Forests temper a stern climate, and in countries where the climate is milder, less strength is wasted in the battle with nature

Uncle Vanya, by Anion Chekov.

Since the beginning of agriculture, 12,000 years ago, humans have had an uneasy relationship with forests. On one hand, the forests provided timber, and good hunting for game. But they also took up space where crops might be grown, and provided a refuge for malevolent creatures both real and imaginary. As farming spread out from its first heartlands in the Middle East, northern China and Central America, forests began to lose ground. Already by the time of ancient Greece 2,500 years ago, deforestation was so extensive that Plato lamented that some mountain lands that had yielded good stout timber were now "good only for bees". Evidence from pollen preserved in lake beds shows that the majority of Europe and China's natural forest was already cleared by this time. The remaining forest in both these regions continued a slow, halting decline and reached a low point some time in the last few centuries. A more recent burst of forest clearance occurred when European settlers arrived in North America from the 1600s onwards. At first, there were huge tracts of almost unbroken forest in the east, yet by the mid-1800s most of this forest had been cleared and replaced by farmland. For example, southern New England was more than 90% forested when settlers first arrived, but by 1870 there was less than 25% forest cover. In Midwestern areas such as the forested parts of Wisconsin, deforestation started later (in the 1830s) as settlers moved west, and reached a low point around 1900 with only about 10% forest cover. The character of the surviving forests was also very different. Uncut old-growth forest, which Thomas Jefferson had suggested held enough timber to last 500 years, was mostly gone by the mid-1800s and essentially disappeared in the eastern IJSA by the 1920s. In its place was younger, rcgrown forest with smaller trees and altered species composition, harvested every few decades for timber.

In the tropics, the main burst of deforestation began later—in the 20th century— and it is still under way. This big increase in deforestation started around 1950, as populations and economics of tropical countries expanded. Thus, for example, in Costa Rica the area of forest was reduced from 67% primary (meaning original, old growth forest) forest in 1940 to only 17% in 1983. Vietnam was about 45% forested in 1943, but this figure had fallen to about 20% by the mid-1980s. So far, it seems that somewhere around 12 to 15% of the primary rainforest of Amazonia has been cleared, although parts of this have now reverted back to relatively species-poor secondary forest. Over each of the last five years up to 2005, deforestation was most extensive in South America, where an average of 4.3 million hectares (10.6 million acres) were lost annually over the last five years, followed by Africa with 4 million hectares (9.8 million acres) according to figures from the UN's Food and Agriculture Organization. Although all of this represents a huge area lost each year, the rate of deforestation has declined in the last decade, giving some hope for the long-term future of tropical forests. An average 7.3 million hectares have been lost annually over the last five years, down from 8.9 million hectares (22 million acres) a year between 1990 and 2000.

The effects that forest loss might have on climate have been thought about for a lot longer that most people would expect. It is a surprise for modern ecologists to find a character in Chekov's late 19th century play Uncle Vanya already talking of the influence that forest cover might have on climate, and advocating tree-planting for the purpose of climate improvement. Yet, the ideas are far older still. Christopher Columbus in the 1490s suggested that the verdant forests of the Caribbean islands helped to produce the abundance of rain that fell on them. His thinking was influenced by folk knowledge of the history of the Spanish and Portuguese islands oil* northwest Africa: the Canaries, Madeira and the Azores. It was felt that the almost complete deforestation of these islands had resulted in a drier, less rainy climate than when Europeans first arrived.

The possible climatic effects of the rapid deforestation of the American colonics were keenly discussed by a succession of English and American scientists from the late 1600s to the early 1800s. In early colonial times, observers of nature echoed Columbus in suggesting that the humidity and frequent thunderstorms of the eastern USA in summer were a product of the abundant forest cover. Later, as the cultivated lands extended, it was suggested that forest clearance was causing rainstorms to become less frequent, and the air was becoming generally less humid than before. Another view at the time was that the climate was becoming more "moderate" as a result of deforestation, with cooler summers and warmer winters. One writer hypothesized that this was because occan breezes could now blow further inland without the trees blocking them. Opinion on the rain-generating influence of forests on climate was by now so deeply held that in the 1790s laws were passed in the Caribbean islands to establish forest preserves. The hope was to increase rainfall, ensuring better growth of sugar cane.

During the 1800s, however, the view that forests had a significant influence on climate had both its advocates and skeptics in the scientific world. By the late 1800s it had lost favor, and mainstream scientists generally agreed that forests were unimportant in shaping climate. So, by Chckov's time this was rather an old-fashioned view that had already been mulled over and rejected by prevailing scientific opinion. However, such ideas did not entirely die out, even if they were no longer scientifically respectable. In the 1970s, for example, the idea that loss of forest in the tropics could dry out the climate over extensive areas was a major fear and a rallying point for the environmental movement.

In the last 30 years, the view that forests are important in making the climate has undergone a remarkable resurgence, backed up by sophisticated modeling techniques. The modern tools for understanding how forests aflcct climate arc the high-powered computer, and complex models of atmosphere, land surface and ocean (Chapter 1), incorporating many of the microclimatic effects of vegetation cover mentioned in Chapter 4. It is looking like Columbus and the natural philosophers of the 1600s and 1700s were not too far wide of the mark, after all. Loss or gain of forest both natural and caused by humans may have all sorts of consequences for climate.


