The Amazon Basin (black outline) stretches across the heart of South America. Much of the area is covered by rainforests. Layer upon layer of amazingly diverse plants and trees reaches from the forest floor to the tree tops. With all this lush plant life, it can be hard to believe that parts of the Amazon go months each year with little or no rain. Even more surprising is the fact that the dry season may actually be the “good season” for tree and plant growth.
A team of scientists led by ecologist Alfredo Huete at the University of Arizona made this discovery by combining five years of vegetation greenness data collected by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite and then subtracting wet-season measurements from dry-season measurements. This image shows those results. Greenness is shown using a scale called the Enhanced Vegetation Index. Green indicates where vegetation flourished during the dry season, and brown shows where vegetation declined during the dry season.
The undisturbed forests greened up when rainy season clouds departed. The greenness of the savannas and grasslands to the north and south declined during the dry season. Pastures cleared in former rainforest areas, such as along the southeastern margin of the Amazon and in isolated pockets along the river, also declined in greenness during the dry season. Apparently, lack of sunlight in the wet season limits forest growth more than lack of water during the dry season. Mature rainforest trees can draw up water from tens of meters deep in the soil. This deep water and plenty of sunshine allow the forest to green up in the dry season, which in some places can last as long as 4 to 5 months.
To learn more about the discovery of the Amazon’s dry-season green-up, read the Earth Observatory feature story Defying Dry: Amazon Greener in Dry Season than Wet.
- Huete, A.R., Didan, K., Shimabukuro, Y.E., Ratana, P., Saleska, S. R., Hutyra, L.R., Yang, W., Nemani, R.R., and Myneni, R.(2006) Amazon rainforests green-up with sunlight in dry season. Geophysical Research Letters, 33, L06405, doi:10.1029/2005GL025583.
Map by Robert Simmon, based on data from the University of Arizona Terrestrial Biophysics and Remote Sensing Lab.