Proposed amendments to a 1994 law preventing the logging of trees with diameters greater than 21 inches could undermine the protection of the region's largest trees.

New research, published Thursday in the journal Frontiers in Forests and Global Change, suggest the widest trees dominate carbon storage in the forests of Oregon and Washington State.

When scientists surveyed the population of wide-diameter trees in study plots on national forest lands in the Pacific Northwest, they found trees with diameters in excess of 21 inches accounted for just 3 percent of the tree population, but stored 42 percent of the total above-ground carbon.

The findings are only the latest to highlight the ecological services provided by bigger, older trees.

Previous studies have shown forests with healthy populations of larger, older trees tend to be more heterogenous and host greater biodiversity.

They also store more carbon than homogenous forest plots dominated by younger, smaller trees. Forests with bigger, older trees are also more resilient to wildfire.

"Large trees represent a small proportion of trees in the forest, but they play an exceptionally important role in the entire forest community -- the many unique functions they provide would take hundreds of years to replace," lead study author David Mildrexler, systems ecologist with Eastern Oregon Legacy Lands, said in a news release.

After surveying the trunk diameters and distribution of tree species in forest plots, researchers used well-established equations for estimating biomass. Roughly half of a tree's biomass is made up of carbon.

Researchers found that trees with trunks measuring more than 30 inches in diameter accounted for just 0.6 percent of the tree population, but these wide-load trees stored 16 percent of the carbon in the forest plots.

The data showed that the older and bigger trees get, the more carbon they store with each year of growth. In other words, over time, bigger trees become more and more efficient at storing carbon.

"If you think of adding a ring of new growth to the circumference of a large tree and its branches every year, that ring adds up to a lot more carbon than the ring of a small tree." Mildrexler said.

"This is why specifically letting large trees grow larger is so important for climate change because it maintains the carbon stores in the trees and accumulates more carbon out of the atmosphere at a very low cost," he said.

According to Mildrexler, the findings suggest policy makers need to strengthen, not undermine, protections for the forest's biggest trees.

"Large trees are the cornerstones of diversity and resilience for the entire forest community," Mildrexler said. "They support rich communities of plants, birds, mammals, insects, and micro-organisms, as well as act as giant water towers that tap into groundwater resources and cool our planet through evaporation."

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