To get very far in understanding the effects of forest cover on climate, we need to break down the complex form and behavior of the forest into simple components. These arc the building blocks of a model that can include the role of forest in making climate. Several of them have already been talked about in Chapters 4 and 5, but it will do no harm to mention them again (Figure 6.1). One important basic aspect of forests is the proportion of sunlight that they absorb. Known as "albedo" (from a Latin word meaning "whiteness") this is important in determining how easily the forest can heat up in the sun. The darker the forest surface (i.e., the lower the albedo) the more solar energy is absorbed, as opposed to being reflected straight back out into space. When it is absorbed, this energy tends to heat up the leaves. Some of the heat then goes to warming the air around the top of the forest canopy. But. in fact, much of the heat energy that is in the leaves just "vanishes"; the leaves stay much cooler than you would expect from all the sun's energy that they are absorbing. The missing heat has not really vanished—it has just been stored for a while in the water vapor that evaporates from the leaves. This is known as latent heat. It is a strange thing that even though the dark forest covcr is absorbing more heat from the sun—compared with a more sparsely vegetated environment—it does not show up in terms of temperature! The more open non-forested environment will nearly always be hotter during the day.

So, this brings us to a second important aspect of how forests affect climate. Transpiration, the evaporation of water out of tiny pores in the leaf surface, takes up heat. This is water that has fallen from the sky, soaked into the soil, been sucked up

Darker forest surface absorbs light

Lighter open arid vegetation reflects light

Evapo-transpfration from leaves

Evapo-transpfration from leaves

H20 ^ H2O H20

Water in soil

Figure 6.1. Some of the ways in which forests modify temperature, (a) Albedo: the dark forest surface absorbs sunlight, warming the air. (b) Latent heat uptake in evaporation cools the air. (c) Roughness helps to feed heat and water vapor to the atmosphere above, cooling the forest. Source: Author.

by roots and carried up within the tree all the way to the cells of the leaves. A single large tree can take up as much heat in evaporation as you'd get from one hundred 100 W lightbulbs burning continuously. This stored latent heat will eventually be released somewhere up in the atmosphere, thousands of meters up and perhaps hundreds or even thousands of kilometers away. What enables the heat of evaporation to be released again is the condensation of water into droplets, forming clouds and eventually rain. In addition to this, there is what is known as "physical evaporation": rainwater evaporating from the surface of the leaves or from the soil surface, without having passed through the tree. In forested areas, the greatest part of the evaporation of water going on is through transpiration from the leaves, rather than physical evaporation.

Climate scientists refer to the proportion of the heat absorbed by the forest that goes into evaporation—rather than just heating up the air—as the "Bowen ratio ". This is something that varies between different forest and vegetation types, but also according to season and even with the most recent weather conditions.

In essence, forests pump heat and water out into the air above them. They do this more effectively than most other vegetation types, and far more so than bare soil. Something that also helps forests act as water pumps to the atmosphere is that they store a lot of rainwater amongst the root mass of the forest, which is rich in spongy organic matter from the decay of leaves, roots and branches of the trees. Water that would otherwise run straight off the land surface and down to the sea is instead held in the soil, to be sucked up by roots and then evaporated from leaves in the canopy. The deepest roots of many trees reach tens of meters down into the ground, and this also helps them to sustain a good rate of evaporation long after the surface soil has dried out. because the trees can continue to tap into groundwater in pores in the rocks below.

IIow do these two processes—heat transfer and water transfer—affect climate? On the local scale, evaporation from all the leaves in a forest canopy makes the surrounding air cooler than it would otherwise be (Chapter 4). The moisture from leaves also affects broader-scale aspects of regional climate. It increases the humidity—giving, for example, the sticky summer climate of the southeastern USA when plenty of heat and plenty of rainfall combine in a predominantly forested landscape. This humidity itself keeps the night warm: as the air cools in the evening, some of this water vapor condenses out yielding heat that helps prevent the air from cooling further. And the water vapor itself acts as a "greenhouse gas", trapping heat radiated by the forest canopy during the night and sending it straight back down to earth.

Leafing out in spring in the temperate forest zone has an immediate effect on temperature due to the onset of transpiration. The progressive increase in temperature into spring is halted for a few days by the transpiration from these newly formed tree leaves (Figure 6.2). These patterns seem to be paralleled more extensively in the tropics, where models and observations suggest that transpiration from forest keeps the climate cooler and rainier (sec below), and less variable between night and day.

So, in at least some cases such as these, Chekov's characters were right after all: the forest docs seem to moderate the climate.

Leaves Come Out Spring Clipart

Figure 6.2. As the leaves come out. the progressive warming into spring halts for a few days because of the latent heat taken up by evaporation from the leaves. From: Bonan.

Days Before/After Leaf Emergence

Figure 6.2. As the leaves come out. the progressive warming into spring halts for a few days because of the latent heat taken up by evaporation from the leaves. From: Bonan.

Another way that forests modify atmospheric processes is through "roughness". The crowns of forest trees often look like heads of broccoli packed all against one another, with bumps on the surface and valleys between them. This uneven surface lets the wind blow down between their crowns but then find its way blocked by others in front, and it tends to set the air rolling, a type of motion known as "turbulence" (Chapter 4). The big trunks and branches also act as barriers for the wind, slowing the wind down and making it more turbulent. All this turbulence set oil* by the forest canopy tends to carry heat and water vapor upwards more effectively. So, the roughness of forest surfaces makes them feed water to the atmosphere more rapidly, compared with smoother surfaces such as grassland, crops or bare desert.

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