Old, Big Trees Account for Disproportionately Large Amount of Forest Biomass

Posted on May 6, 2012

4 Foot Wide Sugar Pine in Yosemite National Park

Scientists have come up with more proof why the biggest, oldest trees in the forest are extra important and need protecting. Researchers have found that very big trees, three or more feet in diameter, account for nearly half the biomass measured at a Yosemite National Park site, yet represent just 1% of the trees growing there.

James Lutz, a University of Washington research scientist in environmental and forest sciences, says this means just a few towering white fir, sugar pine and incense cedars per acre at the Yosemite site are disproportionately responsible for photosynthesis, converting carbon dioxide into plant tissue and sequestering that carbon in the forest. Lutz is the lead author of a paper, published on PLoS One, about the largest quantitative study yet of the importance of big trees in temperate forests.

Lutz says, "In a forest comprised of younger trees that are generally the same age, if you lose one percent of the trees, you lose one percent of the biomass. In a forest with large trees like the one we studied, if you lose one percent of the trees, you could lose half the biomass."

The new 63-acre study site in the western part of Yosemite National Park is one of the largest, fully-mapped plots in the world and the largest old-growth plot in North America. The tally of what's there, including the counting and tagging of 34,500 live trees, was done by citizen scientists, mainly undergraduate college students, led by Lutz, Larson, Mark Swanson of Washington State University and James Freund of the UW. A Facebook page for the Yosemite plot can be found here.

Included was all above-ground biomass such as live trees, snags, downed woody debris, litter and what's called duff, the decaying plant matter on the ground under trees. Even when big trees die, they continue to dominate biomass in different ways. For example, 12 percent of standing snags were the remains of large-diameter trees, but still accounted for 60 percent of the total biomass of snags. Live and dead biomass totaled 280 tons per acre (652 metric tons per hectare), a figure unmatched by any other forest in the Smithsonian Center for Tropical Forest Science network, a global network of 42 tropical and temperate forest plots including the one in Yosemite.

Trees in the western U.S. with trunks more than three feet across are typically at least 200 years old. Many forests that were heavily harvested in the 19th and 20th centuries, or those that are used as commercial forest lands today, don't generally have large-diameter trees, snags or large wood on the ground.

Lutz says, "These trees started growing in the Little Ice Age. Current models can't fully capture the hundreds of years of dynamic processes that have shaped them during their lifetimes."

Photo: Washington State University