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Asking the hard questions… about extreme events and tree response


Friday, July 7, 2017, by Caitlin Keady
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[Photo of tree core showing the Gypsy Moth defoliation of 1981. Photo by Neil Pederson]Who remembers last year’s drought? Well, the trees sure do. Imagine the beginning of spring, when leaves are starting to return and wildflowers are blooming. Then picture a sudden  overnight frost. All those plants and trees that were kicking off their growing season likely went into shock and halted growth. Even though the frost only lasted one night, it may have lasting effects throughout the growing season. Pretty grim, I know. Extreme events such as drought and late spring frosts can cause disturbances among tree populations, resulting in abnormally narrow or wide rings, where each ring represents one year of growth. Droughts behave differently, but we wonder if the timing of these events could have similarly lasting effects throughout the growing season. Maybe drought in a particular week in early June could result in a narrow ring for that year?

[Learning how to core trees. Photo by Neil Pederson]One approach to tackle this question is to ‘ask’ the trees what makes them stressed. In other words, what was the climate doing during extreme narrow and wide ring years? Perhaps some drought years were hotter than others or maybe winter temperatures had something to do with it. To answer my questions, I am using R, a programming language designed for statistical analysis. I have daily data for temperature and precipitation collected right here at Harvard Forest, and annual tree ring data from Harvard Forest’s Lyford plots. If all goes well, I will expand this study to a region in upstate New York and compare my results.

Since I have access to daily climate data, I can be pretty specific with my climate response analysis, which is pretty exciting if you ask me. To hone in on time periods where climate is crucial for growth, I have created some pretty non-traditional climate variables. These include  the timing of the coldest week in winter, the frequency of rainless days in summer, the number of days between the first day warm enough for growth and the last frost, and many more. These variables stray pretty far from variables used in most studies like summer precipitation and average spring temperature, which I will also include in my project. Because I am using such odd variables and because research like this hasn’t been done in this area before, I am not exactly  sure what to expect.

[Trails around Harvard Forest. Photo by Caitlin Keady]Life at the forest is pretty nice too. There are plenty of trails for running and biking, which is important for me since my job is computer based and since the food here is delicious so I’ve  been eating lots of it! I’m a big hiking fan and the White Mountains in New Hampshire are relatively close so I’ve been taking advantage of that as well. I’ve also been given a few opportunities to try out field work and learn how to date trees to understand where my data come from, so I have learned  a lot outside of my project as well. The weekly seminars or career panels have been very helpful for thinking about post grad options. Although I’m only halfway through the summer, I have already learned a bunch, especially about R which I had no experience with before I arrived and I still have a few weekends of outdoor fun ahead of me.

Caitlin Keady currently attends Bates College in Lewiston, ME and is studying Math and Environmental Studies.

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