Forests continue to decline worldwide, according to a new study in the Proceedings of the National Academy of Science (PNAS). Employing satellite imagery researchers found that over a million square kilometers of forest were lost around the world between 2000 and 2005. This represents a 3.1 percent loss of total forest as estimated from 2000. Yet the study reveals some surprises: including the fact that from 2000 to 2005 both the United States and Canada had higher percentages of forest loss than even Brazil.

Counting forest loss due either to human disturbance or natural causes, the study found that North America lost the most forest of the world's six forest-containing continents. Perhaps surprisingly, thirty percent of total forest loss occurred in North America alone. Combined with South America—the largest extent of tropical forests in the world—the two continents represent half of the world's total forest loss. Africa, in turn, suffered the least forest loss.

Forest loss by nation

Of the seven nations that contain over a million square kilometers of forest—Russia, Brazil, the United States, Canada, Indonesia, China, and the Democratic Republic of Congo—Brazil lost the most total forest during the five year time period.

According to the researchers Brazil lost 26,000 square kilometers (10,038 square miles) per year of its rainforest, and 7,000 square kilometers (2,702 square miles) in its dry tropical forests. Over the five years, total forest loss in Brazil came to 165,000 square kilometers (63,706 square miles). In all this represents 3.6 percent of its total 2000 forest cover: half a percent higher than the global average.

Canada was close behind Brazil: losing some 160,000 square kilometers (61,776 square miles) of its forest cover. However, proportionally Canada's forest loss equals 5.2 percent of the nation's total forest cover: higher than Brazil's percentage and over two points higher than the global average.

But the United States had the greatest percentage loss of the seven nations—even more than Brazil and Canada—losing 6 percent of its forest cover in just five years time, a total of 120,000 square kilometers (46,332 square miles). While fire and beetle infestation played a role in Alaska and the western US, large-scale logging in the southeast, along the western coast, and in the Midwest play a big role in the nation's continuing forest decline.

The researchers write that "the often publicized phenomenon of forest conversion within the humid tropics is observed in our results, but significant GFCL [i.e. global forest cover loss] is evident in all biomes. For example, rates of GFCL in regions such as the southeast United States are among the highest globally."

Indonesia lost 3.6 percent of its forest cover during the five years, Russia 2.8 percent, China 2.3 percent, while the Democratic Republic of Congo lost the smallest percentage: 0.6 percent.

The study also highlights other nations with significant forest decline including Malaysia due to palm oil plantations, Paraguay and Argentina to agriculture, and Australia to fires.

Forest loss by ecosystem

Comparing the world's four major forest ecosystems, the study found that the boreal experienced the largest loss in the five years, tropical rainforests came in second, dry tropical forest were third, and finally temperate forests.

In the boreal 60 percent of the loss was due to fires, while the remaining 40 percent was caused by logging, disease, and infestations of pine beetles linked to climate change.

For tropical rainforests the majority of the loss was due to clearing for agriculture in Brazil and plantations in Indonesia and Malaysia. The report found that while the Congo Basin was impacted by some selective logging, large-scale clearing for agriculture was not yet an issue leaving the region with less forest decline.

Dry tropical forests were largely impacted by clearing for agriculture in places like Brazil, Paraguay, and Argentina.

Temperate forests suffered the least amount of total loss because, as the researchers write, "the majority of this biome has long been converted to agricultural and settlement land uses." Yet proportionally, temperate forests came in second of the four forest types largely due to high deforestation rates in the United States.

In measuring forest loss, the study calculated forest cover as 25 percent of canopy cover with trees over 5 meters high.

By Jeremy Hance