Forests

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 Anton 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 USA by the 1920s. In its place was younger, regrown 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 economies of tropical countries expanded (Figure 6.1). 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. A global average of 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 off 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 colonies 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 ocean 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 Chekov'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 affect climate are 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.

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