In 2005, parts of the Amazon Basin experienced extreme drought conditions. There was also a major spike in tree loss that year, and several previous studies have linked these two observations. However, a new study in Geophysical Research Letters has placed the blame for the tree loss to a ‘squall line’ – a long line of severe thunderstorms – that crossed the whole Amazon Basin from southwest to northeast between 16 and 18 January.

The squall line was 1,000 kilometres long and 200 kilometres wide, and the associated winds reached speeds of up to 145km/h, uprooting and snapping trees in the storm’s path. 

A team of researchers, led by Robinson Negrón Juárez of Tulane University in New Orleans, used a combination of Landsat satellite images, field measurements of tree mortality and modelling to estimate the number of trees killed by the storm. They first used changes in forest reflectivity in Landsat images from the Manaus region to find areas where trees had been lost. They then visited five field sites and counted the number of trees killed by the storm. In the central areas of the most affected plots, up to 80 per cent of trees had been killed.

These data, when combined with the satellite data, gave an estimate of 300,000 to 500,000 knocked down trees in the Manaus area. Assuming a similar level of tree loss along the whole of the storm’s path gave an estimate of between 441 million and 663 million trees destroyed across the whole of the Amazon Basin.

September 2010