Last month, I spent several days in Harvard Forest, 3500 acres of woods dedicated to scientific research. The forest is home to dozens of research projects, some short-term, others stretching over decades. I told you a little about how I got to participate in some of these studies, learning how to collect and analyze data in the same ways that ecologists do. Along the way, I ran into something a little weird—trees that were very much alive, but weren't growing.

If those of us who are not tree experts know anything at all about tree life cycles it's probably centered on tree rings. We learned back in grade school that trees form a new ring every year. Chop down the tree, and you can see a record sometimes stretching back hundreds of years—burn marks indicating fire, fat rings during times of plenty, and thin rings showing resource scarcity. And we know that scientists use these rings to learn about the past, to find out what was happening in local environments before human beings started to painstakingly record that information.

When it makes a new ring, a tree becomes a little fatter. Over decades, you should see a change in its diameter. So I was surprised, during my time in Harvard Forest, to run across several red maple trees that hadn't grown an inch in 11 years. Scientists had measured the trees in 2001. We came back and measured them in 2012. In that time, the diameters hadn't changed at all.

Turns out, this was not mere mis-measurement on my part. Neil Pederson is an assistant research professor in Columbia University's Tree Ring Laboratory. He's also found red maples (and other trees) that are living, but not growing, in the Harvard Forest. Pederson calls them zombie maples. He says these trees are really representative of the fact that individual plants can vary from one another as much as individual people—something scientists have to account for in their work. It's also a great example of how complicated even seemingly simple science can become once you start to dig into the details.

Neil Pederson: I was doing research at the Eddy Flux Tower plot to see if we could match tree rings to the carbon flux. The Eddy Flux Tower plot is this highly engineered system of taking up samples above, below, and winthin the canopy of the forest to see how carbon is moving through the forest. There are samples taken constantly, 24-7. I was there in 2003 or 2004 and it had been going for about 11 years at the time. They’d seen that the forest was continually taking up carbon in the form of new growth, and every few years they were going out and measuring the forest to document that. We went out to take cores and look at the tree rings. My idea was to take those tree rings and put them in a regional context by measuring similar trees across the Northeast. I initially focused on red oak because those were the biggest and most dominant trees in the plot.

The Eddy Flux Tower plot is thought to reflect ecosystem productivity. Normally, they measure all the trees. When we did our measurements, we decided to be efficient and see if we could get at the same number by measuring only the most dominant and largest trees. Maybe those would be the most important. Our tree rings didn’t quite agree with Eddy Flux Tower measurements, so that suggested that there were other trees we needed to core to get a good idea of ecosystem productivity.

So we went back and we cored the red maple. These trees aren't big, but they are the most numerous in the understory. With those two species we had a significant percentage of the forest in terms of biomass and numbers of trees. And that’s how I stumbled into it—maples sitting there alive, but not growing.

NP: When we core trees, everyone understand that rings say something about age and growth. But not every tree produces a new ring around the base of the stem each you. You can have missing rings or locally absent rings during times of stress on the tree. Because of that you have to cross date trees. We core different trees and make sure the patterns match. By comparing them you can get a good idea of whether each individual ring is correctly dated. Then you just keep adding trees to the comparison and building up this profile within a population and a species.

I worked up the first five red maples really quick. In like a day. They have a ring structure that isn’t as easy to see as that of a pine or hemlock, but I figured I’d be done in four days.

But then I got to the next tree, and I cross-dated it as best I could but it wasn’t behaving the same. It wasn’t growing there as well. We have a statistical program that helps us cross date and spot the patterns that eyes might miss. The program said we were missing five rings and I thought, "That can’t be right."

I put that core down and went through two or three other trees with no problem. But then the next tree was missing seven rings. And these weren’t old trees, either. They weren't in old age decline. They were maybe only 50 or 60 years old. I started recognizing that in 1981 the trees had a white ring, not caramel like red maple can look. That was the year of the gypsy moth defoliation event. The moth removed leaves. Without the leaves, the tree can't feed itself as well and the wood is less dense. So that's where the white ring comes from.

Once I had found that white ring as a marker ring I started realizing that in the last decade or so before I cored them the trees had just stopped growing.

