Time Traveling Tree Trunks | Coast to Canopy episode 2, with Dr. Jean Huffman, a chat with Ikam Acosta (who came up with a name for the podcast), and featuring music by Tracy Horenbein.
I think the first post I wrote about longleaf pine was titled Why We Burn. I’ve spent years writing about the why. We live in a place where lightning strikes more often than anywhere else in the country. The plants and animals here have evolved to survive in a place that regularly ignites. But longleaf habitats are more fragmented than they had been, and are interspersed among our communities, our homes. Humans have taken control of fire on this landscape, for our own safety and also in an attempt to recreate ecological benefits lost as we clearcut and built over habitat.
If we want to burn for the ecological benefits, we need to know how fire burned before people took control of it. Unfortunately, as Dr. Jean Huffman tells us early on in this episode of Coast to Canopy, humans didn’t bother to take notes back then. We don’t want to burn too often or not often enough. Or at the wrong time of year. Is there a wrong time? Frequency and season matter.
Jean is a dendrochronologist at Tall Timbers Research Station and Land Conservancy in Tallahassee. She studies the history recorded in tree rings, and specifically that of longleaf pine (Pinus palustris). I first met her years ago on the St. Joseph Bay State Buffer Preserve. She was the preserve’s manager back then, and she showed me several rare, fire-dependent plants. She was working on her dissertation, trying to solve a puzzle that had previously prevented the kind of fire history that would let us know how these ecosystems once burned.
How can you build a fire history from a tree whose bark is near-impervious to fire?
Coast to Canopy blog posts are curated transcripts. My notes appear in italics.
Reading the Rings
Dr. Jean Huffman: This is from the Saint Joseph Bay State Buffer Preserve. And it only has about 12 rings. I use it as a demonstration to show kids and other people what a tree ring is and how you can look at fire scars.
So, a tree ring consists of the early wood, which is light colored, a band of early wood, and then a band of late wood that’s dark colored. Those together make one annual ring.
And this section has a fire scar that just happens to be from a fire that I did as the manager at the Buffer Preserve. I conducted a fire, and it happened to scar this tree. You can tell two main things about fire history from these scars.
And [the first] is the year. Dendrochronology is the science of exact dating of tree rings. And so it’s not difficult in this one because it has bark and, you know when it was cut, and it only has a few rings. So that’s easy as far as year. But you can also tell the timing, the seasonal timing of when a fire occurred by where the actual scar occurs within the ring.
This particular scar is at the end of the early wood, right before the late wood starts. I’ve done other research that assigns months to the position of the scar within the ring. And that means that this fire happened about in June.
What Information do Stumps Record?
Climate: Rain and Droughts
Dr. Jean Huffman: In the case of the longleaf and slash pines, the environmental factor that the growth response is most tied to is rainfall.
In years where the rainfall is really low, you’ll have a narrow ring. And where there’s more rainfall, you’ll have a wider ring. That pattern of years and size of the ring is what allows us to make a growth chronology that allows dating of ones that don’t have a known outer date.
The outermost ring represents the last year of the tree’s life. If you know when the tree died, you can count inward and determine the age of the tree when it died, and the years during which it was alive. Jean tries to find areas with multiple trunks. When trunks with unknown dates overlap with trees with known dates, the overlapping years will have the same rain/ dry pattern. If she can find enough stumps with overlapping ages, she can piece together a chronology that dates back hundreds of years.
Dr. Jean Huffman: That’s called cross-dating. It’s a basic thing of dendrochronology, a basic tenant that allows all this work to be done.
Scars show human activity, fires, and insect injuries
Dr. Jean Huffman: Anything that injures the cambium of a tree will cause a scar… In my trees, sometimes there’s a big area of cambium that’s killed and wood removed. And that shows a cat face scar from when people were doing turpentine operations and cat facing the trees or scraping the bark off so that they could extract the resin.
There are human type scars up north where indigenous people stripped bark for various reasons, so that you can get human history, human injuries.
You can get insect injuries. So if, a bug gets under the bark and chomps the cambium a little bit, that will cause a scar (think woodboring beetles). And, then there are, of course, the fire scars.
The Challenge of Finding Old Longleaf Pine Trees
Jean Huffman: When I started doing this work, which was about 1998, there were no fire histories whatsoever for the Southeast. And there are several reasons for this. One is that you need old trees that have recorded these. And because the southeast coastal plain was logged so extensively, there are very few old trees left.
