Top Line: Big old trees play out-sized roles in a forest stand in terms of biodiversity, carbon storage, and carbon sequestration.
This is the first in a series of two Public Lands Blog posts on big old trees. Part 1 explodes the myth that their rate of growth slows as they age and also introduces the concept of tree-related microhabitats. Part 2 examines a new scientific paper on the state of medium-sized and large trees in the United States.
I never met a big tree I didn’t like.
Thus, I was heartened to read a recently published scientific paper (Chisholm and Gray 2025) that reports that the number of big trees is increasing in the United States (although the world is quite another matter). While certainly worth celebrating, this good news needs to be considered in the context of past, present, and potential future logging.
Such considering we shall do in Part 2. First, before we go there, this Part 1 aims to (1) demolish a common myth that old-growth trees decline in their rates of growth as they age, and (2) introduce “tree-related microhabitats” (TreMs) that go hand in hand with big old trees. It will become clear that what is lost in terms of ecological benefits when mature and old-growth trees are logged hasn’t been fully appreciated in the past.
The Myth: As Trees Age, Their Rate of Growth Declines
The myth that trees’ rate of growth declines as they age is simply not true. An article with thirty-eight co-authors that appeared in the scientific journal Nature, entitled “Rate of Tree Carbon Accumulation Increases Continuously with Tree Size,” laid the myth to rest (more like shot and killed it, but it has not yet been buried). This stand of authors examined 673,046 trees belonging to 403 tropical, subtropical, and temperate tree species on every continent with forests. They concluded:
[L]arge, old trees do not act simply as senescent carbon reservoirs but actively fix large amounts of carbon compared to smaller trees; at the extreme, a single big tree can add the same amount of carbon to the forest within a year as is contained in an entire mid-sized tree. [emphasis added]
The myth arose because most measurements of the growth of trees have been done by measuring stands of trees rather than individual specimens of trees. Most of the measuring is of usable wood (usable enough to send to the mill) rather than of total biomass or carbon. Foresters measure not only the useful (to them) wood presently in a stand but also how much this usable wood is increasing.
At a stand level, a forest reaches a point at which the rate of annual growth of usable wood is at its peak, what foresters refer to as “culmination of mean annual increment” (CMAI). It turns out that CMAI is a very useful indicator of when a young forest becomes a mature forest—the point at which timber-volume-maximizing forestry calls for clear-cutting.
As a very general example, Douglas-fir trees in the Pacific Northwest reach CMAI at about eighty years of age, depending on the growing site (the soils, elevation, aspect, and such). As a stand continues to age, individual trees die due to competition from other trees. When that happens, the next amount of usable wood for the forester to tally declines. But to conclude from this that trees’ rate of growth declines as they age is a case of not seeing the forest for the logs.
Introducing Tree-Related Microhabitats
Big and old trees are not simply bigger and older versions of smaller and younger trees. Scientists have identified “tree-related microhabitats” (TreMs) on and in large and old trees, but rarely and usually not at all on and in young and small trees. More than two-thirds of the scientific literature on TreMs comes from French, German, or Italian forests (Martin et al. 2022).
TreMs are defined by Kozák et al. 2022 as
“distinct, well-delineated structures occurring on living or standing dead trees, that constitute a particular and essential substrate or life site for species or species communities during at least a part of their life cycle to develop, feed, shelter or breed” (Larrieu et al., 2018). These microhabitats mainly include cavities, tree injuries, exposed wood, fungal fruiting bodies, and excrescences. Even though the relationship between TreM richness and forest biodiversity is relatively understudied, the occurrence of TreMs appears to be a suitable indicator for many taxa, such as certain insects, birds, and bats. [emphasis added and most citations omitted]
When I first read this, my new word of the day became excrescence (“a distinct outgrowth on a human or animal body or on a plant”). I immediately thought of a conk (according to the Dictionary of Forestry, “the visible fruiting body of a wood-destroying fungus which projects to some degree beyond the substrate”). One person’s defect or imperfection (“wood-destroying fungus”) is another person’s perfection (“tree-related microhabitat”), but both are excrescences!
Big Old Trees Defined
While size matters, it’s all relative. A relatively small big tree can still have an out-sized impact in a forest. What’s large (or old) depends upon the species and the growing site (soils, elevation, aspect, latitude, and such).
Most tree sizes are given in measurements not of the height of the tree but the width of the tree, a.k.a. the diameter at breast height (dbh). (One of my most prized possessions is a “d-tape” I found that some forester, up to no good, left in the woods. The tape goes around the circumference of the tree but is calibrated to measure the diameter of the tree.) A worldwide review (Stephenson et al. 2014) describes the largest trees as measuring >100 cm dbh. A global review out of China (Ali and Wang 2021) defines mid-sized as between 40 and 60 cm, and large as >60 (but sometimes 50+, 70+, 100+) cm dbh.
The new analysis of US forests (Chisholm and Gray 2025) that we will be deeply examining in Part 2 defines a medium tree as 50 to 100 cm dbh and a large tree as >100 cm dbh. The paper reports its results separately for the eastern and western US, as large trees are so rare in the East. For most references herein, we’ll use their definitions of medium and large, but for those of us who will die thinking in imperial units, a medium tree is ~19.7 to 39.4 inches dbh, and a large tree is anything larger than a medium tree.
