Forests help to slow the challenge of climate change, don't they? Only if climate change doesn't fell the forests first.
There could be big problems with national and international plans to plant forests to deal with climate change. One of them is uncertainty about how climate change is going to deal with the forests.
In six new studies of what might be called the plantation carbon conundrum, independent groups of researchers warn that:
- As land surfaces warm rapidly, would-be foresters might see their investments literally going up in smoke
- Poorly-designed policies could deliver plantations that end up absorbing far less carbon than anyone hoped for, and at a cost to natural wilderness and biodiversity
- In temperate zones, rising temperatures and extended drought could mean that many trees will die
- How effectively newly planted forests absorb atmospheric carbon depends on a suite of factors: among them tree species, the history of the land newly-planted, and the type of soil around the roots
- Because natural forests directly or indirectly support four out of every five species living on land, the loss of tree canopy is already changing the numbers and variety of global wildlife
- And finally, as temperatures go up, the appetite of the most prolific species for atmospheric carbon could dwindle - meaning that forests planted to slow climate change will be increasingly ineffective.
That the loss of natural forests worldwide is a driver of global heating and climate change has never been in doubt. And climate scientists continue to count tomorrow's forests as part of the answer to the threat of catastrophic climate change.
But researchers have already warned that a vow to plant one trillion trees is not of itself a readymade answer, and that national plans to conserve existing forest are less than effective.
So the challenge for foresters and ecologists is to decide what works best - and what would not. Researchers in the US argue in the journal Science that governments and policymakers need a masterplan to confront the risks forests face from the consequences of rising temperatures: drought, fire and insect disturbance.
Forests and other natural ecosystems absorb about one-third of all the greenhouse gas emissions that human actions release each year. New forests must be part of the answer, but only if the new timber goes on and on absorbing carbon.
"There's a very real chance that many of those forest projects could go up in flames or to bugs or drought stress or hurricanes in the coming decades," said William Anderegg of the University of Utah, who led the study. "Without good science to tell us what the risks are, we're flying blind and not making the best policy decisions."
The other papers look at aspects of the hazard, and of well-intentioned policies to combat climate change. The Bonn Challenge aims to restore an area of forest eight times the size of California, but 80% of the commitments so far involve plantations of single species or of exploitable species: fruit, for instance, and rubber on what might have been natural forest land, grassland or savannahs that support biodiversity.
Californian and Chilean researchers report in the journal Nature Sustainability that they looked at the role of long-running Chilean government subsidies in afforestation and found an uncomfortable result: exotic species flourished at the expense of native wilderness.
"Chile's forest subsidies probably decreased biodiversity without increasing total carbon stored in aboveground biomass," they conclude, bluntly. And one of the paper's authors, Eric Lambin of Stanford University, spelled it out: "That's the exact opposite of what these policies are aiming for."
"Up until now, forests have stabilised the climate, but as they become more drought-stressed, they could become a destabilising carbon source"
German scientists report in the journal Basic and Applied Ecology that a warmer world has already delivered dramatic consequences for the forests of Germany, Austria and Switzerland.
The past five years have been the warmest in the region since records began, and 2018's summer was the most extreme - 3.3°C above the long-term average. For spruce and other species that was the limit, and by 2019 even beech trees had died.
Since extreme drought and heat will become ever more likely, researchers need to decide what mix of species is going to survive and provide cover for threatened species. "This is going to take some time," said Bernhard Schuldt, of the University of Würzburg.
Chinese and US researchers report in Nature Sustainability that they examined the same problems using a ground-up approach. They looked at 11,000 soil samples taken across 163 control and forested plots in northern China, to find that the carbon capture potential of afforestation schemes may have been overestimated. In soils low in carbon, plantation did increase the density of organic carbon. In those soils already rich in organic carbon, the planting seemed to lower carbon density.
European researchers, too, report in Science that they looked at data collected over 150 years at 6,000 locations to work out what happened to plants and animals as climate change and human intrusion transformed the world's forests. Again, the answers are not simple.
"Surprisingly, we found that forest loss doesn't always lead to biodiversity declines," said Gergana Daskalova of the University of Edinburgh in Scotland. "Instead, when we lose forest cover, this can amplify the ongoing biodiversity change. For example, if a plant or animal species was declining before forest loss, its decline becomes even more severe." Species already doing well, however, seemed to do better.
But there's little guarantee that what works now will go on working, according to Arizona scientists writing in the journal Global Change Biology. So far, forests have helped contain climate change. But they found that North America's most prolific tree, the Douglas fir, will absorb less carbon in future and do less to slow climate change.
They based their finding on examination of 2.7 million tree rings from 2,700 sites in the fir's enormous ecological range. At the southern and warmest and driest end of this range, the decline in annual growth could be as high as 30%.
"More warming for trees could mean more stress, more tree death and less capacity to slow global warming," said Margaret Evans, of the University of Arizona.
"Up until now, forests have stabilised the climate, but as they become more drought-stressed, they could become a destabilising carbon source."