I presented the info to my committee at the time. and they said, “Are they alive?” And I said, "Yeah, but they must be zombies." That’s why I was excited when I saw your tweet about zombie maples. It confirmed that somebody else had seen this through direct measurements. That’s important. It's not our only corroborating evidence. We had a technician here almost 30 years ago who cored red maples in the catskill mountains. I pulled out his cores and measured them and he has scores of missing rings in the decade before those trees were cored. The trees were still alive, but not growing at the base of the tree.

NP: An eco-physiologist on my Ph.D. committee just got fascinated by this and what it means. Are they adding growth higher up the trunk someplace? Are they reusing the old tubes for passing water and nutrients up and down the tree? Usually each new growth ring replaces the old tubes. There’s a a lot of plant physiology questions that could be looked into here. It's an interesting phenomenon.

And we don't know exactly what's causing it. It's not the run-in with gypsy moths. In surviving red oak, for instance, after the gypsy moth defoliation they were growing back like nothing had happened within three to five years. Trees can get back to normal in a few years depending on severity of the disturbance. An earthquake can knock trees back for a decade or more before they recover. Some really severe defoliation events can take a decade or more. But trees are amazingly resilient. They have to be. They can’t run from anything.

We've not found any sign of climactic stress on these trees, either. If anything, since the 1990s in the Northeast winters have gotten warmer and that’s actually less stressful. My hypothesis is that ecology is driving this. These trees are small. They're in the understory and suppressed and they’re getting beat out by much faster growing red oak trees. I’ve seen a lot of missing rings in trees since I first spotted the zombie maples. It's a lot more frequent then the literature would suggest. And I think it’s simply competition. They’re losing out to bigger trees.

NP: Who wants to die? That’s kind of a joke, but it’s kind of not. Trees have the tenacity to grow in unbelievalble conditions. A white cedar can grow for 200 years under normal conditions, but they can also take root on cliff faces and live there for 800 to 1000 years. A chestnut oak I was looking at yesterday, it grew maybe two inches in diameter in 100 years. That's incredibly slow growth. It’s not what you think of when you think of oak.

But it makes sense. In general, trees can’t improve their condition actively the way that things like beavers or alligators can. Some trees can drop needles and promote fires that kill competitors in the understory. Other trees leach out toxins that kill nearby plants. We're finding more and more plants that do have abilities like that, but they're still not as capable as animals to change or move the environment around them. So they just persist. They keep on living for another day.

Forests here in the Northeast are dense. This might just be one survival strategy where you sit in the understory for as long as you can hoping that a neighbor will fall over and give you light and space. That’s painting some very human feelings on a tree, but you get the idea. They’re programmed to survive and reproduce and being a zombie is not a bad strategy for doing that. How they persist in that state, though, that's a really interesting question.

Unfortunately, no. Tree rings have become famous and infamous lately. Dendrochronology has really exploded in the last couple decades because it’s a really good way to understand our climate past. Tree records are so good and they can inform us of so much information and tell us whether today's conditions are normal or unusual. So we core trees for so many reasons and along the way we find all these other things, like zombies. It’s fascinating. But it’s not the main focus of my work. I just learn about it enough to make my work better.

For instance, we have this 36-year-old pitch pine planted in a plantation. We found that after looking at 200 trees, 80-90% of the trees were missing the 1992 ring. We think it was another defoliation event that happened. So we’re going to take cross sections at half meter intervals up the trunk of the trees. We can’t analyze all 600 samples, but we’ll be able to look at enough to see whether those trees formed rings higher up the trunk. Maybe they formed a ring at 5 meters, even if they didn’t form at 1 meter. That will help answer that one question about zombie trees. We do know that trees are more likely to form rings higher up and missing rings become less of a problem as you move up a stem. We know this stuff, but we don’t always have the time or motivation to publish on every detail we learn. We can’t publish on everything, there's not enough time in the day.

Not really. Not if you're doing dendrochronology correctly. Remember when I was talking about cross-dating? That's the key. You have to pay attention to populations, not just individuals.

When we found the zombie maples, we were coring trees in an understory and we were looking at all of the trees. When you're looking for climate signals, you look at the trees that are most likely to capture the aspect you're trying to study. You try to isolate the signal in the environment first. So you find the trees that are less influenced by competition. We target overstory trees that are getting full sunlight and those are much less likely to drop rings like this.

Now there are missing rings even in those kind of trees, but it’s really rare for it to happen across a population. So then we core 20 trees or more in a population. This is how we control for this potential issue. We’ve collected a lot of samples from all around the world. There are times when they’re more or less prone to forming rings, but I can’t think of a single population where all the trees in the population missed a ring.

