The Growth Patterns of Aspens
I've grown up in Colorado, a place known not only for its mountains and skiing, but also for its substantial groves of quaking aspens. For most of my early childhood I had a small grove of my own in the front yard, which grew so quickly and efficiently that we had to remove it for fear that it would crowd out the neat rosebushes and pruned chokecherry trees. My interest in the growth patterns of aspen trees probably started there, though I did not yet realize it.
This particular research project started last year, when I was in seventh grade. Our class was on a two-day-long camping trip, with the assignment to choose an experiment, formulate a hypothesis, and conduct an experiment that either proved or disproved the hypothesis. We asked the question: Is an aspen's diameter affected by its distance from the grove's center? Knowing that a grove is all one organism and that it spreads using "shoots" from its root system (as well as by seeding), we believed that it would make sense for the grove to spread outward, with the oldest and thickest trees near the center and the younger, skinnier trees close to the perimeter. These assumptions lead us to our hypothesis that the farther away from the center of the aspen grove a tree stands, the smaller its diameter will be.
After we formulated our hypothesis, we discussed various ways to go about collecting our data. We agreed on the method: first, mark the perimeter of the grove with flags, then find the northernmost, southernmost, easternmost, and westernmost points of the border. We then measured 30 meters inward from each compass point and marked those points with flags. Within the border of new flags, we would choose the tree with the largest diameter (the eldest tree). That tree would be considered the center of the grove. From this hypothetical center, we would run a string directly north, south, east, and west, marking any tree within one meter on either side of the string with another flag. We would then measure each marked tree and record its distance from the center of the grove, as well as its diameter. (The data was recorded separately for each direction, and was then analyzed back at school.)
We were camped in Golden Gate Canyon State Park and went on a hike to find a grove to study. It was a clear, sunny fall day, and we set out with hopes of collecting the data needed to prove our hypothesis. We stopped a few times along the trail to explore aspen groves, but most of them were oddly shaped, or divided by a road, stream, or path. We wanted a more circular grove so that the number of data points we collected would be close to the same for each direction. We had walked a little more than a mile when we saw the ideal grove. The leaves of all the trees were the same color, which our teacher Susie Steele told us was a sign that the trees belonged to the same organism. Its perimeter shape seemed circular, and there didn't seem to be any complications involved with pathways or streams cutting through it. Out toward the edge of the grove the smaller trees were recovering from their first time being gnawed on by elk. Many people, even those who have lived in Colorado for years, do not know that the black scars that distinguish older aspen trees are caused by hungry elk in the wintertime. The trees toward the center of the grove had fewer new scars on their trunks because most of their bark was already scarred and ripped, torn off in the strips that indicate elk chewing, but also with unique eye markings where their branches had been broken off. The eerie eyes watched us warily as we explored. The dominant grasses that were intertwined among the aspens crunched under our feet, and the canopy of gold watched our movement from above. We decided that this was the perfect site to collect our data.
First, we wrote down our site description -the plant and animal life, the geology of the area, the weather, and other miscellaneous observations; then we broke up into two groups to collect our data. One group measured north and west, the other south and east. Several people from each group measured the trees, while one person recorded the data shouted out by the other group members. When all of the trees had been measured, we gathered and copied all of the data from each direction into our field journals. When we got back to school, another one of our teachers, Lynnette DeSchepper, taught us how to analyze our data using Microsoft Excel. We used a regression analysis to determine whether the data points had a relationship. She told us that there were two outcomes—we could accept the null hypothesis, or we could reject it. Accepting the null hypothesis means accepting that your hypothesis is incorrect. Entering our data (separated by the four directions) into Excel and running the regression analysis-with a 5 percent chance for error (the standard for scientific research) was our first step. We waited with anticipation. However, the results were disappointing. In each direction but north, the data showed that our hypothesis was incorrect. However, we were not convinced that a true conclusion could be reached from so few data points. We ended the experiment by deciding as a group that we needed more data, and until that data was collected, the results were inconclusive. That was where the experiment stopped until the month of October 2003.
I was excited when Susie approached me about continuing the research project. I believed that with more data points we could make a definite conclusion regarding the spread of aspen trees. We quickly set up field trips to gather data, and we set out on the first excursion with both our spirits and our expectations high.
On our first trip we went to White Ranch Park, about 10 miles away from Golden Gate Canyon State Park. The grove we chose to measure was in a long ravine and seemed to have a less definite center than our first grove. We found the center according to our method and began to measure the diameters of the trees. The temperature had dropped the night before, and we found ourselves driving on ice and working in pea-soup fog. We could barely see and had trouble measuring the distance from the center of the grove to the outer trees because of the lack of visibility. We collected all of our data, wrote our site description, and headed back to school.
