Changing Planet: Survival of Trees
(Teacher Instructions)
Grade Levels: 7-10
Overview:
Students investigate the role of gases in plant growth, and how proxy records, field work, and controlled
experiments assist scientists in understanding Earth processes
Materials:
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Student worksheet
Internet access
Large glass cookie jar or bottle
Seal for the jar or bottle
Dry sterilized soil
Gravel
Activated charcoal
3 or 4 moisture-tolerant houseplants
Spray bottle
Time:
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Withering Crops lesson: 2 class periods
Carbon Dioxide : Sources and Sinks lesson: 1 class period
Build a Tree lesson: 30 minutes
Ecosystem Bottle lesson: 1 class period for set up; multiple weeks for data collection; 1 class
period for closure
Learning Outcomes:
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Students will identify the reactants and products of photosynthesis and cellular respiration
Students will understand how some field data can be used to infer about past environments
Students will observe and record changes over time in a controlled environment and draw
conclusions about the processes taking place
Background Information:
Trees contain some of nature's most accurate evidence of the past. Their growth layers, appearing as
rings in the cross section of the tree trunk, record evidence of floods, droughts, insect attacks, lightning
strikes, earthquakes and even changes of carbon dioxide levels that occurred during the lifespan of the
tree. Subtle changes in the thickness of the rings over time indicate changes in length of, or water
availability during, the growing season.
Each year, a tree adds to its girth, with the new growth being called a tree ring. Tree growth
depends upon local environmental conditions. In some areas the limiting factor for growth is water
availability, in other areas (especially at high latitudes) it is the length of the growing season. In areas
where water is limited and the amount of water varies from year to year, scientists can use tree-ring
patterns to reconstruct regional patterns of drought. In areas where the length of the growing season is the
limiting factor, the thickness of tree rings can indicate when growing seasons were longer (during warmer
times) and when growing seasons were shorter (cooler times).
The study of the growth of tree rings is known as dendrochronology. The study of the relationship
between climate and tree growth in an effort to reconstruct past climates is known as dendroclimatology.
As can be seen in this image, tree ring consists of
two layers: (1) A light colored layer which grows in the
spring and (2) A dark colored layer which forms in late
summer.
At locations where tree growth is limited by water
availability, trees will produce wider rings during wet and
cool years than during hot and dry years. A severe winter
can cause narrower rings too. If the rings are a consistent
width throughout the tree, the climate was the same year
after year. By counting the rings of a tree, we can pretty
Image in public domain
accurately determine the age and health of the tree and the
growing season of each year.
Modern dendrochronologists seldom cut down a tree to analyze its rings. Instead, core samples
are extracted using a borer that's screwed into the tree and pulled out, bringing with it a straw-size sample
of wood about 4 millimeters in diameter. The hole in the tree is then sealed to prevent disease.
Computer analysis and other methods have allowed scientists to better understand certain largescale climatic changes that have occurred in past centuries. These methods also make highly localized
analyses possible. For example, archaeologists use tree rings to date timber from log cabins and Native
American pueblos by matching the rings from the cut timbers of the homes to rings in very old trees
nearby. Matching these patterns can show the year a tree was cut, thus revealing the age of a dwelling.
Tree ring data is only collected outside of the tropics. Trees in temperate latitudes have annual
spurts of growth in the summer and periods of dormancy in the winter, which creates the distinctive
pattern of light and dark bands. Tropical trees grow year-round, and thus do not have the alternating dark
and light band pattern that allows us to read tree ring records.
Tree ring records can be combined to create climate records that span a timeframe longer than the
life of a single tree. For example, the data from a living, 200-year old tree could be combined with a data
from wood from a tree that was felled 150 years ago (after living a couple of centuries) to produce a
composite dataset spanning several hundred years.
Trees, alive or dead, are not the only source of wood used to construct such extended records.
Beams from old buildings or ruins, samples from wooden frames of old paintings, and slivers from
violins have all been used to add wood samples from trees long dead to climate chronologies. In some
cases, tree rings enshrined in petrified wood even give us some insights into climate conditions in truly
ancient times.
The oldest trees on Earth, the bristlecone pines of western North America, can live for more than
4,000 years. Dead bristlecone trunks, often well-preserved in the dry terrain upon which bristlecones
grow, can be as much as 9,000 years old.
The proxy climate record preserved by tree ring data spans a period of about 9,000 years. The
resolution of tree ring data is one year. Tree ring records are amongst the highest resolution proxy climate
data types, but they also have one of the shortest time spans over which they apply as compared to other
proxies (like ice cores)
Directions:
1. For background information on how climate change is affecting the survival of trees around the
world, watch the video Changing Planet: Survival of Trees (available on the website). Also
explore these topics on the Windows to the Universe website at the links listed below.
2. Gather the materials needed for each part of the lesson you choose to do, and print out the student
worksheet. Parts of this lesson may be omitted based on the prior knowledge and grade levels of
the students.
3. The focus of this program is on the effects of combined rise in global carbon dioxide levels and
rise in temperature on trees. Dr. Ward uses ice core data along with data from trees preserved in
tar pits (these trees date from the last ice age about 40,000 years ago). She saw that certain plants
can survive in combined low carbon dioxide levels and colder temperatures typical of that time,
and she is using those records as a baseline to project the impact of the increase in these two
parameters on tree growth in the future. Students should have a basic understanding of the
mechanisms of photosynthesis since that is how plants use carbon dioxide. If a review in this
concept is needed, you may want to check out the Windows to the Universe lesson called
Changing Planet: Withering Plants - Stressing Over Lost Water.
