2. Effects of elevated CO2 on trees
Notes to faculty
Outcomes: Students better understand “where the CO2 goes” in a plant (in this
case, trees) – specifically, how elevated concentrations lead to enhanced plant
growth. Students should also improve their figure interpretation skills plus better
recognize ecosystem level effects of rising CO2 concentrations. Additional
questions should give them better appreciation for the uncertainty of longer term
effects of CO2 enrichment and limitations of scientific research.
What students do: They work in groups to first describe and then to interpret a
figure from one of the Free-Air CO2 Enrichments (FACE – URL below) on loblolly
pine. The figure shows a clear effect of elevated CO2 on tree basal area. An
additional set of questions asks them to consider longer term effects of increased
CO2 and questions they have about the experiment.
What to pay particular attention to:
o The “Step One-Two” approach is meant to slow students down and help
them do what comes automatically to you when you interpret figures.
Some students freeze when presented with a graph like this and think that
they should understand it right away. Others jump too quickly to
interpretation without understanding the design, etc. If this is the first time
students have used this approach, you will have to explain the difference
between description and interpretation (and why it’s important).
o For many students the link between higher concentrations of CO 2 and
plant growth (here, greater tree diameter/cross sectional area) may be
very unclear. Therefore in discussion, you could ask a student to “follow
the carbon” from the atmosphere surrounding the tree to new growth in
the vascular cambium. A drawing on the board, overhead, etc. will help
students visualize this. There is much students will not know and the many
details is an issue since this is not a botany course. Some photos of
stomata and cross sections of trees will be helpful.
o Make sure students understand how tree cross sectional area is
calculated from diameter (DBH).
Logistics:
o You could do this in two stages with groups first working on the figure
interpretation (“Step One – Step Two” approach) and then one of the
groups presenting the two steps followed by discussion. After this,
students could go back to their groups to address the second set of
questions.
o If you have less time, the figure could simply be the basis for a discussion
about ecosystem-level effects of CO2, what variation means in ecological
studies, short vs. long term experiments, and/or the FACE project more
generally (however consider “what to pay attention to”). FACE is a huge
project and there are many FACE photos online you could use in a power
point presentation. Students will be impressed with the huge scale and
expense of these investigations. Longer term data are also available
online.
o Information specific to this exercise: need for FACE project, the difficulty of
extrapolation from short-term experiments to long term predictions;
broader application of single location ecology experiments; pathway of
carbon in trees to wood production in the cambium layer).
o Units & Quantitative Skills: Figures as “language” that students must lean.
Representation of variation (+/- SE designation)
HIDDEN CURRICULUM
• Principles: Conservation of Matter – tracking “where the C goes” (in a
tree when carbon dioxide levels increase).
• Processes: Generation (photosynthesis
• Scale & Time: Effect of ecosystem level processes (rising carbon dioxide
globally) on a specific organism (in this case trees); tracking C at molecular and
organismal scales; long-term effects of elevated carbon dioxide.
• Forms & Representations: Ecosystem and global matter flow
Student Directions
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:
o 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?
o All experiments have limitations. What questions do you have about this
one that influence your interpretation of the findings?