Tree mortality in Papua New Guinea's Wanang forest rose after the 2015-17 El Niño drought

Researchers measured the mortality of more than 21,000 trees in the Wanang Forest Dynamics Plot, a 50-hectare (124-acre) research site in northern Papua New Guinea. The measurements covered the periods before, during and two years after the 2015-17 El Niño drought, news.mongabay.com reports. Mortality was significantly higher after the drought than before or during it mongabay.comnews.mongabay.comnews.mongabay.comOpen source ↗.

“It was after the drought, the post-drought period, that you could actually see those trees dying,” said Hayden Wagia, the study's lead author and a faculty botanist at the Papua New Guinea University of Technology's School of Forestry. The authors say the study cannot determine exactly what caused the delayed deaths.

Rainfall and terrain

The drought brought a substantial reduction in rainfall to the forest. Only 5.2% of the area's total precipitation fell during the dry season during the drought, compared with 13.1% before it and 9.6% afterward.

Trees responded differently depending on where they grew. Valley trees had relatively high mortality before and during the drought but recovered quickly afterward, while ridge trees showed mortality that rose after the drought and stayed elevated mongabay.comnews.mongabay.comnews.mongabay.comOpen source ↗. For smaller trees, annual mortality in valleys was 7.8% before the drought, 8.81% during it and 4.91% afterward. On ridges it rose from 4.66% before the drought to 7.07% after.

The researchers suggest that valleys can retain more water than exposed ridges, and that erosion on ridges could reduce water retention.

Possible causes of delayed mortality

Scientists name several possible explanations for delayed mortality. Severe drought can damage the xylem, the tissue that transports water through trees, and trees may deplete carbohydrate reserves needed for recovery and defense against pests and disease.

David Galbraith of the University of Leeds, who was not involved in the study, noted that delayed mortality has been documented elsewhere. In a large experimental drought in the Amazon, researchers excluded about 50% of rainfall from a forest plot, and mortality took roughly two to three years to increase substantially mongabay.comnews.mongabay.comnews.mongabay.comOpen source ↗. “There isn't a one-size-fits-all kind of response of forests to drought,” Galbraith said.

Wagia said that if some species are more vulnerable to drought than others, repeated droughts would shift the structure and composition of the forest.