The world’s forestry cover is being stripped at an alarming rate – destroying a vital defence against global warming.
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A dark green blanket is draped across the freezing northern latitudes of the globe. This is the quiet, vast taiga, a forest of conifers that stretches from Scandinavia, Siberia and Japan across to Alaska and Canada. The trees have just a few short months in which it is warm enough to grow. In the winter, temperatures can plunge to -50°C. Even so, they pump out much of the world’s oxygen.
Travel south, and nature changes from serene to raucous. The jungles of the Amazon and the Congo Basin are hot and tangled, loud with animal and bird life. But in both the far north and the tropics, and in many places in between, forests are coming under new pressures that have the potential to fuel the spiral of a warming climate.
We have chopped or burnt half of the Earth’s original forest cover. Just a third of the Earth’s land mass is now covered by forests, a total of four billion hectares of land. And stripping of that forest cover by humans continues at an alarming rate, particularly in the ecological treasure houses that are the tropical rainforests.
But human encroachment is no longer the only threat. A changing climate is tinkering with the ecological balance in many forests, sometimes for good but often for bad. In cooler parts of the world, such as the area the taiga covers, trees are growing more densely and vigorously because of warming temperatures. They also face new threats as insect predators expand their range.
In warmer zones, trees are coming under greater stress because of higher temperatures, increased risk of forest fire, altered rainfall patterns, new insect threats and more frequent extreme weather, such as severe storms and blizzards. Waves of beetles have ravaged conifer forests from Mexico to Alaska, killing nearly 30 billion trees since the 1990s, in part because warming has speeded up reproduction of beetles in cooler areas. (New Zealand has some species of bark beetles, but none of the rampant kind such as the mountain pine beetle.) And in the inner valleys of the Swiss Alps, drought is thought to have triggered high death rates of scots pines, with 60% of trees lost in some sites. In Spain, tree mortality and defoliation has risen over several decades.
Trees are themselves a vital counter to climate change. Standing forests lock up vast stores of carbon during photosynthesis, retaining them in leaves, wood, organic matter in the soil and leaf litter on the forest floor.
But when cleared or ravaged by drought or fire, forests emit carbon as the wood and organic matter break down. Dying forests have the potential to speed up climate change by pumping out more carbon. Currently forest clearance is responsible for a sixth of global carbon emissions.
If there is a silver lining, it is that the human assault on forests is slowing. In places like China and North America, new forests are being planted. Overall, the amount of land area covered by forests shrank by 8.3 million hectares each year in the decade to 2000. That’s according to the most comprehensive recent tally of the state of the world’s forests, a 2010 report by the United Nations’ Food and Agriculture Organisation (FAO), which uses satellite imagery to monitor the ebbs and flows of the globe’s forests.
But in the decade to 2010, the rate of net forest loss had slowed to 5.2 million hectares a year – an area nearly half the size of the North Island. Another tiny glimmer of hope arises from having more forested land coming under protection. Currently more than 10% of the world’s forests are protected.
Oceania
Forest fires and severe drought in Australia have been responsible for an acceleration of forest loss in the Oceania area. The annual net loss of forested area rose from 700,000ha in the 1990s to a million hectares in 2005-10. What isn’t clear yet is how much of Australia’s lost forest cover will regenerate rapidly, and how much will not return because of hotter, drier conditions as a result of climatic change. Since 2000 there has been an increase in high-intensity megafires, especially in Australia’s open forests.
South America
The largest area of untouched forests grow in the Amazon Basin, reaching across such countries as Bolivia, Brazil, Colombia, Peru and Venezuela. But the rate that these forests are being stripped is among the fastest in the world. Between 1990 and 2000, 4.2 million hectares were felled each year, although that slowed to four million in the following decade. From 2005 to 2010, the annual loss slowed further to 3.6 million hectares. Drought caused by climate change is predicted to cause significant dieback of the Amazon.
Africa
As in South America, the picture of deforestation in Africa is alarming. Four million hectares a year of forests were lost in the decade to 2000. The following decade, that slowed to 3.4 million, according to the FAO. The most precious of the Africa forests are the steamy woods of the Congo Basin. These are the second biggest area of indigenous forests in the world after the Amazon.
North and Central America
The total forested area has remained largely stable since 1990. Although some forested land has been lost in Central America, an extra two million hectares of land were planted in forests in North America, and another one million hectares in the Caribbean. The total carbon stock in forest biomass increased, according to the FAO.
Increasingly large areas have been subject to forest fire over the past two decades. Areas affected by insect attacks declined in the past 20 years, but have begun increasing again. Insect damage reached a new peak in the US in 2005, affecting 22 million hectares, or 3.4% of forested areas. Since the late 1990s, more than 11 million hectares of forests in Canada and the western US have been devastated by the mountain pine beetle. The explosion in the beetle population has been exacerbated by higher winter temperatures, allowing the insect to expand its range into new areas.
Europe
Europe, which contains close to 30% of the world’s forested areas, increased its forest cover by more than 700,000ha a year over the past two decades. Natural expansion of forests into abandoned agricultural land is common in Europe. The total amount of carbon stored in the forests also increased as trees grew more vigorously. However, drought appears to have been behind significant dying off of scots pines in the Alps, and of increased tree mortality in Spain.
Asia
Large-scale tree planting, particularly in China, has turned around the overall loss of forestry cover in Asia. Although forests shrank overall by about 600,000ha a year in the 1990s, in the following decade the forested area grew 2.2 million hectares a year. Overall, the forested area is about 16 million hectares greater than it was in 1990. However, there was still a continuing loss of forests in South Asia and Southeast Asia. Of particular importance are the virgin tropical rainforests that stretch from the Mekong in Laos to Indonesian Borneo. They make up the third major area of untouched forests in the world, after the Amazon and the Congo. In China, logging bans have been put in place in many existing forests.