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.
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 within 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.
When we core trees, everyone understands 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.
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 are 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.
Who wants to die? That’s kind of a joke, but it’s kind of not. Trees have the tenacity to grow in unbelievable 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 hope 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.
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.
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.
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 kinds 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.