with it the airborne water that creates the rains that moisten the forests in the first place — and which is then recycled by the forest into new clouds.
One person who has spent a lot of time looking at how this might work is a British scientist called Peter Bunyard. I've known Peter for years and bumped into him in 2011 at a conference on the conservation of tropical forests in Colombia. He has spent much of his career researching and writing about the Amazon rainforests and he excitedly told me about new experiments he was planning that should help explain what may be a very powerful force created by water and trees working in synergy. At the time Bunyard was researching at Sergio Arboleda University in Bogotá and had become absorbed with understanding how forests were apparently driving atmospheric flow through a previously unexplained mechanism.
The inspiration for Bunyard's experiments came from two Russian mathematicians from the Institute of Nuclear Physics in St. Petersburg: Anastassia Makarieva and Viktor Gorshkov. These researchers had put forward an idea called the 'biotic pump theory.' This explained how the physics of condensation would lead to surface airflows because of a force that enabled living systems to literally pump air. Without it, the humid ocean-derived airflow would dwindle away, leading to less and less rain. Through this theory it was possible to see how the endpoint of widespread deforestation could be desertification.
(Continues…)
Excerpted from "Rainforest"
by .
Copyright © 2019 Tony Juniper.
Excerpted by permission of ISLAND PRESS.
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