What constitutes a forest?
We often think of forests as static collections of trees, along with some shrubs, ferns, fungi, and other “foresty” organisms.
But forests are more than an array of cool critters and plant life. They are dynamic ecosystems that are constantly changing. Sometimes dramatically. As a result, there are times a forest may not look much like a forest at all.
Storms, pests and disease, landslides, and floods are parts of a forest ecosystem—interacting with the organisms that reside there and shaping forest development. Disturbances, such as these, are not only natural, but essential to many forest species—sending forests through complex paths of community change.
In Pacific Northwest forests, fires are an especially important example of these essential forces of disturbance. Forests in the Pacific Northwest evolved with fire and have adapted to the presence of fire on the landscape in a variety of ways.
Andrew Merchel, a dendroecologist from Oregon State University, knows the importance of forest fire to the region all too well. For the past several years, Andrew has been studying Pacific Northwest forests to better understand the patterns in fire frequency and, more recently, how these patterns might ultimately influence forest development.
I was fortunate to get invited along with his crew to a couple of his field sites this summer to see their research in action.
The Long Road to Site 87
It was a warm, sunny morning as I waited for Andrew at our designated pullout. I was parked just off the Santaim Hwy near Longbow Camp. Standing along the road, I passed the time watching the South Santiam River’s ripples catch the light and flash white below me.
Soon, Andrew arrived with his crew of young students, and we headed out to what would be the first of two sites.
As we traveled down the narrow, overgrown forest service roads, Andrew told me a bit about the site.
He explained that the site has had two relatively recent mixed-severity fires—one in 1848 and one in 1868, but before that, the only other fire they found evidence of was in 1535.”
“So, we had a fire a really long time ago, and three hundred or so years without fire,” Andrew emphasized, “and then two fires in the 1800s.”
As we got nearer to “site 87”, Andrew pointed out patches of thinning that had been done around the middle-aged Douglas-fir—an unusually recent occurrence on national forest land for the time—but helpful to us in our fieldwork for the day.
Records in the Rings
And then we were there—site 87. It was time to get to work. Andrew’s field crew made up of college students headed to collect forest development data on one side of the road, while Andrew and I went in search of the perfect stump on the opposite side.
As mentioned earlier, Andrew is a dendroecologist—which basically means he uses tree growth rings to better understand how forest ecosystems have changed over time. As part of that research, Andrew has been using crosscuts from dead trees and stumps to reconstruct fire records for a variety of forests in the West.
The tree rings on each crosscut provide a record of time that can be compared with other crosscut tree records to establish a timeline that goes beyond the lifespan of one tree.
Dendrochronologists can date each annual ring sampled from crosscuts even if a sample is collected from a snag or log that has been dead for centuries.
They date annual rings with a technique called cross-dating, which uses the sensitivity of annual tree rings to climate. Hot, dry years result in thin rings with narrow latewood and moist years result in years with wide rings with thick latewood in the Pacific Northwest.
Each decade has a unique pattern of thin and thick years that can be used like a fingerprint to precisely match a series of tree rings to the exact calendar years when they grew on the tree.
In this way, tree-ring records tell you a lot about the environmental conditions of each forest, including climate, over the recorded years. Most importantly for Andrew’s research, the rings also record fires as scars in the tree rings—providing information about the year, season, frequency, size, and sometimes severity of fires that occurred outside of modern records.
More Frequent
Research at the Forest Service’s PNW Research Station and the tree ring lab at Oregon State University has really shed light on the frequency of fire in Westside forests.
Before, ecologists thought that westside forests experienced fire as a function of lightning; and that fire was historically infrequent in much of the western Cascades and Oregon Coast Range—with forests going 100s of years between fires.
Now, hundreds of fire scars collected from dead trees have shown there are many ways fire exists in westside forests. Fire regimes (patterns of fire) are variable in frequency and in how they shaped forest conditions over time. For example, some westside forests record fire in nearly every decade, while others go centuries without fire. The role of fire historically varied with forest age, Indigenous burning, lightning, topography, and microclimate.
Searching for Scars
Chainsaw in hand, Andrew and I headed down the road and trampled our way uphill through the underbrush to check out some of the thinned areas for stumps to cut into. The goal is to find stumps that show fire scars—a blackened resinous area along a ring.
Andrew and his crew had already sampled the area, but he was hoping to get more samples from older trees. Looking at the age classes of trees in the forest, Andrew suspects there may have been another large fire in the 1820s yet to be discovered.
Not all Stumps are Like the Others
“There are clues about which stumps to cut into,” Andrew explained as we carefully picked our way over the bramble and down woody material. “Trees that scar when they are young, for example, will often scar again with the next fire.”
