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.