When a Tree Falls in a Forest

Photo by David R. Foster

Say you want to know how long a fallen tree takes to completely decompose. You could walk into the woods, cut a tree down, and return to check on it, say, once a month, for 10, 50, maybe even 100 years. Given enough time, you’d see the tree progress through the stages of decomposition until its remains were indistinguishable from the surrounding soil.

Of course, this approach would only tell you about one specific tree in one specific spot with a unique set of environmental conditions. And most of us don’t have the stick-to-itiveness (or lifespan) for this sort of thing.

Fortunately, scientists are looking into this for us. Over the last few years, a group of researchers from the U.S. Forest Service and the University of Minnesota have worked to find a faster, more generalizable method. By monitoring thousands of downed logs in forests across the U.S., they collected data that was used to build a mathematical model that shows how long different tree species take to break down.

Heading up this project is Christopher Woodall, a research forester with the Forest Service’s Northern Research Station in St. Paul, Minnesota. In woods stretching from Minnesota to Maine in the north and from Louisiana to Georgia in the south, technicians catalogued every downed log they found. They ranked each log on a scale of one to five, from freshly fallen to badly decomposed, and then returned to the same forests five years later to revisit and reassess. Using these figures, the researchers were able to create a model of decay for 36 different species of trees. “Some people said it couldn’t be done,” said Woodall, “but we did it.”

The computer model calculates that the “residence times” (how long a tree will take to completely decompose) for conifer species range from 57 to 124 years, while hardwood species are typically around on the forest floor for 46 to 71 years. Warmer, more humid environments promote faster decay than cooler, drier climates.

Besides tree type and climate conditions, the biggest factor in how long a piece of wood takes to decompose is length, with shorter log segments breaking down faster. Woodall speculates that this is because logs that are broken off at both ends offer more opportunities for fungi and microorganisms to colonize their insides. “If somebody cut off one of your arms, you’d have one potential infection point, but if they cut off both your arms, you’d have two infection points,” he said. “You’d have twice the probability of getting gangrene.”

The model can be used to predict how the process of decomposition in forests might change due to global warming. It can also be used to calculate the carbon stocks in a forest, so a forester could, for example, estimate how many years dead wood left by logging operations will take to break down, depending on size, site class, and the area of the country. “It’s good to inform people who manage forests about what effect their management actions will have on the residency of biomass and carbon that they leave behind,” said Woodall.

Decay by the Numbers

Here’s a sampling of the findings from the research in raw numbers. Half-life is defined as the number of years it takes a piece of downed woody debris to lose 50 percent of its initial biomass. Because decay rates are not linear, the residence times of downed woody debris don’t always correlate directly to the half-life. The study used a complex formula to arrive at residence times because, as the researchers noted, the length of time that downed woody debris resides in a forest “is not necessarily marked by a distinct end point.”

  1. Ken Hotopp → in Schoharie County, NY
    Feb 22, 2016

    Forty five years ago we built a house just inside the woods. In the backyard stood a dead 22”, 15’ beech. It fell 5 years ago. Counting the tree’s 80 yr+ life, 40+ years standing dead & 70 yrs on the ground it influenced the local environment for 2 centuries. Shouldn’t we be using a longer view when managing our woods?

  2. Steven → in 05667
    Feb 23, 2016

    As I’m reading this article, I can’t help but wonder about the practical aspects of rot-resistant trees. I look to the species that take the longest to decay, thinking of them as good for construction material, and notice that conspicuously missing from your list are species that are known to be rot resistant: Cedar, Locust, and Chestnut(?). I’m guessing that the research is not focused on discovering the best material for construction, but isn’t rot resistance essentially what is being discussed here for academic purposes?

    Would appreciate more info, if it’s available. Thanks.

  3. Richard Cosby → in Linden Virginia
    Feb 20, 2018

    My wife and I live in a trillium woods on Blue Mountain very near the Appalachian Trail in Linden, Va. We are leaving most of our little piece of land 4.5 acres in a natural state - leaving the dead-fall.  We have been here for over 30 years and have watched fallen tulip poplars, locusts, oaks slowly rot away. We have watched the dogwoods decline and have echos of the American Chestnuts through sprouts which appear less and less.  We are living in fear of emerald ash borers which will devastate this area shortly….Question:  We had five large trees fall across our nature trails through our woods - would you take a chainsaw and clear your path or just find a new route? What would a true naturalist do?  I love the natural woods and seeing the process working over time.  How do you feel about this?  What do you think?
    What is best for the trees?
    More info:  Our woods were clear cut with a few exceptions over a 100 years ago and the tulip Poplars dominate. Some people would say take out some of the Poplars to allow the other species to grow. The Ash will all go soon sadly and make more space.

  4. Doug → in PA
    May 22, 2018

    Looking at the chart at the bottom of the article, what does the column time mean?

  5. Kristin Driessen → in Lake Shore
    Jun 26, 2018

    I have 30 acres of natural forest. At least 4 types of conifers, almost every type of oak, sugar and red maple, poplars and birch. It has remained natural with the exception of putting in a trail.  About 3 yrs ago we had a blowdown and about 70 percent of the trees uprooted or became snags. I’ve learned so much from allowing nature to repair.  Snags are very valuable for peckers and cavity nesters. Woodpeckers, nuthatches, and chickadees all eat beetles and insects. I have all of them. Im hoping they will combat EAB if it arrives. Downed trees and understory hold moisture in soil. I believe this helps PREVENT forest fires (90% are started by humans). Rotting logs provide perfect mediums for new pine seedlings. Many of the uprooted trees still had enough roots in the ground to survive.  I have a green Balsam completely on the ground surviving after 3 yrs.  Mosses, club mosses, ferns, wildflowers and tree seedlings would have all been taken away if I had succumbed to the forestry people. My forest thrives today. I learn something new every time I go there. I did clear my trail again, but put the logs off to the sides.  Bushy pine tree tops, I cut so they dropped to the ground to prevent excessive drying out of needles. There is so much more to a forest than trees.

  6. Sylvan Dyer
    Jul 26, 2018

    I have found some downed trees that are thought to be Eastern Red Cedar in North Georgia Mountains at elevation of 3000 ft.. How can I determine how long they have been laying there on the forest floor?

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