Leaf-litter production in human-modified Amazonian forests following the El Nino-mediated drought and fires of 2015-2016

Leaf-litter production is an essential part of the carbon cycle of tropical forests. In the Amazon, it is influenced by climate, presenting high levels during the driest months of the year. However, it is less established how extreme climatic events may impact leaf-litter production in the long term. Even more unclear is how litter production is affected by human-driven disturbances. Here we examine the effects of the 2015-16 El Nin similar to o drought and subsequent fires in the leaf-litter production of human-modified Amazonian forests, thus investigating the interactions of a climatic extreme with anthropogenic disturbances on this key process of the Amazonian carbon cycle. We sampled leaf litter from April 2015 until March 2019 across 20 plots located in the eastern Brazilian Amazon, in a total of 11,548 samples. Plots were distributed along a pre-El Nin similar to o gradient of human disturbance, including undisturbed, logged, logged-and-burned, and secondary forests. All plots were impacted by the extreme drought caused by the 2015-16 El Nin similar to o, and eight were also impacted by understory fires. We found a significant and non-linear relationship between precipitation and monthly leaf-litter production - above 300 mm of monthly precipitation, the production of leaf-litter becomes independent of rainfall. Surprisingly, this relationship was not influenced by pre-El Nin similar to o forest disturbance class. During the El Nin similar to o, leaf-litter production was higher, decreasing sharply in the following year, especially in El Nin similar to o-fire-affected forests. Between 2017 and 2019, all forests experienced a gradual increase in the production of leaf litter. However, the mechanisms behind this increase remain unclear and are likely different between forests affected only by the El Nin similar to o drought and those affected by both the drought and fires. Our results suggest that while leaf-litter production may be insensitive to past human disturbances, it is affected, in the short term, by extreme climatic events, especially in forests impacted by El Nin similar to o fires.

Automated summary

This study examines the impact of extreme climatic events and human disturbances on leaf-litter production in Amazonian forests, finding an increase in production during El Niño, followed by a sharp decrease, and a gradual recovery between 2017 and 2019.