Tropical forests struggle to recover from El Niño

The extreme weather patterns of the 2015-16 El Niño, among the worst since the 1950s, had crippling ripple effects around the globe, from drastic crop failures, food shortages and disease outbreaks to destructive wildfires and severe coral bleaching.

Focussing on tropical rainforests in Africa, America, Asia and the Amazon, two new studies have found these woodlands are struggling to recover, and the impacts are aggravated by human activities.

The first, published in the journal Science Advances, reports that rainforests in Asia, America and Africa had failed to restore their carbon balance by 2017.

Relentless droughts and intense heat during the El Niño caused mass carbon emissions from the tropical vegetation, write Jean-Pierre Wigneron from the University of Bordeaux, France, and co-authors from Europe, the US and China.

It was anticipated that new growth would start to soak up and store the gases again as cooler, wetter conditions helped the rainforests recuperate. However, models of carbon dioxide output show large variation, impeding efforts to assess their recovery.

To address this, the team used remote sensing technology to map changes in carbon stored by trees and other vegetation between 2014 and 2017 using L-band vegetation optical depth (L-VOD) to measure stems, branches and leaf density via microwave emissions.

Compared with carbon stocks from pre-El Niño and the average stocks from 2010 to 2017 they found the tropics had not recovered, and this was largely attributed to tropical Africa and America.

And while Asia and America demonstrated feeble recovery, Africa showed continued decline. The team also found that Africa contributed to 56% of carbon losses during the El Niño, particularly in the north.

“Northern tropical Africa appeared to be responsible for an unexpectedly large net source of carbon during that period,” the authors report.

Allowing for patterns of deforestation, the researchers still found high losses in areas with no large-scale tree lopping, which they suggest could either be attributed to climate conditions or unidentified deforestation or degradation.

Their observations aligned with other remote sensing indices, supporting the value of using L-VOD technology to continue monitoring the impact of volatile climates on terrestrial carbon stocks virtually in real time.

“These findings have important implications for the long-term vulnerability of carbon stocks in the tropics,” they write, “as large-scale droughts and El Niño events are expected to intensify, in terms of both frequency and intensity.”

Zoning in on the Amazon Rainforest, another international group of scientists from the UK, Brazil and New Zealand investigated the El Niño’s impact on biodiversity and insect populations.

The team reports in the journal Biotropica that numbers of the ecologically vital dung beetle had fallen by more than half during the intense droughts and wildfires, and the effects persisted for at least two years.

Dung beetles (Coleoptera: Scarabaeinae) spread nutrients and seeds and can give important insights into the health of an ecosystem. 

The researchers counted more than 14,000 of the industrious insects from 98 species across 30 forest plots in the Brazilian state of Para, surveyed between 2010 and 2017, and monitored their level of dung removal and seed dispersal.

They counted around 8000 beetles in 2010 but only 3700 after the El Niño in 2016. The populations continued to nosedive, numbering only 2600 in 2017.

The largest declines were in forests that had burned as a result of human activities, including deforestation and predatory logging.

“Our investigation provides important insights into how human activities and climate extremes can act together and affect tropical forest biodiversity and ecosystem functioning,” says lead researcher Filipe Franca from Lancaster University, UK.

He expressed concern that these declining numbers could reflect a broader impact on mammal populations.

“Dung beetles depend on mammal poo for nesting and feeding, therefore declines in beetles are likely associated with the loss of mammals due to that El Niño’s drought and fires.”

It could also indicate repercussions for forest regeneration, as the remaining beetles struggled to spread nutrients and seeds.

The researchers stress the importance of addressing the direct and indirect impacts of human activities on forest fauna, including insects, and the ecosystems they help to keep alive.

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