Abstract
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 Niño 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 Niño 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 Niño, 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 Niño forest disturbance class. During the El Niño, leaf-litter production was higher, decreasing sharply in the following year, especially in El Niño-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 Niño 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 Niño fires.
Highlights
Extreme climatic events are becoming increasingly common as the 21st century progresses (IPCC, 2007)
We considered how the relationship between monthly leaf litter and precipitation varied across pre-EN forest disturbance classes, El Nino impact, and years (i.e. 2015/16, 2016/17, 2017/18, and 2018/19)
Leaf litter was significantly and non-linearly related to precipitation – in months which precipitation levels were greater than c. 300 mm, leaf litter was largely independent of it (Fig. 1)
Summary
Extreme climatic events are becoming increasingly common as the 21st century progresses (IPCC, 2007). Within the Amazon, temperatures reached 1.5–2 ◦C above the maximum temperature observed during previous El Ninos (Jimenez-Munoz et al, 2016). This increase in temperature was accompanied by a severe reduction in precipitation, which resulted in widespread drought and a subsequent increase in the occurrence of understory forest fires One million hectares of forest (Withey et al, 2018). The importance of these extreme climatic events has resulted in a growing number of studies evaluating how intense drought and result ing understory fires affect carbon cycling rates in Amazonian forests. Litter – the aggregate input of leaves, twigs, fruits, flowers, and seeds onto the soil surface – for example, is much less studied despite its crucial role in the functioning of tropical rainforests, including nutrients cycling (Osborne et al, 2020), reducing seed predation (Cintra, 1997), influencing seedling establishment (Molofsky and Augspurger, 1992), and providing habitat for a broad range of vertebrates and invertebrates (Gascon, 1996; Vitt and Caldwell, 1994; Silveira et al, 2013)
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