The only time I know of where all but two didn’t form a ring was in a population that experienced an insect defoliation in 1748. All the trees but two failed to produce a ring that year. But we don’t use that population for climate studies precisely because we know that the insect signal disrupted the growth.

Scientists measure trees for a wide variety of reasons. When I visited the Harvard Forest last week, I measured them as part of studying carbon sequestration by plants. But you can't just go out into the woods with any old tape measure and expect to collect some significant data.

That's because where you measure the tree matters. If you want to compare the diameters of two trees, you have to make sure you're measuring them in the same place. If you measured one tree at the wide base and the other further up the trunk, where trees usually get narrower, the comparison wouldn't mean much.

That's where diameter breast height (DBH) comes in. It's a way of standardizing the measuring process.

As the name implies, DBH is meant to be a diameter measurement of a tree trunk taken at, roughly, breast height on an adult. Of course, where exactly "adult breast height" is varies greatly from person to person. So DBH has been set to a standard height—1.4 meters in the United States.

In a research forest, you'll often see some kind of marker on the trees showing where this official "breast hight" is, so people can quickly move through the woods, taking diameter measurements, without having to measure vertically on each tree. In some cases, DBH is marked with yellow spray paint. In others, metal bands. These metal bands actually help measure diameter, too. Set with springs, the bands expand as the tree does, so all researchers have to is measure the distance between two dots on the band and see how far apart the dots have moved since last time.

Read all the Dispatches from Harvard Forest

Earlier this week, I showed you how scientists can use a simple, hand-operated tool to collect stratified core samples of mud at the bottom of a swamp. The deeper the samples go down, the older the mud is—until, eventually, you're looking at 6000-year-old muck, the remains of a lake bed that filled in with sediment and became swamp.

The core samples are narrow logs, each 50 cm long. (In all honesty, they looked like less-colorful versions of the 3 pound gummi worm I ordered for my 30th birthday party last year.) For the most part, they're some variation on the shade of brown, with occasional streaks of red and burnt umber, until you get to the very bottom. There, the samples turn grey. Put a bit in your mouth, as I was encouraged to do by Harvard Forest director David Foster, and you'll taste clay and feel grit between your teeth.

That's all well and good. But what do you do with core samples once you have them? For this installment of Dispatches From Harvard Forest I'm going to leave the woods and head into the lab, to see what happens to the parts of the Forest that scientists take home.

Step one: Make dirt cupcakes

We cut samples out of the samples. (Insert your "yo dawg, I heard you like samples" joke here.) Every 25 cm, so twice for each core, we cut off a little hunk from the side. We put the pieces into ceramic cups that had been weighed and labeled, so we'd know later where in the chain each sample had come from and what the samples weighed.

Then we baked them.

Seriously. The Marine Biological Laboratory (or MBL as it prefers to be known these days) has a great big industrial oven. The cups went in a roasting pan. The roasting pan went into the oven. Several hours later, all the liquid had been cooked off and we were left with dry samples.

Out of all the little samples, there were really just three main types. Near the top, we had a lot of crumbly black earth, studded with roots and sticks and fibers.

Further down, that petered out, and you ended up with solid lumps. The lumps had some stuff in them, but not nearly as much. By the time mud is this old, a lot of the biological material in it has decomposed. These samples looked brown when we first cut them off the mud cylinders. After baking, they turned greyish-green, mottled with brown spots.

Finally, at the very bottom, was the grey clay. After baking, I could see that the grid I'd tasted was actually mica. It made the whole sample sparkle.

We weighed the baked samples and we wrote down a short description of what they looked like. This being science, "I think this lump of dirt looks kind of bluish-green" was not considered to be an accurate description.

How do you take something subjective, like color, and bring it into the world of the objective? This looks like a job for official color charts.

The Munsell Soil Color Chart book is like Pantone for dirt. You just take your sample and match it up to one of the color chips. The number of the chip is what gets recorded. That way, other people can go back and verify (or challenge) your interpretation.

Step 3: Burn off all the carbon

Next, the samples go back in the oven and the heat gets turned way up—hot enough to burn away all the organic material. What your left with is stuff like minerals, metals, and rock. If you weigh the samples and then compare that to what they weighed after first baking, you know how much of the sample was organic material and how much wasn't.