Another reason for this was that that you could look at a cross-section of a stump or a tree and there’d be no fire scars, because longleaf pines are super fire-resistant. And actually, if they do get an injury, like a cat face or something like that, they tend to burn up and die because the resin on the face and all.
Longleaf pine ecosystems burn regularly, so these trees are, of course, fire resistant. This means the trees wouldn’t often get damaged enough to form scars, kind of a challenge when you’re trying build a chronology. Luckily, Jean had a breakthrough. All she had to do was look lower down on the tree.
The Stump Revelation
Jean Huffman: When I started collecting [cross sections] from old stumps at the Buffer, my husband, Neil (who does all this work with me) and I were able to to sample some of these stumps all the way down to the ground. They were turned up and so we could sample them right at the base, right at the interface of the roots and the shoot, the bowl of the tree.
What we found there was lots of fire scars… Whereas if you sampled up just six inches up, there are no scars. But right there at the base, there were lots of scars.
Things fall at the base, like pine cones, branches, lots of pine needles. So the intensity of the fire is or can be very hot in different places. And so, that’s why they have scars there when they don’t have scars anywhere further up.
All of the plant debris Jean mentions, especially pine needles, is fuel for fires. Because longleaf ecosystems had burned so frequently, there was less available fuel, and fires would not usually have been very large. They burned low to the ground, which is where Jean finds the most scars on the trees.
Where fire are less frequent, there is more fuel on the ground, and they can be much more intense.
Jean Huffman: A fire out West, say, or up north in a type of habitat that burns every 30 or 40 years, has very intense fires that do scar trees. And, here [in north Florida], historically, the fires were so frequent that they don’t get those high, fuel accumulations that would have made scars.
The Age of the Buffer Stumps
At the Buffer Preserve, Jean learned where to find fire scars on longleaf pine stumps. Not only that, but the stumps were hundreds of year old.
Jean Huffman: What we found is they dated to the, 1500s, like, 1540, 1530, for the oldest ones and went only went up to the early 1800s. Some of these stumps, the outer date is like the 1700s.
Especially in trees that were cut… by people, the tree does some kind of process that makes the stumps full of resin, like, sort of like concrete. And they really persist for hundreds of years in the right conditions. So, the Buffer study kind of brought the fire history work back to the 1500s right away.
The stumps came from trees that pre-dated Spanish occupation of the Florida panhandle. An ill-fated expedition led by Pánfilo Narvaez reached Apalachee Bay in the late 1520s, and Hernando de Soto’s expedition famously wintered in the Apalachee village of Anhaica, in present-day Tallahassee, in 1539. But the Spanish would not take control of the panhandle for several decades.
The information recorded in Jean’s Buffer Preserve stumps are a glimpse of a Florida before Europeans and their descendants controlled the landscape.
How Often to Burn
One of the biggest questions land managers have regarding fire is, how often should they burn? Older stumps can show us how often forests burned when they were allowed to burn naturally.
Jean Huffman: One of the things that makes me happy about doing this work is that it is useful to managers, that information from my dissertation. Previous to that, FNAI, which is the Florida Natural Areas Inventory, that kind of sets standard of different habitats and how often they would have burned and all that, they had, I believe it was either a two/ three to five year interval. This is what they thought it was. But after my research showed this, they actually changed the interval to one to three [years].
In other words, forests should burn at least every three years, and as often as one year. Under the previous regime, forests would go longer between burns.
Jean Huffman: That made me very happy that this work has some influence and application as we’ve learned about these systems… But what’s been really interesting is my most recent research, that’s at the Tyndall Air Force Base, near the Buffer Preserve… with my husband, Neil Jones, and Kevin Robertson and Monica Rother, other people from Tall Timbers.
We have collected many trees and sampled them low and have a really complete record. And we have been very, I have to say, surprised to find that the fire interval during the period from the late 1500s to the late 1700s, even into the 1800s was annual. The fire frequency was very, very, very frequent.
In other words, fire burned every year.
What happens when you don’t burn?
Jean Huffman: It turns out that this three year threshold is very important. So looking at pine flatwoods… if you have gaps that go more than three years, it shifts towards woody dominance. Towards increased palmetto, gallberry, all kinds of woody species.
