The “21-inch rule” protects trees that are 21 inches in diameter or larger at breast height on six national forests east of the Cascade Mountains crest in Oregon and Washington. It was implemented in 1994 as a wildlife and habitat protection measure in response to a century of intensive logging of large trees and old forests. It does not affect private forestlands.

The national forests covered by the 21-inch rule extend across Oregon’s largest ecoregion — the Blue Mountains, which connect the Cascades to the Rocky Mountains. The Morgan Nesbit project area, referenced in a recent article, overlaps the heart of a critical linkage area between the Blues and Rockies.

I’m an ecologist who studies forest landscapes and the significant values they provide. I will share some of the science specific to the largest trees and their unique role in water and energy cycles which underscore their benefits to the forest and local community when they are kept alive and growing in the forest.

Large trees hold huge volumes of water and store massive amounts of carbon compared to smaller trees. In an analysis of carbon storage in the six national forests covered by the 21-inch rule, my coauthors and I found that big trees of five dominant species with trunks 21 inches DBH or larger make up just 3% of the total trees in these forests but store over 42% of the aboveground carbon. The same dynamic holds for water. A rule of thumb is that about half a tree’s weight is water. Large trees dominate aboveground biomass stocks, and thus water storage as well. Some researchers describe carbon-dense forests with big trees as “deep pools.” Imagine diving into a big, deep pool of water vs. standing in a shallow one. The big, deep pools stay colder longer than the shallows. Similarly, older forests with big trees create their own microclimate conditions that help buffer species from a warming world.

Because large trees have the deepest roots, they can access groundwater unavailable to shallow-rooted plants. This is critical during drier months when large trees can access water needed to photosynthesize, and thus continue storing more carbon from the atmosphere, while cooling the surface temperature as water evaporates from foliage. Water released to the atmosphere contributes to downwind moisture content and rainfall. We call the forest-driven transport of water from coastal forests inland the biotic pump. Large trees are the engine of this work, especially during the dry season.

To appreciate the cooling power of forests, consider that forest canopies measured from satellite at 1 p.m. in full direct sunlight can register summer surface temperatures more than 30° F cooler than adjacent non-forest cover types. Among the most abundant forest types in the Blue Mountains, fir/spruce forests have average summer maximum surface temperatures 12° F cooler than ponderosa pine forests.

Large trees preserve longer-lasting snowpack due to shade cast by their huge canopies. This extends water supplies that support fisheries and agricultural uses. Intact forests with large trees are associated with increased late-summer streamflow and clean drinking water. When large trees die, their outsized role continues. Large standing dead trees are one of the most important habitats in the forest, and large downed logs help anchor snowpack and retain moisture that creates cool microsites on the forest floor. Large wood in streams is critical to the integrity of the system.

Some argue we need to increase logging of large-diameter trees to reduce fire risk. But in actuality, diameter limits such as the 21-inch rule have limited impact on fire hazard metrics because large trees are relatively rare and they are the most fire-resistant trees in the forest. An ingrowth of small trees is more relevant to fire and drought vulnerability, and small trees hold far less carbon and water.

We need large trees on the landscape now, helping create healthy watersheds, fighting climate change, and safeguarding species in a warming world. We would do well to protect large trees where we can and especially on national forest lands.

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