We went back to Golden Gate Canyon State Park for the second field trip. We measured two sites that day. The first grove was also in a ravine, but the ravine was shallow and the grove was quite a bit rounder than the grove at White Ranch. We measured and found the center, then started to
measure the distances and diameters. When it came time to measure north, we found that there were only two trees that would fall on the line. As a consequence, we moved our line to 18 degrees north instead of 0 degrees north. This line provided many more trees to gather data from. We wrote up our site description and moved on to our final grove. That final grove was located on a steep mountainside, and seemed to be nearly symmetrical. We had just started to measure when we saw a huge cloud in the distance. We expected it to reach us in about half an hour, but as we watched, it rolled down the mountainside like lightning. In about 15 seconds we were engulfed in swirling snow. We hurriedly finished, wrote up our site description, and returned to school.
Once back at school, I looked over our data to get a feel for it. It didn't take very long for us to realize that it would be difficult to dissect the data without the help of Microsoft Excel. Lynnette again offered to help. She showed me how to do the regression and analyze the data that we had collected using Excel. It took time to analyze each direction from each site. We kept getting the same answers. With a few exceptions, the regression analysis showed that there was no clear relationship between the distance of a tree from the center of the grove and the diameter of the tree. We were going to leave it at that, but we kept seeing a negative number in the t-statistic box (the t-statistic, when looked at in relation to the degrees of freedom and the t-critical value, tells you whether the data is significant).
We were curious about the negative t-statistic, so Lynette set up a meeting with her college statistics professor, Dr. Fogleman, so that we could talk to him about it. When we went to the University of Denver campus, I had hoped he would tell us that the negative t-statistic changed all of the data, and that it really did support the hypothesis, though I doubted this would ever happen.
The professor told us that the negative t-statistic did not, in fact, change the data; it just showed a different slope of line. I was very disappointed after I heard that, but my excitement came back immediately when he suggested that we forget about the directions and just run one regression analysis on all of the data points together. He said that a few outliers were probably messing up the data in the smaller quantities. It made a lot of sense-outliers wouldn't matter as much in a large quantity of data as they would in a small one. After returning to school, we compiled the data into two columns, one for the distances from the center of various groves, the other for the diameters of the tree. We ran the regression with a standard 5 percent possible error factor, and found that not only was the data relevant, but it was relevant up to a 1 percent possible error factor. Even the data from our first trip, which we had thought was null, turned out to be significant when looked at in this new way.
Though I believe that the conclusion I came to is correct, there are still many variables that could have changed the data in some way. Variables such as the slope of the hill, the sources of water and sunlight, soil conditions, or human and wildlife impact could have skewed the data one way or other. Aspens shoot more roots toward sources of water, therefore sending up more sprouts in that direction. This could mean that the "center" we found might not be the first tree of the grove after all. Also, one of the only ways to tell the difference between groves is by leaf color in the fall. We measured the last three groves in the wintertime, so it is possible that one of the groves we measured might not actually be a single grove after all. Soil conditions and human development could stunt a grove on one side, forcing it to expand in the other direction. Given more time and resources, I would definitely conduct an experiment that measures more groves and takes more variables into consideration.
Now when I go up into the mountains in the fall, and I see the aspen trees gently swaying to some unheard melody written by the wind, I won't think just of how beautiful they are. I will see beyond the soil to the mass of compact roots; beyond the bark to the flowing sap of life; and beyond the present grove to the grove as it grows and expands into the future.
Mutel, Cornelia F., and John C. Emerick. From Grassland to Glacier. Boulder, CO: Johnson Books, 1984 and 1992.
DeSchepper, Lynnette. Interviewed by Elsie Baum. Denver, Colorado. 20, November, 2003.
Dr. Fogleman. Interviewed by Elsie Baum. Denver, Colorado, 4 December 2003.
Steele, Susie. Interviewed by Patti McKinnel's class. Golden Gate Canyon State Park, Colorado. 26, September, 2002.
Aspen. Encyclopedia Britannica. Retrieved December 10, 2003, from Encyclopedia Britannica Online. http://www.search.eb.com/ eb/ article?eu=10018
Baranski, Michael J. " Aspen " World Book Online Reference Center, http://www.worldbookonline.com/ ar?/na/ar/ co/ar033700.htm, December 11, 2003
More About This Resource...
This winning entry in the Museum's Young Naturalist Awards 2004 investigates the growth patterns of aspens. Elsie's narrative essay (with photographs, illustrations, and references) discusses:
- how her research project started last year with a class project, that sought to answer the question: Is an aspen's diameter affected by its distance from the grove's center?
- how she resumed the project a year later with the goal of finding more data points concerning the spread of aspen trees
- how a consultation with a statistics professor gave her the needed insight to understand how outliers were messing up the data
Less than 1 period
Supplement a study of biology or statistics with an activity drawn from this winning student essay.
- Send students to this online article, or print copies of the essay for them to read.
- Divide the class into small groups, and have them design an experiment that would measure more groves and take more variables into consideration.
OriginYoung Naturalist Awards
SubtopicTools and Methods