Rare and ancient trees are key to a healthy forest | Science

About 800 years ago, a giant oak tree in England’s Sherwood Forest helped shelter Robin Hood from the corrupt sheriff of Nottingham. Though the tale is likely a myth, the tree is not: It still stands as one of the world’s oldest oaks.

Such ancient trees—some dating back more than 3000 years—are key to the survival of their forests, new research shows. Rare trees—some so scarce scientists have yet to find them—are also critical to forest health, another new study reveals.

Together, the research suggests conservationists should do more to protect the world’s oldest and rarest trees, says William Laurance, an ecologist at James Cook University, Cairns, who was not involved with the work. Because forests are such important ecosystems—storing carbon, curbing water runoff, and providing food and shelter for many species—such a strategy isn’t just good for the health of the forest ecosystem, he argues, it’s critical for the health of the planet.

In the old trees study, Charles Cannon, who researches ecological evolution at the Morton Arboretum, wanted to get a sense of the percentage of ancient trees in an average forest. “Ancient” is a matter of perspective and species, however. In oak forests, where most trees live less than 100 years, the ancient ones survive to almost 1000; with longer lived bristlecone pines, ancient trees can exceed 3000 years.

Cannon and colleagues decided to define a forest’s most venerable trees as those 10 to 20 times older than their brethren. They used statistics to predict how many ancient trees they would expect to find in forests with different tree death rates. Unlike animals, which tend to have specific life spans, trees can grow indefinitely until felled by lightning, fire, chainsaws, or disease. So, using forest death rates ranging from 0.5% to 5% per year, as measured by foresters, Cannon ran a computer program that calculated the age distributions that would be achieved through time in a simulated forest.

Each year a certain percentage of trees die. Over time, therefore, Cannon knew there would be few trees that reached old age. But he says he was surprised just how few survive to be “ancient.”

When annual death rates exceed 3%, no trees make it to an “ancient” age, he found. When mortality hovers between 1% and 2%, just 1% or fewer of the trees become that old, Cannon and his colleagues report today in Nature Plants.

That could be bad news because these trees harbor genes critical to the survival of the entire forest, Cannon says. Ancient trees may contain DNA that makes them less likely to be toppled by wind, for example, or more resistant to fungal diseases. And because these individuals have survived hundreds—if not thousands—of years of climate fluctuations, old-timers that sprouted in a very different environment provide a way for the forest to survive should climate swing back to former times.

“Once they are gone, they are gone,” Cannon says. “We can’t just replant ourselves back to a healthy forest.”

“Ancient trees are an irreplaceable hub of biodiversity,” agrees conservation ecologist Gianluca Piovesan, a co-author at the University of Tuscia. Unusual insects and other species make their homes in them, he says. “We absolutely must preserve old-growth forests and ancient trees to transition to an ecologically sound future.”

“That’s a message that needs to go out not just to the scientific community, but to the whole world,” says David Milarch, co-founder of the Archangel Ancient Tree Archive. For several decades, Milarch, who was not involved with the work, and his group have been cultivating and planting clones of ancient trees from 130 species, including oaks, maples, and redwoods, hoping to ensure their genes are perpetuated.

Meanwhile, Peter Reich, an ecologist at the University of Michigan, Ann Arbor, and the University of Minnesota, Twin Cities, has homed in on the diversity of tree species found in forests. He and scores of collaborators have compiled a data set of 40 million trees from more than 90 countries, coming up with what they say is the first scientific estimate of the total number of tree species.

In all, the world hosts approximately 73,000 tree species, of which about 9000 have yet to be named and cataloged, Reich and his colleagues report today in the Proceedings of the National Academy of Sciences.

Surprisingly, one-third of the known species are rare, with just one or two individuals in a survey, as are even more of the unnamed ones. And because rare trees, like ancient ones, may hold genes that can benefit the entire forest, conserving these species is critical as well, Reich says.

Tree biodiversity—the number of species per forest—is greatest in South and Central America, Africa, Asia, and Oceania, the team found. This helps “establish a benchmark to help us to prioritize conservation efforts,” he adds, suggesting those continents need the most help.

Making sure ancient trees survive could help preserve forest diversity, Cannon says. That diversity is the difference between new species evolving and old ones going extinct. The longer a tree lives, the less likely its species is to go extinct and the more likely that new species will arise in its lifetime.

So, when places like the Amazon rainforest are stable, they accumulate more diversity through time even if rare trees are not reproducing very well, Cannon explains. Thus, “Ancient trees delay the extinction process,” he says, particularly if they belong to a rare species: They may live long enough for the climate or environment to once again allow these genetically rich trees to share their bounty with the rest of the forest—a bit like the famed outlaw of yore.