Naturally, the results changed as you moved from the surface down. Barely any weight remained in the uppermost samples. The lowest ones had barely changed. That's the difference between soil filled with plant material, and lumps of mica-filled clay.

This is, to say the least, probably not a huge revelation. But it leads to something really cool. After the carbon was burned off, the samples looked amazing. Some were chalky moonscapes, others had turned into piles of dark red fibers.

The fibers, pictured above, are what you should be paying attention to. Because they don't really make sense. We just burned off all the carbon-based material...which should include plant fibers. So, then, what in the sam hill are those things?

According to Rich McHorney, one of my advisors in the MBL Science Journalism Fellowship, the red color is from iron oxide—rust. What you're seeing here isn't plant fibers, but a shell of rust that had formed around plant fibers that were on their way to fossilizing. We burned away the plants. But the iron oxide remained. In a way, it's a bit like the casts of bodies from Pompeii. How cool is that?

Read the rest of my Dispatches from Harvard Forest

Seventy-one feet above the Harvard Forest, you can stand on a plywood platform attached to a slightly swaying tower of metal scaffolding, and look out over miles of hemlock groves. On the ground, the trees are massive—trunks reaching up and up and up. From the top of the tower, though, the view feels a bit like hanging out in a Christmas Tree farm. All you see are the friendly, conical tops.

The Hemlock Eddy Flux Tower is one of four research towers in the Harvard Forest. Since 2001, data collection systems on the top of this tower have measured carbon dioxide, water vapor, and wind currents. These measurements are made five times every second.

Thanks to this system, we now know that even a relatively old forest like this can still capture and store a decent amount of carbon dioxide. The hemlocks around the tower are pushing 230. That's not terribly old by tree standards, but it's old for this part of North America—most of which was once clear cut. It's also old enough to challenge some previously held conventional wisdom about what kinds of forests are best for carbon sequestration. Previously, scientists thought only young forests, where the trees were still growing rapidly, did that job very well. Sites like the Hemlock Tower have shown a different story.

Also: It's rather terrifying to climb. The tower lives, it is not stationary. A network of steel cables keep it from toppling over, but you can still feel it tilting one way and then the other underneath you. And, at every landing on the stairs, there's a precarious little gap you have to step over. I took my camera with me in one hand as I made the ascent. About partway up, the filming quality takes a notable turn for the worse as I found myself clinging a bit more tightly to the hand rails. How's that for an awesome tool of science?

Video Link

I spent last weekend in the Harvard Forest, participating in hands-on science experiments as part of the Marine Biological Laboratory's science journalism fellowship. The goal was to give us an inside look at what, exactly, scientists actually do. When you're reading a peer-reviewed scientific research paper, where did all that data come from?

Sometimes, it comes from a swamp.

On Saturday, we walked into the Forest's Blackgum Swamp to take core samples out of the muck. There was no standing water in this swamp, at least not when we visited. But I wouldn't call the ground "solid", either. Instead, it was more like a moss-covered sponge. With every step, the ground beneath me would sink and smoosh. In some of the lower patches, that meant a shoe-full of water. In other spots, it was just a disconcerting sensation.

Taking core samples involves a little machine that's like a cross between a shovel and a straw. Made of heavy, solid metal, it has an extendable handle on one end. At the other, there's a hollow, cylindrical chamber that can be opened and closed by turning the handle counterclockwise. You drive the chamber into the ground, turn the handle, and then pull it back out. Once everything is back on the surface, you can open the chamber and see a perfect cylinder of earth, pulled up from below. That cylinder is removed from the chamber, wrapped in plastic wrap, labeled, and put in a long wooden box. Then you do all of that again, in 50 centimeter increments, until you hit stone. We got to about 475 centimeters—15 feet deep. By that point, you'll have collected 1000s of years of layered sediment.

This is not as easy as it sounds.

Second, as you get deeper, it becomes harder and harder to pull the coring tool back out of the hole. First off, there's the handle. Each extension adds weight and unwieldiness. Then, there's the swamp itself—water-saturated mud that forms a suction around the coring tool. By the end, it took three people to remove the tool from the ground—two to pull it out, and one to catch the comically tall handle as it emerged from the Earth. When the tool came out of the ground, it came with a gush of water, like we'd just struck oil, and indescribably hilarious squirpple-plushhh-blurp sound.

