UM study: Losing the world’s largest trees would degrade forests and carbon capture

Graduate students from UM’s Franke College of Forestry and Conservation visit the Ross Creek Cedar Grove about 20 miles south of Troy. Some of the largest diameter trees known in Montana are in this grove. (University of Montana photo)

The largest 1 percent of trees in mature forests make up 50 percent of the planet’s forest biomass, and losing those trees could degrade the world’s forests and their carbon-scrubbing power, according to research conducted by a team of international scientists, including the University of Montana.

“These big trees store a disproportionately large amount of the carbon in the forest,” said UM forest ecology professor Andrew Larson Larson. “They’re the elites, and because of their age and size, they’re very difficult to replace.”

The team of 98 scientists from 21 countries and territories collected data from 48 large forest plots around the world to complete the study, “Global importance of large-diameter trees.”

Forests are a key component of the global carbon cycle, as trees draw carbon dioxide from the atmosphere through photosynthesis and stores that carbon as wood in large, old trees.

The team found that in temperate forests, the small number of tree species represent the largest trees. In contrast, tropical forests tend to have more species capable of attaining large size.

“The finding that large diameter trees tend to belong to just a few species in temperate forests really caught my attention,” Larson said. “The implication is that loss of one or a few species capable of growing to large size could result in structural and functional degradation of forests.”

Those big, old trees can also be found in western Montana, including the world champion western larch near Seeley Lake.

Western larch grow to the largest size among all tree species in Montana’s subalpine forests. The champion larch near Seeley Lake is the largest western larch, and also the largest individual larch tree of the 10 larch species worldwide.

“Imagine introduction of a non-native insect or disease that wipes out western larch,” Larson said. “This would cause an irreversible structural simplification of our forests, resulting in reduced wood production, less carbon storage and lost habitat for animals that nest or den in large hollow trees. Temperate forests may be more vulnerable to this type of large tree loss than tropical forests.”

The researchers also found that both the concentration of biomass in the largest 1 percent of trees and forest density declined with latitude.

Although tropical forests are known to have many more species than temperate forests, the study found that temperate forests have higher structural complexity, both in terms of different tree sizes within an area and also between adjacent areas of forest.

“The distribution of big trees has not been well explained by theory,” said Tucker Furniss, a doctoral student at Utah State University and the paper’s lead author. “Our results emphasize the importance of considering these rare, but disproportionately important ecosystem elements. We clearly need more applied and theoretical research on these important big trees.”