(Note to self: medium tree = ~20-inches-plus dbh; large tree = ~39-inches-plus dbh.)
There is generally a relationship between the size and age of a tree, especially on the same growing site, but trees of the same age can vary hugely in size in the wild. I’ve wrapped my hand around the trunk of an old-growth ponderosa pine that had lived for centuries in a thick understory. Just a few feet away, two of us could not wrap our arms around the trunk of an old-growth ponderosa pine that was just about the same age.
Old Trees for Biodiversity, Big Trees for Carbon Storage
In terms of ecological benefits, what counts the most: oldness or bigness? Through the lens of biodiversity, oldness counts the most (remember the TreMs!). Through the lens of carbon storage, bigness counts the most.
Biodiversity
The older and larger a tree, the more biologically diverse is the tree and the forest it lives in. Big old trees are full of tree-related microhabitats (TreMs), which are biologically diverse themselves and indicators of the biodiversity of the forest the TreMs are in. According to Kozák et al. (2022):
Consequently, the largest trees and the oldest trees differ, but they synergistically support the integrity of forest functioning through the provisioning of specialized microhabitats, which, in turn, promote the viability and persistence of dependent, niche-differentiated flora and fauna. [emphasis added]
TreMs extend beyond the life of the live tree. They exist long after the tree dies and for as long as the tree corpus is present, either standing or fallen. There is more life in a dead tree than a live tree.
Carbon Storage and Sequestration
The evidence is strong that big old trees disproportionately contain most of the aboveground carbon in a forest. In a journal article pertaining to the eastside forests of Oregon and Washington, Mildrexler et al. (2020) reported:
Pooled across the five dominant species [Douglas-fir, Engelmann spruce, grand fir, ponderosa pine, and western larch], large trees accounted for 3% of the 636,520 trees occurring on the inventory plots but stored 42% of the total [above ground carbon]. [emphasis added]
Chisholm and Gray (2025) note:
Only 0.1% of all western trees were larger than 100 cm, and 1.9% were between 50 and 100 cm, with the implication that the top ~2% largest trees accounted for 75% of the live tree carbon storage in the western United States. [emphasis added]
(More on this in Part 2.)
Other studies have come to similar conclusions.
Ali and Wang (2021) indicate that such also appears to be the case in tropical forests:
Big-sized trees, which accounted for 2.5% of the stand density in the forest, contributed ~ 25% of aboveground biomass.
Stephenson et al. (2014) considered the annual forest growth mass rather than the existing forest mass with similarly astounding results:
The rapid growth of large trees indicates that, relative to their numbers, they could play a disproportionately important role in these feedbacks. For example, in our western USA old-growth forest plots, trees 100 cm in diameter comprised 6% of trees, yet contributed 33% of the annual forest mass growth. [emphasis added and citations omitted]
The point to take away here is that both biodiversity and carbon storage and sequestration take an out-sized hit when mature and old-growth trees are logged. With that in mind, we will proceed in Part 2 to look at implications of the new finding that the number of big trees is increasing in the US.
For More Information
Ali, Arshad, and LiQiu Wang. 2021. “Big-Sized Trees and Forest Functioning: Current Knowledge and Future Perspectives.” Ecological Indicators 127:107760.
Chernevyy, Yurij, et al. 2024. “Environmental and Economic Significance of Big, Old-Growth Trees.” International Journal of Environmental Studies 81(1):475–488.
Chisholm, Paul J., and Andrew N. Gray. 2025. “Populations of Large Diameter Trees Are Increasing Across the United States.” Environmental Sciences 122(11):e2421780122.
Kozák, Daniel, et al. 2022. “Importance of Conserving Large and Old Trees to Continuity of Tree-Related Microhabitats.” Conservation Biology 37(3):e14066.
Martin, Maxence, et al. 2022. “Tree-Related Microhabitats Are Promising Yet Underused Tools for Biodiversity and Nature Conservation: A Systematic Review for International Perspectives.” Frontiers in Forests and Global Change 5:818474.
Mildrexler, David J., et al. 2020. “Large Trees Dominate Carbon Storage in Forests East of the Cascade Crest in the United States Pacific Northwest.” Frontiers in Forests and Global Change 3:594274.
Moomaw, William R., Susan A. Masino, and Edward K Faison. 2019. “Intact Forests in the United States: Proforestation Mitigates Climate Change and Serves the Greatest Good.” Frontiers in Forests and Global Change 2:00027.
Stephenson, N. L., et al. 2014. “Rate of Tree Carbon Accumulation Increases Continuously with Tree Size.” Nature 507:90–93.
USDA Forest Service and USDI Bureau of Land Management. 2024 (revised). Mature and Old-Growth Forests: Definition, Identification, and Initial Inventory on Lands Managed by the Forest Service and Bureau of Land Management in Fulfillment of Section 2(b) of Executive Order No. 14072 (pdf). [Note: my editor found that this report is no longer available on the Forest Service website. Hmmm, I can’t imagine what could have happened to the link…😉 Fortunately she found it is available on a website that is an arm of Big Timber.]
Leave a Reply