The planet’s forests are vital to us all. For one thing, without them, global warming would be a lot worse. Forests pull vast amounts of carbon dioxide out of the atmosphere. They also foster untold biodiversity and deliver large human benefits — close to one out of six people on Earth “directly depends on forests” for food and other services, according to a recent report by the International Union of Forest Research Organizations. If recent research is to believed, trees even improve people’s mental health and well being.
But the world’s forests are in serious trouble, according to a suite of papers out in this week’s issue of the journal Science. The research systematically examines how forests are being damaged by the combined impacts of a changing climate and more human incursions.
“These papers document how humans have fundamentally altered forests across the globe and warn of potential broad-scale future declines in forest health, given increased demand for land and forest products combined with rapid climate change,” note Susan Trumbore of the Max Planck Institute for Biogeochemistry and two coauthors in an overarching introduction to the suite of studies.
The Science papers take the forests of the world one by one — tropical, temperate, boreal (northern), and one we don’t often think of: planted forests, sown by humans for their own use. It turns out that all four types are facing major threats, albeit in different ways:
Tropical Forests. Forests of the tropics are, in some ways, faring the worst. These crucial forests, which serve over a billion of the world’s people and provide a home for more than half of the planet’s species, are being severely impacted by logging, the transformation of forests into farmland, resource extraction, and more. “Across the world’s extant tropical forests a recent estimate suggests, 24% are intact, 46% fragmented, and 30% otherwise degraded,” write the University College London’s Simon Lewis and two colleagues in the Science paper focused on these forests.
In fact, a staggering 100 million hectares of tropical forest — roughly 386,000 square miles — were turned into farmland from 1980 to 2012, according to Lewis and colleagues. That’s an area bigger than the size of Texas — indeed, roughly half the size of Alaska.
The consequences of this change range from losing more carbon to the atmosphere and thereby worsening global warming — growing forests pull carbon in, but their loss releases it again — to killing off large numbers of species. What’s more, all of this also compounds the risk of gigantic fires. These don’t typically occur in thriving, wet tropical forests, explain Lewis and his colleagues — but they do occur if those forests are chopped up, dried out, and filled with people, who tend to start blazes accidentally.
Tropical forests aren’t unique in the problems they face. The other forest systems are in similar straits, although the details naturally vary a great deal.
Temperate Forests. Also struggling are temperate forests, such as those found in much of the lower 48 United States. For these forests, a changing climate is bringing about a particular kind of threat multiplier — droughts.
The Science study on temperate forests, by the U.S. Forest Service’s Constance Millar and the U.S. Geological Survey’s Nathan Stephenson, explains that many are now facing the threat of a new kind of intense drought — which then sets the stage for not only more burning, but also more sweeping insect and pathogen attacks. Drought also stresses and sometimes kills trees directly by reducing the amount of water that they can draw from the land through their root systems.
“In recent decades, outbreaks of insects and pathogens have resulted in millions of hectares of forest defoliation, canopy dieback, declines in growth, and forest mortality in western North America and Europe,” the authors note. “In many cases, climate was a direct or indirect trigger for these other agents of megadisturbance or influenced the severity and extent of outbreaks.”
And then, of course, there are fires — more specifically, megafires. It’s not just that they can burn up hundreds of thousands of acres of trees — it’s also a question of what grows back afterwards, and whether it provides the same services as the prior forest did, whether to humans, animal species, or the climate. “To the extent that these large fires increase in the future, the potential for shifts to new forest types and nonforest vegetation will accelerate,” Millar and Stephenson write.
Boreal forests. Perhaps the hardiest are the globe’s boreal forests, comprising 30 percent of forests overall and concentrated in the United States (Alaska), Canada, and most of all Russia. These are remote and unique regions where conditions are freezing for much of the year, and where the ground beneath is often permanently frozen (thus, featuring “permafrost”).
Few people live in these remote forests — they contain gigantic tracts of simply untouched forest land, some of the best preserved on the globe. But they’re still struggling — because of all forests on Earth, global warming is happening the fastest in this region.
Average temperature increases across many boreal forests are already 1.5 degrees Celsius, write the Canadian Forest Service’s Sylvie Gauthier and colleagues in the Science paper on these forests. And much more warming — and drying — is projected going forward, meaning that “disturbances are generally predicted to increase in extent, frequency, or severity over the same time frame, although uncertainties in the projections remain.” When forest scientists say “disturbances” they mean not only fires, but also factors like pest outbreaks.
What’s more, there’s a double whammy because these forests grow atop permafrost, which can also thaw and release carbon — a process that large wildfires can accelerate. “In Russia alone, the release of C from the thawing permafrost by the end of the century could potentially be several times larger than that of current tropical deforestation,” write Gauthier and colleagues.
“The health of the immense and seemingly timeless boreal forest is presently under threat,” they conclude.
Planted forests. And then come the world’s planted forests – also often known as “tree farms” or plantation. These make up a surprising seven percent of the world’s total forests, and could comprise much more than that by the end of the century.
Planted forests tend to be of the same type of tree species, and are managed for their resources. And what’s more, note a team of researchers from South Africa and New Zealand led by Mike Wingfield of the University of Pretoria in their Science paper, they are often comprised of trees of a species type, like Eucalyptus, that is not native to the area in which they are planted.
This sets up a really big problem, note the authors:
Non-native trees in plantations are in part successful because they have been separated from their natural enemies. However, when plantation trees are reunited with their coevolved pests, which may be introduced accidentally, or when they encounter novel pests to which they have no resistance, substantial damage or loss can ensue. The longer these non-native trees are planted in an area, the more threatened they become by native pests.
Unless major action is taken on this front, the authors continue, “pest problems will continue to grow and will threaten the long-term sustainability of forests and forestry worldwide.”
Put it all together, and it’s hard to read through the Science papers and not fear for the forests of the globe. “Many of the trees alive today will experience temperatures and CO2 levels outside the range to which they are adapted,” note Trumbore and her colleagues.
To be sure, they observe, trees will always be with us, but perhaps in a form or distribution that is, overall, quite changed. And not only are we causing all of this — in the end, it would hurt us most of all.
“Human concerns about forest health mostly reflect our dependence on the continued availability of the products and services that forests provide,” the authors conclude.