In this series of photos, you can see journalist Eric Niiler, one of my co-fellows in the program, push the coring tool into the ground and begin to take it back out. Bear in mind, this is early in the process. The handle was MUCH longer than this by the end.

In these final two shots, Harvard Forest director David Foster and scientist Rich McHorney open the cylindrical chamber and move a fresh core from the tool to a plastic wrap sheath.

Video Link

Read the rest of my series of dispatches from the Harvard Forest

I'm currently attending the Marine Biological Laboratory's 10-day science journalism fellowship. As part of that, I get to do some hands-on science experiments and get a better perspective on how the work of science is done and how data is collected. Along with five other fellows, I spent last weekend collecting A LOT of data in Massachusetts' Harvard Forest—3,500 acres of extremely well-documented wilderness.

All this week, I'll be posting some of the highlights from my trip—videos and photos that will introduce you to the Harvard Forest, how science is done in the field, and to some of the key ideas that I'm learning during my time here.

This will be the central access point for all those posts. Check back every day to see what's new.

In This Series:
Scientific Research in a Forest
How Past Land Use Affects the Current Landscape
How To: Collect 6000-year-old swamp mud
Climbing a rickety stair to the top of the forest
What's your diameter breast height?
The secret world of swamp mud

Do you see how the ground level is higher on the left-hand side of this photo? To the right of the stone wall, the ground distinctly drops by a foot or more.

That wall is more than 200 years old. It marks the border between what was once a plowed field (on the left) and grazing pasture (on the right). Today, this site is woodland—part of the Harvard Forest, the most-studied forest in the world. But for generations, this land was farmed by Jonathan Sanderson and his descendants. And, even two centuries later, you can still see the way different uses of the land changed the land.

For instance, the ground level is higher on the left because plowed fields erode more easily. This site is on a slight slope. Water runs downhill, toward the right hand corner of the photo. As it did that, it carried bits of plowed field along with it—sediment that washed up against the stone wall and stayed there. Over many years, the effect changed the level of the land.

This isn't necessarily a catastrophic thing. But it is change. I spent last weekend in the Harvard Forest, participating in science in a hands-on way as part of the Marine Biological Laboratory's science journalism fellowship. One of the things I learned during my stint in the forest: The past ain't past. History is recorded in geology and ecology as surely as it's recorded in books. Very cool stuff!

I spent Friday, Saturday, and Sunday in the Harvard Forest—the most-studied forest in the world. It's an interesting place, with a complicated history. Originally forest, it was clear-cut in the decades following European settlement. By 1830, less than 90% of this part of Massachusetts had any forest left. But that trend had already begun to reverse itself by 1850, spurred by urbanization and cheaper, more-efficient farming in the "West" (i.e., Ohio).

What is now the Harvard Forest was farmland for many years. Then it was used for tree plantations. Then it became forest again, studied first by Harvard University's forestry program in the early 20th century, and then by ecologists and other environmental scientists beginning in the 1980s. Today, these 3,500 acres are home to dozens of individual studies and long-term, interdisciplinary projects led by scientists from more than 15 universities and institutions.

This particular study, led by Dr. Jerry Melillo of the Marine Biological Laboratory, is studying the nitrogen and carbon cycles of forests, and how those cycles are affected by rising soil temperatures. They're trying to understand how climate change will affect the growth of wild plants, and how it will affect those plants' ability to absorb and store carbon dioxide. I'll get more in-depth on this study later. Right now, I thought that this site offered a really great view of what a research forest looks like—it's a chance to see detail-oriented science and wild nature interacting and overlapping.

Here's a couple more photos that will give you an idea of the kind of things you might find in a research forest.

They include electronics in odd places. This junction box delivers power that runs several sensors driven into the forest floor. Another key feature are various home-built collection systems meant to capture leaves and debris that fall off of trees. These baskets, and what's inside them, can help scientists back-calculate the volume of leaves the trees in this area grow in a given year.

Also common in a research forest: Lots and lots of signage and color-coded tags. In order for science to happen, you have to know exactly what you're looking at. When you make comparisons at the same spot over time, you have to know you're dealing with the same plants, or the same section of the woods. Signs help. You can't see it here, but the trees, themselves, are also labeled. In this particular study, every tree has a number that it wears tacked to its trunk like a little dog tag.

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