"Building Better Forests" was produced by QUEST Wisconsin's Andy Soth.
Trees grow slowly. The climate is changing quickly. That combination could be deadly for the nation’s forests -- but new research may show how to make those forests a little tougher.
Three-quarters of a billion acres in the United States are covered in forest. And it’s not just sitting there. Forests store and filter water, exchange the carbon dioxide we don’t want in the air for the oxygen we do, provide habitat for animals and smaller plants, and fuel the country’s $30 billion timber industry.
But David Mladenoff, a professor of forest ecology at the University of Wisconsin who is featured in the video posted above, explains that some key tree species -- especially those in northern forests -- might not be able to survive in warmer conditions. More heat-tolerant species could move northward to replace them, but climate change may outpace the ability of southern species to migrate to higher latitudes. If that happens, the total acreage of northern forests could drop dramatically by the end of the century.
Trees that survive these changes will still have to contend with invasive species and new diseases. Pests that used to be eradicated by cold winters will be more likely to infiltrate new territories, attacking trees already stressed by higher temperatures, drought, and storms.
But there is still hope for these trees. Scientists believe that we might be able to bolster forests’ chances of survival if we can figure out how to create forests that are more resilient. As Mike Dombeck, a former chief of the U.S. Forest Service, explained, “That resilience is always in diversity.”
Diversity confers resilience because some tree species will weather climate change better than others. Some will succumb to invasive pests; others won’t. A diverse forest can hedge its bets: even if one species is lost, the forest will persist.
For a lesson in how to cultivate a diverse and resilient forest, scientists and forest managers have turned their attention to a 220,000-acre Wisconsin forest managed by the Menominee tribe.
Dombeck calls the Menominee enterprise “a real success story in forest management.” The Menominee have been harvesting wood from their forest for 150 years, but the reservation holds even more timber today than it did when they started logging. Young and old trees in a variety of species attest that the forest has retained its protective diversity, too.
The tribe credits its success to its celebrated 19th-century chief Oshkosh, who advised the Menominee to harvest only sick trees, mature trees, dying trees -- leaving plenty of large, healthy ones behind. This strategy is called “selective harvest.” Forester David Mausel points out that it mimics nature’s method for culling the forest, where the weaker trees are the first to succumb to insects and disease. The forest, not the lumber market, determines which trees get cut.
But most of Wisconsin’s forests did not reap the rewards of Oshkosh’s advice. Throughout the 19th century, the expanding U.S. timber industry pushed logging ever farther westward. By the turn of the 20th century, the Great Lakes states were being stripped of their old-growth forests in what Dombeck describes as “cut and run” forestry.
The consequence, said Mladenoff, is that state is mostly left with 100-year-old forests -- barely out of babyhood by forestry standards. “Some have already been cut once or twice again so they're even younger. It means we have a very homogenous forest,” he explained. That homogeneity makes the forest vulnerable.
Mladenoff is investigating whether these newer, low-diversity forests can be persuaded to behave like the resilient old-growth forest the Menominee manage. They’ve targeted several features that could help young forests act a little older. “We know that old-growth forests have a very different structure than all our younger managed forests,” Mladenoff said. “They have large trees. They have different sized openings in the canopy. There's many more logs on the forest floor.”
Growing large trees takes time, but the other changes are easier to make. At their study site in the Flambeau River State Forest about 150 miles northwest of the Menominee reservation, Mladenoff and his team cut down carefully selected trees to create gaps in the canopy, and leave old logs to rot slowly on the forest floor. The effect is similar to what the Menominee achieve with selective harvest.
The experiment will run for 50 years, but some effects are already obvious. New, light-hungry tree species are springing up under the gaps in the canopy, increasing the forest’s biodiversity -- maybe this young forest’s best defense against a warmer future.
If the forests survive, they can help combat climate change at its source. Forests absorb billions of pounds of CO2 every year, sequestering it in leaves and branches and tree trunks. Carbon is also stored in tree debris on the forest floor and in the soil -- in fact, the soil holds more than half the carbon locked up in the forest. Mladenoff’s experiment will measure how the new canopy gaps and extra logs affect the forest’s ability to store and release carbon, to determine if these diversity-enhancing strategies could also make the forest a better carbon sink.
For years scientists assumed that only young, quickly growing forests were good at storing carbon. But more recent research has suggested that old forests absorb significant amounts of carbon dioxide, too. Mladenoff explains that even though old trees grow more slowly, their greater mass means that they can still soak up a lot of carbon. A forest where some trees are allowed to reach a ripe old age, and where biodiversity is encouraged, could be both more resistant to climate change and one of our best weapons against it.
“The most important thing,” Dombeck said, “is to take care of the goose that lays the golden eggs, and that's the forest.” Mladenoff’s results may one day help ecologists create a healthier and more resilient “goose” that can weather whatever changes lie ahead.