Basically, a three year [regime] tends to keep it at equilibrium. Less than three year makes it less woody. And more than three years, it gets woodier. And you can see this extremely clearly at the Tall Timbers fire plots, where they for many decades have burn plots of one, two, three, four and more intervals that were set up by Herbert Stoddard.
Historically these areas burned frequently and they were open like you describe. But when fire is excluded, what moves in is new tree species like laurel oaks and water oaks, things that wouldn’t survive a fire. Historically, they weren’t in the system because they couldn’t get established because of the frequent fire.
[Tall Timbers] found… that when it got over three years, it became so shrubby and so closed-in, the fire wouldn’t really even carry anymore. It was only the one to three [year plots] that maintained it at all. And the three was borderline.
So those things move in. And then the shrubs that are there naturally, that are kept low by frequent fire, just explode. So it gets full of hardwood tree species and shrubs that weren’t there originally.
Seasonality of Fire
These days, people manage fire on the Florida landscape, and they do so with various goals. Prescribed fire reduces fuel loads in the forest, and prevents catastrophic wildfires. Jean mentions burning for cattle, to keep pastures from becoming overgrown with woody shrubs. What Jean is looking for in the tree ring record is a fire regime with the most ecological benefits, one that mirrors the regime that plants and animals and the ecosystem evolved with.
Jean Huffman: Most of the the burning that’s gone on in the last 100 years by people for cattle in south Florida/ south central Florida, and used to be cattle in North Florida as well, and for forestry, mostly occurs in the the winter dormant season of trees. And we were curious as to what was the season of past fires.
A lot of people say that indigenous people burned. And for some reason they think that they would have burned in the winter, like post-settlement people did. And so a lot of people expected there to be a lot of dormant season scars. What we found was that pre-euro-settlement, pre 1800s, the vast majority of the scars were in the lightning season.
They were in the May, June, July period of time. And that’s the transition of the early wood to late wood, and it was a surprise to see that, that this was the case over this 300-350 year period of time.
Fire in the Wetlands
As north Floridians, we’re used to seeing and smelling smoke in the winter and early spring. This is when a lot of prescribed fires happen. And yet, tree rings show that fire occurred later in the year, in the summer months. Do plants and animals respond to fire differently in different seasons?
Dr. Jean Huffman: So, December, January, February, March, for current burning, or even into April and, and as a manager and fire ecologist, I’ve seen that there’s a big difference in what happens when you burn in the one season versus the other. When you burn in the summer versus the winter.
I have a couple examples of ecological changes that occur with changes in fire seasonality, and one is fire in wetlands, how fire moves across the landscape.
So the historic fires occurred right at the end of the dry season. They were ignited by lightning, in the beginning of the wet season. So the landscape was really dry, very dry. And, a stray fire would start outside of where it was raining, and burn a lot and burn so frequently. It would burn across the grassy wetlands and, that’s what the landscape was like, that most of the wetlands, a lot of wetlands burned as well.
Jean is referring to ephemeral wetlands. One type of ephemeral wetland, the type where we find frosted flatwoods salamanders, are low spots in mesic flatwoods. They hold water seasonally, and when they’re dry, fire can sweep through them (above).
Other wetlands in the National Forest are more directly connected to the water table, and can stay wet or dry for longer, depending on the fullness of the upper aquifer. They are entirely open and grassy, containing few, if any, sizable trees. These wetlands are also kept open by fire. (below).
Fire and the Frosted Flatwoods Salamander
Dr. Jean Huffman: When the fire season with cattlemen and forestry shifted to the winter wet season, it stopped going into wetlands… wetlands that had open grassy edges and and even interiors with cypress, which is very fire tolerant. They were very open and grassy. But when the fire was switched to the winter and these fires didn’t go into the wetlands, they became shrubby.
Dr. Jean Huffman: So we have a shrub here called titi that just went crazy. Chuck Hess, who worked in the Apalachicola National Forest, has mapped the National Forest from the old aerials, when the wetlands were still open, to current [times]. I believe it’s over 1,000 acres of wetland that shifted from open grassy to absolute dense thickets of titi.
