First, have someone in your group read the title of the paper, the general description, and the figure legend (below the figure). Then work together to address the questions.
The graph below is from a paper titled "First-year growth response of trees in an intact forest exposed to elevated CO2" by LN Shawna and EH DeLucia published in the journal Global Change Biology in 1999, Volume 5: Pages 609-613.
General Description: In this experiment scientists wanted to know how elevated concentrations of carbon dioxide in the atmosphere might affect tree growth. They increased CO2 concentration in the air around trees growing outside about 200 ppm (parts per million) to about 550 ppm in a 15 year old loblolly pine plantation, North Carolina in a FACE experiment (Free-Air CO2 Enrichment; see http://www.bnl.gov/face/faceProgram.asp for more information). There were 3 plots with elevated CO2 and 3 control (ambient) plots left unchanged. The graph shows average values for tree Basal Area plus/minus standard error (the vertical bars - a statistical measure of variation) of all the loblolly pine trees in the 3 experimental (dark circles) and 3 control (open circles) in 1996 and 1997. Basal area is calculated from the diameter of a tree 1.5 meters above the ground (approximately chest height). Basal area is the cross section of a tree.
Figure Legend: Percent increase over initial values (measured in April 1996) of Basal Area (BA) for loblolly pine trees growing in ambient (open circles; n = 103) and elevated (filled circles; n = 100) plots for 1996 (pretreatment) and 1997 (the first year of treatment). The date of initiation of the elevated CO2 treatment is indicated by the vertical line. Approximate dates of initiation and cessation of growth are indicated by arrows on the bottom axis. Error bars are plus or minus one standard error of the mean.
1. Interpret the Graph:
First, look at and then interpret the figure in the following two steps:
Step One: Make sure you understand all parts of the figure - such as, the x and y axes, the legends, all the words - plus the question being addressed and the design of the experiment. Then describe the findings - the data. Write your description below plus any questions you have (e.g. things you don't understand).
Step Two: Interpret the figure. Using what you know about carbon dioxide and plants, why did the scientists see this response to elevated carbon dioxide? What is the implication of this finding? (So what?) Write your interpretation and any questions you may have.
Question: In the space below, draw a cartoon showing what happens to the carbon in the CO2 molecule - how the elevated CO2 results in trees with greater cross sectional area (e.g. "fatter" trees).
2. Additional questions:
- This experiment lasted a year. Of course elevated carbon dioxide will have ecological effects lasting many, many years. What do think might happen over a longer time period (10-100 years) in this experiment? Why?
- All experiments have limitations. What questions do you have about this one that influence your interpretation of the findings?