A graph depicting ring width over time for over one hundred trees
When I started this summer, I treated gathering my data as the end goal. Like the “X” marking the treasure on a map, I assumed that if I could just get all the data - collect all the samples, measure all the ring widths, cross-date all the trees - that everything else would fall into place.
I was wrong, of course. Half the battle of research, it seems, is knowing what to do with the data once you have it. I learned that not all of it was usable, like all the ring widths from before 2000, while some was also irrelevant to the story I was trying to tell, like tree heights. Then I was still faced with the struggle of how to analyze the rest of the data. There’s linear regressions, and generalized additive models, and generalized additive mixed models, and each one outputs its own mix of p-values, t-scores, and coefficients. To be honest, I’m still a little confused by it all. It feels like I could analyze the data a hundred different ways and get a hundred different results and it can be overwhelming at times, which is a reminder that though this summer has taught me a lot about research, I still have even more to learn.
Through it all, I was lucky to have so many wonderful people supporting me. I want to thank Dr. Christy Rollinson for teaching me all that I know about forest ecology, Dr. Silvia Alvarez Clare for helping me write my first research paper, and Christine Carrier for creating such an amazing fellowship program. I also want to thank the Center for Tree Science and Morton Salt, Inc. for funding this summer. Lastly, I want to thank all of you lovely readers who took the time to share this journey with me though my blog posts.
While measuring and dating (or matching a year to each tree ring) the oak cores I collected earlier in the summer, I noticed an abundance of ring anomalies that aren’t supposed to be common in oaks.
In my last blog post, I described the process by which a tree forms tree rings; however, it’s not quite as simple as that. Sometimes trees create extra rings, commonly known as “false rings.” This happens when the tree stops growing in the middle of the growing season and then starts again, leaving two rings for one year.
Besides being annoying when trying to date (or assign a year to each ring), these false rings also pose an interesting question. Most of the rings were found in trees with fire in their history, however it would take more research to verify the exact locations and timings of the prescribed burns in comparison to the trees. Is it possible that fire caused these false rings? As much as I would love to know, this is one of the questions that I just don’t have time to answer.
I was warned near the start of this internship to avoid chasing data down rabbit holes. While it’s important to be curious and follow where the data leads, there isn’t time to explore every unanswered question in forest ecology in the space of one 10 week internship - as much as I wish there was. Maybe the mystery of the false rings will be the project of some future research fellow, until then I’ll just have to wonder.
A portion of a black walnut tree core as seen through the lens of a microscope
I’ve spent the a majority of my time since my last blog peering down a microscope at tree rings, so I figured now was as good a time as any to share what I’ve learned.
For starters, trees grow out, not up. A branch that was five feet off the ground in 1998 will still be five feet off the ground tomorrow. Furthermore, trees grow from the outside, laying down layers of new wood just underneath the bark; rather than from the center. Especially in places like the midwest that have distinct seasons, trees alternate between periods of growth in the summer and dormancy in the winter. The harsh lines formed between growing seasons are what cause the visual rings.
Since trees create a new ring every year, by counting backwards from the bark you can determine how old a tree is! I wouldn’t be surprised if many of you already knew this, but this isn’t all tree rings can tell us.
The width of a tree ring can also tell us a lot about how the tree grew in that particular year. Large rings mean that tree was able to grow a lot; this could mean lots of sunlight, copious amounts of rain, or appropriate pruning. Similarly, small rings are from years that the tree didn’t do as well, often from injury or lack of rain. Most of the trees I’ve measured have had distinctly small rings in 2012 due to the drought.
Thus, tree rings allow scientists to look into the history of both a tree and it’s environment. In my particular case, I’m comparing the tree rings of trees from parts of the forest under different prescribed burning treatments in order to see if the varying fire regimes have any effect on tree growth. Fire is an important management tool in midwest oak forests so I’m excited to know if it is actually as good for the trees as we think is.
I found a slug when we were out doing fieldwork in the East Woods
These past few weeks, dear readers, have been an exercise in persistence. I’ve spent the better part of my time recently doing intensely repetitive work.
In my previous blog post I described the permanent plots in the East Woods that I am studying this summer. In addition to those four plots, I also set up six smaller temporary plots. Across the 3,100 square meter area that these plots cover, I have over 100 trees to collect data from!
These data are in the form of tree cores which are collected using a special hollow-hand drill called an increment borer. The idea is similar to stabbing a straw into a watermelon and retrieving the cylinder of fruit from inside the straw.
The tree cores are stored in paper straws for transport from the field to the lab. Once the cores are dry they need to be mounted on a block of wood to stabilize them. Each core then needs to be sanded, starting with a very coarse grit of 80 and progressively working towards finer grits, with the finest being 1500. When this has been done, a microscope can be used to measure the tree rings to an accuracy of .001 millimeters!
Since I collected at least two cores from each tree, I had a total of 225 tree cores, all which needed to be cored, mounted, sanded, dated, and measured. Coring alone took eight days and that was with the help of six other people. There were times when the work seemed endless, but despite that it is definitely worth it and I can't wait to see where the data leads.
An increment borer in the process of coring a tree
All of the tree cores being stored in straws to dry out
These cores have been mounted and sanded so the rings can be easily seen
I’m Sierra Lopezalles, one of the research fellows working at The Morton Arboretum’s Center for Tree Science this summer. My project is to study the effects of fire and prescribed burning on trees in the Arboretum’s East Woods under the mentorship ofDr. Christy Rollinson.
For me, these first two weeks have been full of surprises and new experiences.
Back at Caltech, where I’m a rising sophomore studying biology, I worked in a microbiology lab studying bacteria. Even though forest ecology is just another branch of biology, working with trees could not be more different. Where a new plate of bacteria can be grown in the span of a few hours, most of the trees I’m working with are many years older than I am.
Perhaps the most foreign part though, was how much physical effort forest ecology requires. Working with bacteria could be exhausting at times but it didn’t get much more strenuous than frantically searching the lab for a bottle of chemicals that someone had put on the wrong shelf. On the other hand, in the past two weeks I’ve spent about twenty hours in the forest, hiking through dense undergrowth to collect data.
A tree identification tag nailed to a Black Cherry
I started off the summer setting up plots in the forest with the help of one of the other fellows. After locating the center of the plot with help of a GPS, we used a compass and tape measure to lay out a 20 meter grid. Each tree inside this grid was measured and tagged with a unique identification number.
The data sheet we used to record tree heights
We did this for a total of four plots, each one in a section of the forest under a different prescribed fire regime. One of plots is burned annually, while one of them has never been burned before. This will allow me to compare how the trees respond when exposed to different frequencies of prescribed burning.
I am very excited to see what the data will reveal about how trees interact with their environment. This internship has reminded me, once again, of how fascinating biology can be (as well as how much I loathe mosquitoes).