Attack of the zombie maples

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.

Maggie Koerth-Baker: When did you first encounter the zombie maples? Was this something you were looking for, or just something you found in the course of other work?

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.

MKB: How could you tell the trees weren't growing just by looking at the core samples?

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.

