How do trees use oxygen isotopes to record climate?

I was sitting at the computer cross-dating cores I scanned into Windendro and that’s when it all went downhill. The files weren’t saved as .tiff and I lost all the chronologies I spent weeks building. I had never considered myself a Dendrochronologist, but there I was, going through what every Dendrochronologist has experienced, having to re-do chronologies. It was a good thing the cores weren’t very long because I’m pretty sure I would have quit on the spot if they were. But how did I get to this point?

It all started on a nice spring day in Montana, well as nice of a spring day as you can get. I decided that I wanted to continue going to school to get my PhD studying trees, but I clearly didn’t know what I was getting myself into (in a good way). I got accepted into an amazing lab at Montana State University studying plant physiology. However, my advisor decided that it was time to transfer from Montana State to The University of Arizona. So, I packed my things up and found myself in the desert, the perfect place to study plant physiology.

Earlier I mentioned that I never considered myself a Dendrochronologist. Well, that is because I’m really a plant physiologist. So, how did I start dabbling in Dendrochronology? Plant physiology and Dendrochronology really aren’t too far apart. My research consists of tracking the stable isotopes of oxygen and hydrogen from the roots of trees to the leaves and back down to the cellulose that builds wood. Since the stable isotopic composition of oxygen in wood cellulose can help us reconstruct temperature, relative humidity, precipitation, and even hurricane activity, it is important to understand the tree physiological response to climate to improve our modeling ability, and ultimately our confidence in these models, under different contexts.

Okay, so you might be wondering what I actually do to help improve models that dendrochronologist can use. Well, I study conifers in the Rocky Mountains to the Colorado Plateau, as well as a variety of tropical trees found in the Sonoran Desert (in the Biosphere 2 rainforest, of course). I start by understanding how these trees use water, where they get their water from in the soil, what types of precipitation (i.e., snowmelt, monsoon rains, etc.) they use, if their water source changes throughout the growing season, that kind of stuff. Then, I look at how the stable isotopes of that water can change at the leaf level. Relative humidity plays a huge role in the changes in the oxygen and hydrogen stable isotopes at the leaf level. Because relative humidity can impact those stable isotopes, that signal gets incorporated into the sugars that are produced from photosynthesis. Those sugars then make their way to the site of cellulose formation. But before they turn into cellulose, some of the oxygens can be exchanged with the source water the trees are using, washing away some of that relative humidity signal. So, it is important to understand how different species of trees, even trees of the same species growing in different locations, are responding to the environment to better understand how these stable isotopes can be altered before they are solidified in wood cellulose.

So, I might not technically be a Dendrochronologist but I have branched out (pun credit: Brandon Strange) into the world of tree rings and have grown (another pun...I know) to appreciate those annual rings!