Oddly shaped stumps that are oblong tend to be good candidates. And of course, the stump needs to be solid without too much decay.
Another factor that affects scarring is that each tree species has its own resistance to scaring and the ability to preserve long records of past fires.
“Many Douglas-fir are not good recorders of historical fires,” Andrew remarked “The initial burn needs to be severe enough to form a first scar on a tree before it develops thick bark that prevents fire damage and the formation of a fire scar.”
Andrew leaned over one large stump and wiped away the smut that had accumulated on top of it with a brush with metal bristles.
“You can see an injury right there,” he said pointing to a white resin-filled gap between a couple of growth rings—a scab around the wound. “That looks like mechanical damage,” and probably not a fire scar, he concluded. Mechanical scars often go across rings, while fire scars form neatly along a single row of cells.
The search continued.
Making the Cut
We moved out of the thinned area and into the denser forest for a bit, looking for promising-looking stumps.
Soon we came across another with sampling potential.
“There is some rock-solid wood right there,” he remarked as he examined the stump—perhaps the product of resin released as the tree scarred.
It was time to make some cuts. Andrew and I put on our earplugs, and he began slicing horizontally through the stump several inches below the original cut. The whorl of the chainsaw and fine woody dust filled the air space.
It was over in just a few minutes.
Andrew removed the top he cut off and began sweeping away at the newly cut surface.
Nothing.
Just a few old branch whorls. No scars.
How to Scar a Tree
The fact of the matter is that trees don’t always scar.
Contrary to what you might think, fire scars form from heat, not flames.
“It is heat transmitting through the bark for a long enough period of time to kill the cambium locally,” explained Andrew. “It is more burn residence time and the ability to transmit heat for long enough that records the fire.”
Perhaps for this reason, if a tree scars when it is young, it will often continue to scar on the same line. They also tend to occur really low on the trunk near the ground where heat is transmitted to the tree bole from surface fuels.
Patterns of Variability
As we searched for another crosscut stump to sample, I asked Andrew to tell me more about his research findings. After all, this was not his first rodeo. Andrew and his team, at the time, had sampled up to 50 sites in Westside forest with 15-20 cross-sections from each one.
Yet, despite all the sampling, it was still difficult for Andrew to identify any environmental patterns.
“We don’t have enough to relate patterns of fire to different environmental settings still,” says Andrew with a sigh, “because there is so much variability.”
Consistently Inconsistent
However, Andrew admits there are some consistencies.
For example, low-elevation sites near major rivers, like the McKenzie and Clackamas, that are hotter and dryer tend to have more frequent fires. These also may be sites where Indigenous People used fire stewardship to produce vital cultural resources.
“Some sites are shocking—the amount of fire they have.”
On the flip side, there are higher elevation sites in the Silver Fir Zone, like Gordon Lakes, where fire is very infrequent. These places will record one high-severity burn, followed by one or two reburns, and then go more than a century without fire. Rinse and repeat.
It is at mid-elevation around 3,000 feet where it gets really challenging to predict.
The hope is to eventually look at the variation in topography, elevation, and other site factors in combination with fire histories to try and understand how forests develop in different ways based on their specific context.
Catface
Andrew and I moved back into the open with fewer trees and shrubby underbrush.
“Come on guys,” he says, leading the charge. “Where are the fire scars?”
As we search, we come across a burnt-out western redcedar with a charred opening in the lower trunk—a catface. A catface forms when the cambium on the tree is killed by fire, and in the next 10 or 15 years the bark falls off, leaving the tree susceptible to future fires.
“The earlier scars are burnt off by the later scars in there,” Andrew explains.
Cedars are interesting trees in fire. They get consumed by fire a lot.
“Cedars really do chimney,” explains Andrew, “Fire gets in there and burns them out on the inside.”
At the same time, “It doesn’t necessarily die,” said Andrew, “or rot” for that matter—instead cedars tend to stand as ghosts of fires past.
Fool Me Again
The stumps were easier to spot in the more open forest and Andrew found a couple more promising candidates. He was really hoping to find an older stump that might provide evidence of a 1759 fire he was fairly certain had occurred.
The chainsaw ablaze, Andrew sliced into another stump and then another, but to no avail.
One of the stumps was massive and took a lot of effort and some careful wedging to remove the cut surface. He cut a couple of cross-sections from the large stump.
“This one is going to frustrate us,” he declared before making this final attempt.
Unfortunately, it ended in sweat and sawdust, but luckily no tears.
Is it Severe?
At this point, we were low on gas and Andrew decided it would be best to start heading back down to the road to meet up with the crew.
As we made our way through the salal, ferns, and other shrubby species—a product of the thinning that occurred here—I asked Andrew about how fire severity fits into his research.
