The Amazon rainforest makes up nearly half of tropical vegetation worldwide. It currently processes more than twice the amount of carbon dioxide belched out in fossil-fuel emissions. It's one of Earth's major assets, and yet we understand very little about how this incredible resource operates. Previous satellite observations have indicated that the forest has seasonal patterns, but it isn't clear how real this signal is, given that satellite measurements are disrupted by cloud cover and smoke from burning.

Now, Matthew Jones from the University of Montana, US, and his colleagues have used a new remotely sensed microwave vegetation optical-depth parameter – developed by the Numerical Terradynamic Simulation Group at the University of Montana – to get a clearer view of seasonal patterns in the Amazon rainforest. They compared their remote sensing data, which spanned nine years (2003–2011), with field and flux tower data from four locations in the Brazilian Amazon rainforest.

The results show that the forest has clear seasonality, and that the timing of the seasonality changes as you travel from west to east. In the western Amazon of Peru there is plenty of rainfall. "There are only two months of dry season here," said Jones. However, the high volume of rainfall also means that the forest is often shrouded in cloud. "In this case we found that the maximum leaf flush occurs during the dry season, when there is a peak in sunlight levels," he added.

In the eastern Amazon, in contrast, the dry season lasts more than six months. To have enough water for growth, Jones said that "the trees here have to produce their leaves when water is at a maximum, during the wet season – the pattern is pretty much the opposite of what we see in the western Amazon". So the timings of water availability, canopy biomass growth and net leaf flush are more in synch.

Moving from west to east along a transect through the Amazon rainforest, Jones and his colleagues found that there was a gradual change in seasonality, from a dry-season leaf flush in the west to a wet-season leaf flush in the east.

The study demonstrates that trees have an adaptive mechanism, and adjust their behaviour according to their environment. Looking ahead, there is unease about how climate change might affect the Amazon rainforest, and how the trees might respond to drought. "Climate change is a real concern, and our study shows that the adaptive phenology of these trees may provide a level of resilience if the changes in climate happen at a slow enough rate," said Jones. "Furthermore, we also observe regional susceptibility to extreme droughts in the short term."

The team reported their results in Environmental Research Letters (ERL).

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