Big trees rule the forests

big tree

Washington State University's Mark Swanson pulls a tape tight around a 4-foot-wide sugar pine, one of the 34,500 live trees counted and tagged for long-term study in a Yosemite National Park study plot.

A new study has shown that in the forest all trees are not equal when it comes to contributing to ecosystem services such as carbon capture. The study of trees in the Yosemite National Park demonstrated that the largest trees made up a substantial proportion of the biomass in the study area. 

The study indicated that 50% of the tree biomass in the forest is made up from just 1% of the trees. With biomass being the result of photosynthesis it means that the loss of just a few trees in a forest can have big impacts on the ability of the forest to capture carbon. [pullquote]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.[/pullquote]

Big trees contribute substantially to forest carbon capture.

This importance of big trees – whose trunks are over 3 feet or more in diameter – towards ecosystem services is of concern to conservationists because many big trees are dying off and the cause is not fully understood. With big trees living for hundreds of years the impact of their losses can be long-lived also.

In one study published in 2009 it was discovered that Yosemite National Park density of big trees had dropped by 25% between 1930 and 1990 despite there being no logging in the area.

Loss of 1% of trees could lead to loss of 50% of biomass.

The latest study, published in PlosOne, was undertaken by a team of field researchers from the University of Washington and was led by James Lutz, a researcher in research scientist in environmental and forest sciences.

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,” Lutz said. “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.” 

Trees and above ground biomass plotted in study area.

To try and understand the impacts of big trees on the forest environment and their ecological services the team have established one of the largest fully plotted forest study areas in the world. The site covers 63 acres of the western part of the Yosemite National Park and all 34,500 live trees have been individually plotted.

The field work was undertaken by 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. 

The plotting did not just include the trees but also all above ground biomass including dead and decaying plant matter. this was important as big trees still make an important contribution towards forest biomass even after death. 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. 

Understanding role of big trees can help forestry management.

Understanding the valuable contribution that big trees make to the environment and their role in capturing carbon from the atmosphere can help forestry managers and officials make better decisions in commercial woodlands.

An example of this highlighted by the authors of the paper is the clearance of undergrowth or under-story brush using controlled burns. 

Before the fires were started, crews raked around some of the large trees so debris wouldn’t just sit and burn at the base of the tree and kill the cambium, the tissue under the bark that sustains trees,” Lutz said. 

They also point out that forest managers in new plantations and woodlands may also want to look at leaving a few trees and encouraging them to grow to their full maturity and size.

The biologists also point out that forest models may not fully understand the way in which these big trees develop over time. Current forest growth models are more suited to younger and more uniform forests. “These trees started growing in the Little Ice Age,” Lutz said. “Current models can’t fully capture the hundreds of years of dynamic processes that have shaped them during their lifetimes.” 

External sites:

Plos One: Ecological Importance of Large-Diameter Trees in a Temperate Mixed-Conifer Forest.

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