4. After students understand the role of photosynthesis in plant growth, introduce the concept of
dendroclimatology, and how trees "record" past climates in their rings. Visit the links below for
additional information on this topic, and have students Build a Tree (this is an online interactive
tool that models the effects of temperature and moisture on tree ring growth). An important point
made in this activity is that other variables besides temperature and precipitation can affect tree
ring growth. Brainstorm other factors that may influence tree growth and ask students to think of
ways they could measure those factors. Students should be thinking about factors such available
carbon dioxide, available sunlight, and available nutrients, as well as events that could impact
growth such as insect infestation, wildfire, or local competition for light or nutrients. Coming up
with ways to measure these impacts may be challenging, and students will probably suggest
controlled laboratory experiments to test some of these factors (this is similar to what Dr. Ward is
doing to test her hypothesis about the effects of rising carbon dioxide and rising temperatures on
plants).
5. This episode lends itself well to exposing students to a variety of scientific practices - from the
determination of proxy records based on evidence gathered in the field to controlled laboratory
experiments to the creation of models of the future. Students learn about the challenges of
applying experimental results gathered from a laboratory to what actually takes place in the field.
As mentioned above, there are numerous abiotic and biotic factors that affect the growth of
plants, not all of which can be controlled in the laboratory simultaneously to truly model what
takes place in a natural ecosystem. To foster scientific thinking in students, ask them to how they
can test Dr. Ward's hypothesis in the classroom. More than likely students will be challenged to
come up with a controlled experiment given that there are so many factors that may influence
plant growth. List all the factors that may influence plant growth and development.
Show students the bottle or jar that will be used to create a model ecosystem. Ask the students to
create the ecosystem with you. Choose a jar or bottle big enough to comfortably hold 3 or 4
moisture-tolerant plants while allowing room for them to grow. The plants should not touch the
walls of the bottle. Place a mixture of activated charcoal and gravel at the base of the bottle; next
add the soil to the bottle to a depth that will comfortably hold the root system of the plants as they
grow. Spray the soil with enough water to dampen the soil but not make it wet. Add the plants to
the bottle using a long stick to assist with the effort if the neck of the jar or bottle is narrow. Spray
the plants to moisten them. Add a few pieces of bark or twigs to the surface, and then seal the
bottle with a cap or lid and place the bottle in indirect sunlight. Monitor the plants over the next
few days to ensure they are healthy. You may need to replace one or two plants or add a little
water as your ecosystem gets up and running.
6. Ask students to compare their model ecosystem with a natural ecosystem looking for responses
related to open and closed systems, and the recycling of nutrients, gases, and water. Ask them to
take out their notebook to create a journal of changes in their ecosystem over time. Ask the
students for ideas on what factors could be monitored (number of leaves - healthy, dying, or dead;
plant width and height; moisture; temperature inside the jar, etc.), and every day or two have
students record their observations. Have students make observations over a period of a couple of
months. Lead students into a discussion about the processes related to the exchange of gases
within the bottle along with the recycling of water. Ask students how they can confirm that an
exchange of gases is taking place, and they may suggest that because the plants look healthy they
must be getting carbon dioxide from the air in the bottle. Ask students to hypothesize what would
happen if the quantity of carbon dioxide in the bottle were to increase. To increase the carbon
dioxide levels in the bottle, try lowering a small container (test tube or flask) of dilute
hydrochloric acid and marble chips into the ecosystem bottle (allow the reaction taking place to
create carbon dioxide for about 30 minutes before removing it). Have students continue their
observations for another couple of weeks. If you have access to a carbon dioxide probe, the
quantity of carbon dioxide can be measured over time and correlated to the collected data.
7. At the end of the demonstration period direct students to the student worksheet to answer
questions related to the strengths and weaknesses of their model ecosystem. Scientists realize the
limitations of laboratory experiments when studying natural ecosystems and create "Free Air
Carbon Dioxide Enrichment Study" (FACES) ecosystems to study the response of trees to
increased levels of carbon dioxide. At these FACES sites around the world an excessive amount
of carbon dioxide is forced into plots of trees and then the trees are monitored for changes in
phenology, biomass, and numerous other parameters. Visit the link below to learn more about
these studies.
ASSESSMENT:
Assess students' notebooks/journals for their conclusions about plant growth and photosynthesis, and be
sure to challenge any thoughts that are not correct by reminding students that observations of the
ecosystem bottle represent the reactions taking place during photosynthesis. Also refer students to their
observations during the Carbon Dioxide: Sources and Sinks lesson. Note the quality of the students'
journals and be sure that they include dates and times with each entry, along with detailed observations.
When developing a test for this topic, be sure to include test items about the science practices employed
in this lesson.
LAB SAFETY:
Always use safe laboratory practices, especially when using chemicals. Follow the supplier's
recommendations for the safe disposal of all used chemicals.
Adapted by NESTA/Windows to the Universe team members Missy Holzer, Jennifer Bergman, and
Roberta Johnson from various Windows to the Universe activities