And the other thing that happened when the seasonality of fire shifts like this is that the fires don’t go in and consume the organic materials. When there’s no fire in the wetlands, the leaves and everything goes to the ground and decays and makes what they call duff. So in the past, the bottoms would have been sandy, and, now they’re covered with a thick layer of duff.
It’s ecologically completely different. And so things like flatwoods salamander that, that had to have that open habitat, they can’t survive. You have the species, flatwoods salamanders that were very common in this landscape of fire that went in wetlands as well as uplands and was very frequent. And, now they are next to extinction, right?
Timing Fire to the Seasonality of Animals
Dr. Jean Huffman: The plants and animals in the system evolved with this type of fire. For some species, when you move fire out of that window that it was most common in… it can really negatively hurt them. For instance, in the fire record, there are almost no fall fires.
Now in my fire history records from the scars, most of the fires were in the June area, but there were some dormant season fires, some fires at other times of the year, but very few in the fall.
And it turns out the flatwoods salamanders, their adults migrate to the wetlands in the fall. They live underground most of the time, but they’re up on the wiregrass. And if you burn in the fall, you’ll burn up the adults.
Wiregrass Flowers After a Fire
Dr. Jean Huffman: There are other species that also respond to that certain season. Wiregrass, which is a dominant species of undisturbed, old growth ground cover sites. One of the predominant natural grasses in these systems.
It was thought for a long time that it never flowered, but that was because the burns were all done in this winter to very early spring time, which doesn’t stimulate the flowering and seeding of the species. But as soon as people started burning in May, June, it flowers amazingly. And it’s because it has [an] adaptation to the specific seasonal timing of fire.
Rebounding after winter vs. growing season burns
Dr. Jean Huffman: When you burn in the winter, it takes a while for things to sprout back. They do. But when you burn in the spring/ summer time, it just comes back really rapidly. So it may be green [at the time of a burn], whatever. They lose that little bit, but they just come back gangbusters, and it stimulates flowering, actually stimulates synchronized flowering.
So if you burn it that season, in the fall, you’ll have a phenomenal bloom of Liatris (the blazingstars) and all kinds of asters, and it’s synchronized. It’s really kind of cool, also, is that, that it synchronizes to flower… in these longleaf pine systems when the butterflies migrate through.
Dating Cypress stumps
Dr. Jean Huffman: In this, process of finding stumps and people telling us about stumps, some people that have land in South Georgia told us about a lake that they have that used to be kind of a cypress flat, but a long time ago it was dammed and made into a lake, in the 1800s.
They drain it every 12 years. And the bottom of the lake is just full of cypress stumps. When they drained it this time, they asked if we might be interested. And so, we went to cross-section these cypress, and cypress gets very old. One little tree trunk that’s about 12 inches had 500 rings.
This presented a situation where we could test this idea about basal scars and sample the stumps at the base. I was very excited to find evidence of fire scars in cypress. Nobody has ever looked at fire frequency in cypress systems, and it’s much less understood than in uplands.
Fire in wetlands is much less studied, much less known and very interesting. And so it’s exciting to find these. We would love to do the fire history work from them, but we haven’t been able to because I have so much work to do finishing up these studies that I’ve already started. We’re looking for funding to help do it, but, it will be really interesting to expand the work into real wetland systems like cypress.
Expanding the Scope of Fire Histories
Dr. Jean Huffman: We’ve collected [from] over 50 sites within Florida, South Georgia, and just this past year, we’ve gone also to North Carolina and central Georgia. We’ve collected from the Seminole Reservation in South Florida to South Georgia and all many places in between… More recently, we’ve done collections in cooperation with other people, like at Goethe State Forest with the Florida Forest Service staff, the manager there who was really interested in knowing what the fire history was there and helped us collect.
And so I have a pretty large collection. Definitely the largest collection of tree ring records for the southeast anywhere.
The thing about the stumps is that it’s rare that you find what we found at Tyndall, the 30 stumps in one little area. And that’s what you use to make a good fire history reconstruction. Since all of the trees would have been recording fires that have one little small area. But a lot of times what you find is one stump here or one stump there, and you can’t do a fire history reconstruction from that, because any one tree only records some fires.
You need a number of trees to really get the accurate record of how many fires were going on. I’m looking forward one day to taking all these stray samples from all over and looking at the seasonality of fire for them, because you have a record of when the fire occurred that you don’t need a series of trees for that.
Well, maybe someday down the road.
