Humans And The Carb0n Store 0f Plants

No single species alters the world's habitats as much as humans do. Even in distant prehistory, the arrival of people in a particular part of the world could be heralded by an increase in burning. For example, in Australia there was a sudden jump in fire frequency around 45,000 years ago, right around the time when humans first show up in the archaeological record. From what we know of the Australian Aborigines and other hunter-gatherers when they were first contacted by Europeans, humans burned the vegetation for many different reasons. Fire was used as an aid to hunting, in driving frightened prey towards an ambush or over a cliff. It was used to clear brush to provide a clear line of sight for hunting, and to encourage green regrowth that would attract grazers to an area. Fire was also used in warfare, and as a signal, and sometimes fires were just set out of boredom. In many parts of the world, frequent burning must have altered the structure of vegetation and changed its carbon storage. In Australia, for example, the burning led to a spread of grasslands at the expense of woody scrub, and a change in the species composition of the wooded areas that remained. In areas of central Africa where humans have probably been burning the savanna for well over a million years, the fires have tended to keep back the forest while maximizing the extent of grassland.

When agriculture spread out from its several areas of origin over the last 12,000 years, the alteration of the world's vegetation became far more intense. Forests were now particularly susceptible to clearance, because humans had such an interest in the fertile land beneath the trees which can be used for agriculture. The denser populations of humans that agriculture can support also placed a huge demand on wood and other products of trees, resulting in frequent thinning and harvesting of the natural forests. Whole regions—such as Europe, China and India—had almost all their lowland forest cover removed and replaced by fields. In the uplands, where it is more difficult to farm, the forests would sometimes come back after cutting if fires were not too frequent, and if there were not too many sheep and goats to eat the seedlings. In many areas where these factors conspired against it, the forest was not able to come back and the upland landscape remained as scrub, grassland or (in very moist climates) peat bog.

These changes caused by humans occurred against a background of natural shifts in climate that also tended to alter the vegetation, giving less forest and more desert (sometimes amplified by vegetation-climate feedbacks, see Chapters 5 and 6). These natural climate changes alone would have been enough to reduce global carbon storage in forests by some 10%, and humans may have reduced the total by another 10% as they removed the forests of Europe and China during ancient times. The tens of billions of tonnes of CO2 were released rather gradually over thousands of years, slowly enough for it to be swallowed up by the rest of the carbon cycle rather than accumulating in the atmosphere. Most of it must have gone into the oceans, into the alkalinity reservoir.

In North America, the main phase of clearance was recent enough to be recorded in the contemporary writings of naturalists. Although the American Indians had farmed there for thousands of years, in most areas their population densities were still low enough that they did not make very much impact on the landscape. When the first Europeans arrived, not only was the eastern USA mostly forested (at least 90% was forest-covered), but the physical structure of the forest was quite different too. The earliest times of settlement on the eastern seaboard are not well recorded, but later accounts as the frontier moved inland through Ohio, Indiana, Illinois and Wisconsin make it clear that big trees—very tall and of large girth—were far more abundant than they are nowadays. Particularly impressive were the chestnuts, tulip poplars, hemlock and elms which towered like the columns of a cathedral. Other common species not normally thought of as being especially large—for example, white oak, red oak and sycamore—often reached a much greater size than one would be used to seeing nowadays. Much of the reason that we do not presently see such large trees in the American forests is that there has not been time for them to reach full maturity before being cut for timber. The forests have been kept in a young state by continual harvesting of trees as soon as they became large enough to be useful. The last of the "old-growth" areas to be exploited in the east seem to have been in the Smoky Mountains, in the early 1900s. Nowadays, there are only a few small fragments that may resemble the original virgin forest, mostly in small, steep-sided valleys in the southern Appalachians.

In many areas the forest that the European settlers encountered was cut down and allowed to rot or to dry out and burn, to make way for fields. Either way, the carbon that had been held in the trees ended up being released as CO2 to the atmosphere. The abundant organic carbon in the humus of forest soils would mostly break down to CO2 over the first several decades of cultivation. Old-growth forests in North America and elsewhere also tended to have a lot of carbon stored in fallen branches and trunks of dead and rotting trees, with the stumps and roots of dead trees remaining below ground: a reservoir known as "woody debris''. Even if the settlers did not deliberately burn this, it would have broken down and oxidized during the first decades after the forest had been cleared. In logged areas that were allowed to remain as forest, the debris would have decayed at its natural pace but would not have been replaced by new material; in a harvested forest trees do not fall over and die but instead end up at the sawmill before they can reach old age. Although much of the timber that was extracted from the original forests of North America was used in construction and not immediately allowed to rot, over centuries many of these buildings fell into disrepair or were consumed by fire. Hence, by various routes the carbon of the original forest would eventually have been released as CO2. It is thought that the rapid clearance and exploitation of American forests in the mid to late 1700s and early 1800s would have released about 50 billion tonnes of carbon, enough to contribute to an initial up-tick in atmospheric CO2 that occurred after 1750, beginning the rise that continues to the present (Figure 7.10).

The tropical forest areas of the world have also been exploited for agriculture and timber for many thousands of years, although in most areas the infertility of the soils prevented large-scale farming. The only major exception (at the drier margins of the tropical rainforest zone) seems to have been the Mayan civilization which grew up in the Central American lowlands, using careful cultivation techniques that recycled nutrients. Nevertheless, in the end this civilization too seems to have declined partly in response to a gradual loss of soil fertility.

It is only really in the last century or so that extensive logging and clearance of tropical forests has taken place. Initially, colonial powers began to take useful timber for export, and then (from the 1950s onwards) clearance increased enormously as populations and economies of tropical countries exploded with modern medicine and agricultural technology. The Amazonian forests of Brazil are now being cleared faster than ever, and so far around 15% of the original old-growth forest has been lost.

Figure 7.10.

Ice core record of atmospheric C02 since 1000 ad, Law Dome,

Antarctica ice cores. Source: Etheridge et al. (CDIAC).

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Air age (year ad)



1250 1500 1750

Air age (year ad)


Many other tropical countries—such as Vietnam, Panama, Nigeria and Costa Rica— have already lost the majority of their forest cover in the last 60 years (see Chapter 6). Many of the areas that remain forested may also have undergone a more subtle depletion of carbon storage over the last century or so, as loggers have partially exploited the forests for their best timber. For example, Sandra Brown of Winrock International has found indications that the forests of the Malay Peninsula and eastern Amazonia have had most of the really large trees extracted, except in the most inaccessible areas where these forest giants still exist, enabling the comparison. The areas that have had the big trees removed still remain as forest, but they have substantially less carbon within them.

The amount of carbon presently stored in the world's living trees alone is about 300 billion tonnes. If all the world's forest cover were to be cleared and burned, the oxidation of the trees would raise the C02 concentration in the atmosphere by nearly half above what it is today. Oxidation of soil organic matter from under the cleared forest land could raise it by as much again. As it is, it is unlikely that all of this carbon would ever be released. However, since deforestation contributes a major proportion (about a quarter to a third) of the C02 entering the atmosphere (see below), anything that alters its pace is of immediate importance.

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