Fire severity is a measure of the magnitude of the immediate impacts of fire on the vegetation and living soil. In forest ecology, it is typically based on tree mortality—about 0-30% tree mortality for low severity, 30-70% mortality for moderate severity, and anything above that is high severity.
“Anything killing the fire-resistant trees is moderate [or high] to me,” Andrew suggested.
When it comes to fire severity, like frequency, Andrew has found that it too is not so easy to predict.
“I would agree that 2020 fires are nothing new,” explained Andrew, “We have always had big blowups, but we are missing fire events outside of these conditions all over the West Cascades that poke holes and do very different types of burning.”
In other words, though large high-severity fires do occur in the West, it would be a mistake to forget that there are many other types of fires that have shaped this forest type historically—fires that vary in both frequency and severity.
These differences in fire severity also occur on a much finer scale, according to Andrew.
“Here is an old-growth tree next to an early seral shrub,” he illustrated, “and they exist right next to each other—and that is normal.”
It is patchy. And that matters because that patchiness increases variety and biodiversity.
Forest Development Implications
Andrew told me about a paper out of Oregon State University by Chris Dunn that looked at the implications of fire severity for forest development in the Willamette and Umpqua National Forests.
In general, Dunn found that the severity of fire results in very different trajectories of forest development.
A low-severity fire may not result in a new cohort of trees, or it will result in shade-tolerant species including western hemlock developing on the site. In a high-severity fire, grasses and shrubs will make up the post-fire community with Douglas-fir the primary cohort of trees able to establish.
Then, there is moderate severity. This is where it gets interesting. It is in moderately burned forests that you can end up with the most biodiversity post-fire—with sites that have up to 17 different tree species established after fire in Southern Oregon. Having both canopy gaps and live trees remaining post-fire allows for greater variability in the forest community as different species find their ecological niche.
“One thing this project is going to do is we are going to core all these hemlocks and true firs and see if they actually link to fire,” explained Andrew, referring to the coring work his field crew is working on. “If they do, then I think we will interpret that low-severity fire was really important to the development of these species in westside forests.”
Forest Management Implications
Andrew spoke strongly about what this means in terms of forest management. If fire is variable, the way we manage the structure of forests should also be variable.
“We can’t just do one thing in plantations where we are trying to restore heterogeneity and biodiversity,” said Andrew. “We should be doing everything from removing 10% of trees to 90% of trees. That is historically probably what happened—creating a lot of variability.”
The result would be real; and probably pretty messy.
“Instead of distinct edges, you would have a constant mosaic,” Andrew described.
What we are doing now in plantation settings is not natural.
“The plantation is completely artificial… cut at 40 years or so and planted uniformly at a high density—this is not one of the development trajectories. It is not mimicking historical disturbance processes or stand development.” Looking ahead, the ecosystem that develops from a plantation will be much different than the ecosystem it replaced.
Making the Cut
We were just about back to the road when Andrew noticed another stump that had been cut the last time he visited. Having not found any stumps with fire scars yet, he led me over to this one—hopeful that I might observe the scarring up close.
After wiping away needles and other debris, we got down to stump level. There we could see two scars—one from the 1868 fire sitting just above the other from 1848. “Which is sort of classic,” remarked Andrew, as far as fire scars go.
The scars showed up as what looked like a break in the annual rings (cambial necrosis) with resin separating the blackened tissue from the wood wound put down during the healing process.
Much of the cut area was also covered with a white rot—the forest ecosystem eager to restart the decay process.
Back on the Road
Soon we were back on the road to meet up with the rest of the field crew. After checking out one last stump on the opposite side of the road, we all piled back in the vehicles for another long twisty ride to field site number two for the day.
Forest ecosystems are dynamic. But they change on timescales that are often outside of human experience. Understanding fire as an agent of change in our forests requires long-range data sets, like what Andrew has tirelessly been collecting—helping fill in our knowledge gaps.
We may not have succeeded in finding a fire-scarred tree that day, but I am grateful to have experienced the forest through Andrew’s eyes—to understand its wonderful complexity and the secrets it retains deep beneath the bark
Andrew Merschel is an ORISE postdoctoral fellow working with the USFS PNW Research Station and he leads the tree ring lab at Oregon State University. Andrew uses tree rings to develop a shared understanding of how different forest ecosystems function over time. He is particularly interested in how disturbances (mostly fire) and forest management have shaped and will continue to shape forest ecosystems in the Pacific Northwest. Andrew lives with his family (Vanessa, Aldo, and Sawyer) in Corvallis, Oregon and they enjoy a mixture of fishing, hiking, wildlife ecology, and chainsaw repair in their spare time.
