I’ll Go on a Cross-Date if You Show Me Some Rings
By Ana Camila Gonzalez
Ever since I’ve started learning to cross-date tree core samples, I’ve learned I have a type. I prefer my tree cores to be black oaks, middle-aged, with some nice big rings to show me. Alright, fine, I can deal with some smaller rings every now and then. As long as they’re some nice marker rings.
Unfortunately, the trees don’t seem to be trying to impress me.
I was told on a fifth grade field trip that you could tell the age of a tree by chopping it down and counting from the ring on the outside, which represents the current year, to the inside ring, which represents the year it started to grow. I’m coming to learn at the Tree Ring Laboratory of Lamont-Doherty Earth Observatory that there are a few problems with that statement.
Primarily, you don’t have to chop the tree down. I learned while doing fieldwork that coring a tree does not damage it at all. More importantly however, you can’t always find the exact age of a tree by simply counting the rings backwards. One has to verify the years you assigned to each ring against other samples, and, occasionally, against known climatic or ecological events. Sometimes a ring can be missing, possibly from either a very dry year or insect defoliation that causes a lack of growth on the side of the tree you’re looking at. Sometimes a ring is there, but it’s tiny; so small you need a microscope to see it: a micro ring. And this is where cross dating comes in.
I sit down to cross date my first batch of samples, black oaks from 2003, with rings I can see without using a microscope. I use the microscope regardless, of course, because sometimes what looks like a ring from far away can actually be a false ring: an “extra” late wood growth caused by an early freeze, early warming, or some disruption to ‘normal’ seasonal weather. The microscope helps me see whether these bands have defined edges or seem to fade, and I’ll know that only the truly defined ones are rings.
I seem to be lucky, however, as none of the Black Oaks seem to have any false rings. I’m actually eager to find some missing rings and micro rings, but I don’t find any of those either; missing rings in oak are so rare that you’ll likely be able to plant your own oak forest and watch it grow to maturity before you find one. This is so easy, I think. I feel like I have it in the bag.
I finish measuring the rings on my samples and labeling them with the years I assigned hypothetically to each ring from my cross dating. Now I’m ready to run the measurements through COFECHA, a program that gives me the correlations between individual samples and finally the correlation between all of the samples. When I first run the program with every sample, I’m told something between 0.5 and 0.6 is the expected correlation for ‘good’ black oaks (in other words, there is a 50 to 60 percent chance that given the ring-width measurements on one sample, you’d be able to predict the measurements on a second sample from the same batch). I get a 0.3 correlation. What could I have possibly done wrong?
I soon find that although Black Oaks don’t usually produce missing rings, micro rings or false rings, it is still a possibility, for reasons I didn’t understand at that time. There is also the possibility of human error resulting from mounting the samples incorrectly, missing pieces of the sample after coring and so on. (Editor’s note: one of the biggest issues dating oaks is jumping from one side of a ray to another while moving down an increment core. Sometimes the rings that are aligned across this division are not!).
What I was doing up until this point was just writing down the years where I found narrow and wider rings as marker rings and trying to find a pattern with everything I wrote down. It was helpful, but I needed to learn more about cross dating to make a few problem samples correlate with the population.
First, I was told I could take a step back and get my nose off of the microscope. By holding up a problem sample to one with a good correlation, I could try and find where patterns aligned visually, and this was usually more helpful than just trying to find the patterns in a sea of numbers I had written down. Second, I was focusing too much on individual samples and not remembering that multiple cores are often taken from the same tree: before a sample can correlate well with an entire forest it is easier to make sure it correlates against the others from the same tree. Finally, I learned that some trees—the very young, the very old, and the trees that constantly get outcompeted for resources—just don’t conform: the rebels, the grumpy old men, the proud nerds. Very suppressed rings won’t correlate well with a series, and neither will very wide rings that signal a release from competition from neighboring giants. Sometimes a 0.3 or a 0.4 correlation is the best you can get for a sample, and I had to learn how to know whether to accept that or keep trying further.
That first batch took me a week and a half to finally cross-date. You should’ve seen the look on my face when I saw my first correlation in the 0.5 range.
And that was just the black oak.
I decided to continue coming to the Tree Ring Lab over winter break, and at first it was incredibly peaceful. A few days of sanding and stabilizing some pines really put me in the Christmas spirit. And then I met Baldcypress, which made me more of a Grinch.
At first, baldcypress and I were really only going to be a one-time thing. I was only told to measure three or four batches from the 80s as a side project, but after I logged all the measurements the COFECHA results were cringe-worthy. I was told I had to try my hand at cross dating the cypress.
If I thought the black oak population had trouble samples, I reconsidered. While Quercus velutina hardly ever displays missing rings, false rings or micro rings, Taxodium distichum seems to want to flaunt them. My first batch had mostly been false rings, but I also learned what a micro ring actually looked like.
I remember staring at a set of what should have been ten rings for 20 minutes, but only seeing nine. I finally asked my advisor and then watched as Neil marked a band relatively darker than its surroundings a cell wide as a ring. If any ring could be called a marker ring, it was this one. Sometimes finding a micro ring where I knew, from the chronology, that a narrower ring should be, was actually a relief. 1966, a heavy drought year for most of the Northeastern US, quickly (and morbidly) became my favorite year.
I dealt with so many false rings that I felt like I was five and my fingers were all turning green (I’m glad no one ever showed me this; I always felt like a princess). Every time I thought a sample couldn’t have any more missing rings I found more. I started thinking everything was a micro ring.
The black oak took a week and a half. I’ve gotten through three batches of baldcypress, and I’m on my fourth: I started over winter break and it is currently spring break. Of course, I’ve been working on other things as well, including a poster presentation on my black oak samples for the Northeast Natural History Conference, but it feels as if the baldcypress just doesn’t want to leave me alone.
Yes, I do have a type. I like real rings, I like big rings and I like rings that conform. In the end, however, I’ve learned more from the “problem children” than the ones that worked out like I wanted them to. I might even admit that the baldcypress has been much more rewarding to work through.
Shhh, don’t tell the black oak.
Ana Camila Gonzalez is a first-year environmental science and creative writing student at Columbia University at the Tree Ring Laboratory of Lamont-Doherty Earth Observatory. She will be blogging on the process of tree-ring analysis, from field work to scientific presentations.