Study: Largest 1 Percent of Trees Make Up Half of Mature Forest Biomass Worldwide

May 10, 2018

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.MISSOULA – New research from a team of 98 scientists in 21 countries and territories has found that the largest 1 percent of trees in mature and old forests comprise about 50 percent of forest biomass worldwide.

The team, including University of Montana forest ecology Professor Andrew Larson, used data from 48 large forest plots around the world. Jim Lutz at Utah State University is lead author of the new paper, published May 8 in the journal Global Ecology and Biogeography. The paper, titled “Global importance of large-diameter trees,” is available at http://dx.doi.org/10.1111/geb.12747

A Forest Ecology summer field class visits an ancient western larch on the west side of the Swan Valley. This tree is special because it has a broken top and rotten center with a large cavity that is used by bears.  “These big trees store a disproportionately large amount of the carbon in the forest,” Larson said. “They’re the elites – and because of their age and size, are very difficult to replace.”

Forests are a key component of the global carbon cycle. Trees draw carbon dioxide out of the atmosphere through photosynthesis, storing some of that carbon as wood in large, old trees.

The team found that in temperate forests, the largest trees are from a small number of tree species. Tropical forests, in contrast, tended 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.”

 “We’re lucky here in western Montana to be surrounded by some of these big, old trees – like the champion western larch near Seeley Lake,” he said.

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 well known to typically have many more species than temperate forests, this 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, Ph.D. student at Utah State University and co-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."

The Smithsonian ForestGEO program provides a rich, consistent source of long-term data on forest structure. Larson has worked on the Yosemite Forest Dynamics Plot, one  of the sites used in this study, for the past decade.

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Contact: Andrew Larson, associate professor of forest ecology, UM W.A. Franke College of Forestry and Conservation, 206-799-1253, a.larson@umontana.edu.