Amazon Rainforest Grows faster in the Dry Season

Contrary to previous beliefs, new evidence suggests that the Amazon Rainforest actually grows more during the dry season. Contrary to most of the vegetation around world when the rain stops growth slows, it appears the opposite is true for parts of the Amazon Rainforest. However it appears this only occurs in regions of the forest that are undisturbed. Those regions that have been used for other reasons due get browner, as would be expected.

"Huete, a UA professor of soil, water and environmental science.

Huete 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.

Figuring out the metabolism of the Amazon rainforest, the largest old-growth rainforest on the planet, is crucial for understanding how rainforests and other tropical biomes 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 affects the natural sequestration and release of carbon dioxide, a major greenhouse gas.

The finding that converted forests grow differently from undisturbed forests has implications for understanding fire regimes in the tropics, including the fires that sometimes rage in tropical areas during El Nino years, which bring drought to many tropical areas, including the Amazon.

The research team figured out when the intact forest grows by analyzing five years of satellite images from the MODIS (Moderate Resolution Imaging Spectroradiometer) instrument mounted on NASA's Terra satellite and by cross-checking with information from local sites on the ground. The research was funded by NASA and is part of the Brazilian-led Large Scale Biosphere-Atmosphere Experiment in Amazônia (LBA).

The paper by Huete, Scott Saleska, a UA assistant professor of ecology and evolutionary biology, and their colleagues, "Amazon Rainforests Green-Up with Sunlight in Dry Season," is scheduled for the March 22 issue of Geophysical Research Letters. NASA funded the research. A complete list of authors can be found at the end of this release.

The MODIS instrument began collecting data in 2000. Once a day, MODIS takes a picture of each spot on the Earth. Each pixel in the images represents a square of about 820 feet (250 meters) on a side. If it's too cloudy at one spot one day, the next day's picture may be fine. Five years' worth of pictures means the scientists have at least one good image of every spot for every month of the year.

To be able to figure out when the Amazon rainforest is growing, Huete's lab used a new measure, called Enhanced Vegetation Index (EVI), for detecting greenness in MODIS images of very highly vegetated rainforests. Greenness is an indicator of active plant growth.

Plants are green because they contain the photosynthetic pigment chlorophyll. Growing plants generate more chlorophyll and therefore look greener."

The increase in greenness is a way of looking at the Amazon's health.

The researches next steps include looking at other rainforests around the world to see if they behave in a similar manner.

Tags: amazon rainforest, rainforest, dry season, vegetation