Tropical forests quite resilient to warming

In an ambitious, long-term study of more than half a million trees, a virtual army of scientists has found that tropical forests are more resilient to warming temperatures than most climate models predict – but only up to a point.

Once temperatures pass a 32-degree-Celsius threshold, the forests – a major carbon reservoir – become four times more sensitive to the heat, causing them to release carbon dioxide more rapidly.

“Tropical forests store vast amounts of carbon in their trees,” says lead author Martin Sullivan from the University of Leeds, UK.

“If all this would be released at once it would be equivalent to around 25 years’ worth of global fossil fuel emissions.”

What triggers this is therefore a major concern, yet understanding how sensitive tropical forests are to rising temperatures is one of the greatest sources of uncertainty in climate modelling, the researchers write in the journal Science.

Forests release carbon dioxide into the atmosphere when the amount of carbon gained by tree growth is less than that lost through death or decay through environmental impacts.

The fact that trees are long-lived has been a key obstacle to measuring how they respond and adapt to global warming over time, so all models until now have been limited by short-term insights.

The team of 225 researchers from across the globe took a different approach.

First, they measured the diameter and height of trees from 590 permanent tropical forest plots in South America, Africa, Asia and Australia and identified nearly 10,000 species. Each tree was tagged so they could track their rate of growth and death over the years.

This enabled the team to calculate the rate at which forests take carbon from the atmosphere and how long they retain it, factoring in the different climates.

“By relating the amount of carbon forests store to the climate the trees grow in we could see how climate controls forest carbon stocks,” Sullivan explains.

“Comparing forests in different locations allows us to observe how forests grow in a particular climate after having had time to adapt.”

The researchers then looked at another 223 forest plots to confirm the relationships they observed.

They found that while trees adapted well to minimum night-time temperatures, maximum daytime temperatures had the biggest impact on carbon storage, mainly by reducing their growth rate, followed by tree mortality from droughts.

Daytime temperatures below 32 degrees had little effect, suggesting forests are less sensitive than most models predict, in part because their high biodiversity enables more tolerant trees to replace less well-adapted species.

The biggest impacts were predicted in South America, with the highest warming, which would drive more than two thirds of the biome over the threshold.

While carbon dioxide fertilisation could, to some degree, mitigate the impact, this would be thwarted by rapid temperature increases, habitat fragmentation, logging and fires, which increase the trees’ vulnerability.

“Our results indicate that forests are surprisingly resilient to small increases in temperature, but this resilience is only up to a point,” says Sullivan.

“To ensure this resilience, we need to protect and connect forests so tree species can move to new locations, as differences in species composition between different forests is likely to be key to their ability to adapt.”

Meanwhile, the researchers say, it is critical to limit emissions and stabilise the Earth’s climate.

Already, a quarter of tropical forests are facing temperatures over the 32-degree threshold, and even the 1.5- or two-degree targets would push the majority of trees over the edge and trigger excess release of carbon into the atmosphere, causing spiralling feedback loops.

But the coronavirus lockdown offers a unique opportunity to take action.

“By not simply returning to ‘business as usual’ after the current crisis we can ensure tropical forests remain huge stores of carbon,” says co-author Oliver Phillips.

“Protecting them from climate change, deforestation and wildlife exploitation needs to be front and centre of our global push for biosecurity.”