While storms have long been recognized as a cause of Amazon tree loss, the new study is the first to produce an actual count. The losses are much greater than previously suspected, the study’s authors say. This suggests that storms may play a larger role in the dynamics of Amazon forests than previously recognized, they add.

Previous research had attributed a peak in tree mortality in 2005 solely to a severe drought that affected parts of the forest. The new study says that a single squall line (a long line of severe thunderstorms, the kind associated with lightening and heavy rainfall) had an important role in the tree demise. This type of storm might become more frequent in the future in the Amazon due to climate change, killing a higher number of trees and releasing more carbon to the atmosphere.

Tropical thunderstorms have long been suspected to wreak havoc in the Amazon, but this is the first time researchers have calculated how many trees a single thunderstorm can kill, says Jeffrey Chambers, a forest ecologist at Tulane University, in New Orleans, and one of the authors of the paper.

In 2005, there was a spike in tree mortality in the Amazon. Previous studies by a coauthor of this new paper, Niro Higuchi of Brazil’s National Institute for Amazon Research (INPA), showed the second largest upsurge recorded since 1989 for the Manaus region. Also in 2005, large parts of the Amazon forest experienced one of the harshest droughts in the last century. A study published in the journal Science in 2009 pointed at the drought as the single agent for a basin-wide increase in tree mortality. But a very large area with major tree loss (the region near Manaus, in the Central Amazon) was not affected by the drought.

From 16-18 January 2005, a squall line 1,000 kilometers (620 miles) long and 200 kilometers (124 miles) wide crossed the whole Amazon basin from southwest to northeast, causing several human deaths in the cities of Manaus, Manacaparu and Santarem. The storm’s associated strong vertical winds, with speeds of up to 145 km/hour (90 mph), uprooted or snapped in half trees that were in their path. In many cases, the stricken trees took down some of their neighbors when they fell.

The researchers used a combination of Landsat satellite images, field-measured tree mortality, and modeling to determine the number of trees killed by the storm. By linking satellite data to field observations, the researchers were able to take into account smaller tree blowdowns (less than 10 trees) that otherwise cannot be detected through satellite images.

Looking at satellite images for the area of Manaus from before and after the storm, the researchers detected changes in the reflectivity of the forest that they suspected are indicative of tree losses. Undisturbed forest patches appear as closed, green canopy in satellite images. When trees die and fall, a clearing opens, exposing wood, dead vegetation, and surface litter. This so-called “woody signal” only lasts for about a year in the Amazon; the time it takes for vegetation to re-grow and cover the exposed wood and soil. Thus, the signal is a good indicator of recent tree deaths.

After seeing disturbances in the satellite images, the researchers established five field sites in one of the blowdown areas, and counted the number of trees that had been killed by the storm—researchers can usually tell what killed a tree from looking at it.

In the most affected plots, near the centers of large blowdowns (contiguous patches of wind-toppled trees), up to 80% of the trees had been killed by the storm, they concluded.

By comparing their field data and the satellite observations, the researchers determined that the satellite images were accurately pinpointing areas of tree death, and they calculated that the storm had killed between 300,000 and 500,000 trees in the area of Manaus. The number of trees killed by the 2005 storm is equivalent to 30% of the annual deforestation in that same year for the Manaus region, which experiences relatively low rates of deforestation.

The team then extrapolated the results to the whole Amazon basin.

We know that the storm was intense and went across the basin. To quantify the potential basin-wide impact, we assumed that the whole area impacted by the storm had a similar level of tree mortality as the mortality observed in Manaus.

—Jeffrey Chambers

Squall lines that move from southwest to northeast of the forest, like the one in January 2005, are relatively rare and poorly studied, says Robinson Negron-Juarez, an atmospheric scientist at Tulane University, in New Orleans, and lead author of the study. Storms that are similarly destructive but advance in the opposite direction (from the northeast coast of South America to the interior of the continent) occur up to four times per month. They can also generate large forest blowdowns, although it’s infrequent that either of these two types of storms crosses the whole Amazon.

This work was funded by NASA and Tulane University.

Resources

• Robinson I. Negrón-Juárez et al. Widespread Amazon forest tree mortality from a single cross-basin squall line event. GRL In Press