Changing Planet: Survival of Trees
Students investigate the role of gases in plant growth, and how proxy records, field work, and controlled experiments assist scientists in understanding Earth processes
Adapted by NESTA/Windows to the Universe team members Missy Holzer, Jennifer Bergman, and Roberta Johnson from various Windows to the Universe activities
Student Learning Outcomes:
Demonstrations, interactive Internet investigation, data collection and analysis
- For background information on how climate change is affecting the survival of trees around the world watch Changing Planet: Survival of Trees. Also explore these topics on the Windows to the Universe website at the links listed below.
- 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.
- 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. To assist students in understanding the process even further, consider using Part 3 and Part 4 of Carbon Dioxide: Sources and Sinks where students use a chemical indicator (BTB) to detect the presence of carbon dioxide.
- 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).
- 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.
- 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.
- 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.
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.
Always use safe laboratory practices, especially when using chemicals. Follow the supplier's recommendations for the safe disposal of all used chemicals.
Do not re-use materials from the ecosystem bottle, although if plants are healthy (they may last a couple of years in their sealed bottle), replant them into larger pots and enjoy them. If the other lessons are used along with the ecosystem bottle, be sure to follow the recommended clean-up procedures provided with those lessons.
- Consider using Trees: Recorders of Climate Change to identify seasonal and annual growth in a cross section of a tree; understand that thickness of a tree ring is affected by environmental conditions; understand that evidence of past climates is recorded in series of tree rings; learn to interpret past climate conditions from tree ring thickness; and collect and analyze tree ring data, testing a hypothesis and drawing conclusions.
- Also consider using Paleoclimates and Pollen where students analyze a model of pollen grains in soil samples from Washington or Colorado to determine the type and amount of "pollen" in the samples, the type of vegetation and age of their samples, and conclude the likely climate at the time the pollen was shed.
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:
A light colored layer which grows in the spring
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 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 large-scale 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).
RELATED SECTIONS OF THE WINDOWS TO THE UNIVERSE WEBSITE:
- Climates of the Past
- Effects of Climate Change Today
- Geologic Time: Our Earth Is Old!
- Growth Spurt in Tree Rings Prompts Questions About Climate Change News story originally written on November 13, 2009
- When Nature Strikes: Wildfires
- Wildfires - Why are They a Challenge to Stop? Classroom Activity
- Free Air Carbon Dioxide Enrichment Study (FACES)
- How Do Trees Record Time? (Exploring Earth)
- Laboratory of Tree-Ring Research
- Life of a Tree - Arbor Day Foundation
- NOAA Paleoclimatology Tree Rings Slide Set
- NOVA Build a Tree-Ring Timeline
- Paleoclimatic Data Before 2000 Years Ago
- Teachers Domain: Life of a Tree
- Time & Cycles - Dendrochronology - from Project Learn
- Tree Rings - What They Are and Why They Vary
- The Ultimate Tree-RIng Web Pages