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Amazon rainforest greens up in the dry season

The Amazon rainforest puts on its biggest growth spurt during the dry season, according to surprising new research. According to Alfredo Huete (University of Arizona), lead author of the new study, "Most of the vegetation around the world follows a general pattern in which plants get green and lush during the rainy season, and then during the dry season, leaves fall because there's not enough water in the soil to support plant growth. What we found for a large section of the Amazon is the opposite. As soon as the rains stop and you start to enter a dry period, the Amazon becomes alive. New leaves spring out, there's a flush of green growth, and the greening continues as the dry season progresses." This finding holds true only for the undisturbed portion of the rainforest. Areas where the primary forest has been converted to other uses or disturbed "brown down" in the dry season.

The study suggests the deep roots of trees in the undisturbed forest can reach water even in the dry season, allowing the trees to flourish during the sunnier, drier part of the year. In contrast, plants in areas that have been logged or converted to other uses cannot reach deep water in the dry season and therefore either go dormant or die.

The researchers say that figuring out the metabolism of the Amazon is crucial for understanding how rainforests and other tropical environments function and how deforestation affects biodiversity and sustainable land use in the tropics. It will also help scientists better understand the global carbon cycle, which includes the natural sequestration and release of carbon dioxide. The finding that converted forests grow differently from undisturbed forests has implications for understanding the effects of fires in the tropics.

The research team made the discovery by analyzing five years of satellite images from the MODIS (Moderate Resolution Imaging Spectroradiometer) instrument on NASA's Terra satellite and cross-checking with information from sites on the ground. Their paper, with eight coauthors from the United States and Brazil, was published in Geophysical Research Letters. The research was funded by NASA and is part of the Brazilian-led Large Scale Biosphere-Atmosphere Experiment in Amazonia (LBA).

University of Arizona
Harvard University
Boston University
Instituto Nacional de Pesquisas Espaciais, Brazil
NASA Ames Research Center


Walrus calves stranded by melting sea ice

Scientists have reported an unprecedented number of unaccompanied and possibly abandoned walrus calves far offshore in the Arctic Ocean, where melting sea ice may be forcing mothers to abandon their pups as the mothers follow the rapidly retreating ice edge north. The sightings suggest that increased polar warming may lead to decreases in the walrus population.

Nine lone walrus calves were reported swimming in deep waters far from shore by researchers aboard the U.S. Coast Guard icebreaker Healy during a cruise in the Canada Basin in the summer of 2004. Unable to forage for themselves, the calves were likely to drown or starve, the scientists said. Lone walrus calves far from shore have not been described before, the researchers report in the April issue of Aquatic Mammals.

The researchers found evidence of warmer ocean temperatures that may have rapidly melted seasonal sea ice over the shallow continental shelf where walruses dive to feed on bottom-dwelling animals such as clams and crabs. Adult walruses use the sea ice as a resting platform; mothers leave the calves there and dive to the bottom for food.

"If walruses and other ice-associated marine mammals cannot adapt to caring for their young in shallow waters without sea ice available as a resting platform between dives to the sea floor, a significant population decline of this species could occur," the research team wrote. The lead author of the study is Lee Cooper (University of Tennessee); coauthors represented five other institutions (see below).

Cooper and colleagues made the unexpected walrus calf sightings during a cruise to investigate the impact of global climate change on the oceanic ecosystem over the continental shelf of Alaska. The focus of their study ranged from the shallower waters of the continental shelf in the Chukchi Sea to deeper waters in the Beaufort Sea of the Western Arctic Ocean. The project was funded by NSF and the Office of Naval Research.

Sea ice normally forms over the continental shelf north of Alaska and persists even in summer. In 2004, however, the researchers measured a mass of water as warm as 7°C (44°F) moving onto parts of the shelf from the Bering Sea. This warm-water intrusion was more than 3°C higher than temperatures at the same time and location in 2002. The warmer water apparently caused seasonal sea ice to melt rapidly over the shallow continental shelf. Ice remained in deep water, with the bottom up to 3,000 meters (about 9,300 feet) deep—too deep for even adult walruses to dive to feed.

University of Tennessee
Woods Hole Oceanographic Institution
University of Miami
University of Maryland
University of Rhode Island
Oregon State University


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