Proposed forest management of the Mount Emily Recreation Area has raised important questions about large trees. I’m an ecologist who studies forest ecosystems and the significant values they provide. I’d like to share a few findings specific to the largest trees in the forest that underscore their outsized benefits to both the forest and local community.

Large trees are crucial in ecosystem water and energy cycles. Large deeply rooted trees tap groundwater resources not available to shallow-rooted plants. During drier months roots lift deep soil water up to shallow, drier portions of soil and release it, sharing water to the ecosystem, including neighboring plants of different species. A study in old growth ponderosa pine found that during July and August this process accounted for approximately 35% of total daily water usage from the upper soil, adding weeks of water during drought. This allows the ecosystem to continue photosynthesis, storing more carbon, and cooling the forest canopy as water evaporates from foliage. Forest canopies can register summer surface temperatures more than 30°F cooler than adjacent non-forest cover types, and large trees are the engine of this work. The water released to the atmosphere contributes to downwind moisture content and rainfall. Intact forests with large trees are positively associated with cool summer temperatures, increased late-summer streamflow and clean surface drinking water.

Among the more remarkable recent discoveries is that massive root systems of large trees link belowground ecosystems via mycorrhizal fungal networks and myriad soil microorganisms, forming an interconnected resource sharing and communication network. Large trees function as focal centers of this underground system, revolutionizing our understanding of the complexity and interconnectedness of forest ecosystems.

Globally, a 2018 study found that the largest-diameter 1% of trees hold half of all the aboveground carbon stored in the world’s forests. In a recently published analysis of carbon storage in six national forests in Eastern Oregon, my coauthors and I found that big trees, with trunks more than 21 inches in diameter (DBH), comprise just 3% of these forests but store 42% of the aboveground carbon. The dominant tree species at MERA, Douglas-fir and ponderosa pine, both had the same proportion of stems ≥ 21 in DBH (about 3.7 %), yet these stems held 37.5% and 45.8% of total species aboveground carbon, respectively.

As trees grow larger, small increases in diameter add a relatively large amount of volume — the overall effect being that carbon stores increase rapidly with tree diameter. For instance, doubling tree diameter from 10 inches to 20 inches led to a 5.3 to 6.2-fold increase in carbon, whereas tripling diameter led to a 13.8 to 18.2-fold increase. The very large trees (≥ 30 in DBH) are exceptionally rare in eastern Oregon — less than 1% of the total stems in the forest. But these giants held an even greater proportion of carbon relative to their small numbers, demonstrating the importance of letting large trees grow larger and accumulate more carbon for climate mitigation now and into the near future.

From its beginning, logging removed the easily accessible low-elevation, large, old-growth ponderosa pine, Douglas-fir, and western larch trees. Today, all remaining old trees are incredibly valuable. Even certain diseases disparaged from a timber production point of view add to the complexity of these inherited treasures from an ecological perspective. Small trees pose the greatest fire risk and are most vulnerable during drought relative to mature trees that have reached full root, bark and canopy development and respond to climate variability better. Large trees are the safe vault to store carbon.

It’s only through large live trees that large-diameter snags come into existence and provide crucial habitat for a diversity of wildlife species in Eastern Oregon’s forests. Snags and fallen trees also contribute to complex long-term carbon and nutrient cycling, serve as substrate for the next generation of seedlings, and contribute legacies that link forest generations. On the ground they act like sponges, absorbing and retaining water and slowly releasing it during the summer to the soil and atmosphere. A 2020 study found that across Pacific Northwest forests there continues to be a long-term deficit in large live trees and snags from 20th century logging.

Large trees are cornerstones of diversity and resilience for the entire forest community, and they provide many services important to society. We would do well to protect large trees where we can, and a sufficient supply of those that will soon reach large diameter.

Do you have a point you’d like to make or an issue you feel strongly about? Submit a letter to the editor or a guest column.