Part 3 - Understanding Long-term Streamflow Using Tree-Ring Science
Much of what we know about global and regional climate on long-term time scales comes from proxy data is a past climate record like tree rings and ice cores used to interpret paleoclimate. Recorded meteorological records in general are short and typically pertain to only the past fifty years or so. Therefore, we must rely on proxy data to foster our understanding of past climate variability.
There are many different types of proxies, including, tree-rings, pollen, ice cores, and sediments. Fundamental to all proxy records is that they are indirect information about climate variations from biological and/or geological evidence. These proxies are affected by changes in the surrounding environment, which affect the growth or composition of the proxy, and are recorded sequentially through time. By understanding how variability in the environment affects the proxy, it becomes possible to reconstruct climate conditions through time for hundreds to thousands of years and more.
Tree rings are often used to understand streamflow histories, because tree growth is often controlled by the same climate-related factors, including precipitation and evapotranspirationthe process by which water is transferred from the land to the atmosphere by evaporation from the soil and other surfaces and by transpiration from plants, that in turn influence variations in streamflow. Since precipitation also influences the amount of water in streams and rivers, these trees are useful for reconstructing annual streamflow. You can think of these kinds of trees as individual recording stations quantifying the overall water balance in a river basin, and measuring the level of streamflow in a basin.
The trees that provide the very best information about streamflow variability in the Colorado River basin—those particularly sensitive to variations in moisture—are species such as ponderosa pine, pinyon pine, and Douglas fir, growing on dry and rocky sites above the river where soil moisture storage is minimal. Note the distance above the river valley the dendrochronologist is working in the image above. Trees growing in these sites are also less likely to be subject to non-climate disturbances, such as fires and insect infestation, and the effects of competition from nearby trees. In addition, the oldest individuals of these species tend to be found on these sites. Tree-ring scientists take care to avoid sites that show any signs of disturbance as well.
Exploring the Sites
Using tree rings to reconstruct a streamflow record begins with selecting a suitable site that contains the best trees to core and analyze. Trees that grow very close to the river aren't much use for learning about climate history, because they can access water so easily they pack on wide new rings of wood even in dry years. To find trees that feel the same climatic pulses as the river, trees whose rings widen and narrow from year to year with the river itself, scientists have to climb up the steep, rocky slopes above the valley and look for gnarled, ugly trees, the kind that loggers ignore. In this region, dendrochronologists take sample cores from pinyon pine, and Douglas fir trees that grow high above the river. In this first part of this lab, your class will investigate three different sites in New Mexico: J9, Pueblita Canyon and Four Wheel Drive. Trees from these sites were used to reconstruct Colorado River streamflow for hundreds of year in the past.
In this part of the lab, you will be broken into teams and be assigned a job within your team, work as a group through a series of tree-ring activities, explore the research sites, measure tree rings, compare your work to others, and produce a Powerpoint to share with your classmates.
Your instructor will place you in groups of four or five. You will be assigned one of the three study sites to focus on. When you are in your group, elect a member to act as the "PI" (Principal Investigator) of your research group. The PI will organize and guide the completion of your final report. Then decide within your group who will act as:
Each member of your group will take part in the data analysis and answering of the Stop and Think questions from this lab. Your instructor will tell you which area you and your group will focus on.
Instructions
1. Explore the Sites
Click on the link below to see a 360 degree view of your study site.
2. Look at the terrain in the region. Take note of the types of vegetation that exist and pick out a few trees that you think would be good candidates for core sampling. The predominant tree type found at the J9 site is Douglas fir, at the other sites you will see mostly pinyon pines.
3. Your group will produce a short PowerPoint presentation describing your study site. Information that you collected on your study site will be placed in your final report at the end of Part 3. Include the following in your report:
4. Developing Tree-Ring Chronologies
A 'chronology' is a tree-ring record that has a large number of samples, has been processed and checked by a dendrochonologist, and has been statistically processes to remove age-trends. To create a tree-ring chronology, cores from the sampled trees at each site are first cross-dated (that is, patterns of narrow and wide rings are matched from tree to tree) to account for missing or false rings, so that every annual ring is absolutely dated to the correct year. Note how accurately the tree-ring record (green) captures the actual streamflow measurements (blue) in the graph below. The Colorado River Compact of 1922, that we learned about in Part 1, was made during a wet period, a fact only revealed when tree-ring records over the past several hundred years were developed.
In this activity, you will use a quantitative method to measure the width of tree rings (using ImageJ) to produce a graph of raw tree-ring measurements that reflect the climate record in the Colorado River basin over the past 120 years. The cross-dating has been done for you. Your group will be using some of the same procedures followed by dendrochronologists to build a record that can be used to identify changes in climate such as droughts. Then you will compare your graph with stream flow data to determine how well tree rings represent changes in the flow volume of the Colorado River. Can tree rings offer a window into the past history of the mighty river? Can knowledge of streamflow in the past inform decision making regarding future water allocations in the basin?
Team Activity Instructions
1. Decide which members of your group will be compiling the tree-ring data-- the data crunchers. All group members should familiarize themselves with the method for measuring tree rings below as the data crunchers work through their core sample.
2. For data crunchers: Open ImageJ. (If you don't have it on your desktop, you can download an "all platforms" version here. If you've never used ImageJ before, and would like a review of its basic features, view go to the ImageJ Beginner's Tutorial. Note: to run ImageJ, you may have to also download a Java Bundle, if so, you will be prompted to do this. Some computers may also require you to go into System Preferences/Security and Privacy and allow apps from "App store and identified developers" in order to allow the download.
3. Before you begin work, look at the 12 core samples shown below. Two core samples have been taken per tree with two trees sampled at each site. Can you tell which of the eight trees is the oldest? The youngest? Hint: The first group of three dots on the left of each core sample represents the year 1900. Notice the differences between the samples at each site. What do you think caused these differences?
4. Each core sample tells a story of the environmental conditions the tree endured from year-to-year. Occasionally however, due to very harsh or stressful conditions, a tree may not produce a ring all the way around its perimeter. These are called missing or locally absent ringsare growth rings that are discontinuous around the stem so that it is absent at certain points; also referred to as a partial ring or missing ring. Trees can also produce what scientists call a false ring a layer of wood less than a full season's growth and sometimes not all around the trunk when compared to a normal annual ring. False rings usually occur during a warm snap after the tree starts to go into dormancy. Note, that the once you zoom into the cores, they have been marked to help you identify tricky rings.
5. Your instructor will assign your group a tree (two core samples) that was sampled at one of the three tree-ring sites that you explored above. Right click on your designated core sample, then click on it again to get to the highest resolution image. Download this image to your desktop. In ImageJ, go to File/Open and open your sample image in the program.
J9 site in Colorado
![[creative commons]](/images/creativecommons_16.png)
Four Wheel Drive site in New Mexico
Pueblita Canyon site in New Mexico
6. Click on the straight line measuring tool. 
Draw a line from the left hand edge of the image, parallel to the reference dots, to the right edge of the image (the bark). Note: If you would like to see an example of how to measure in ImageJ, watch the first portion of the ImageJ tutorial.
7. Go to the Analyze Menu/Set Scale. The length of the line you drew should be displayed. Enter the known distance listed next to the sample core you are working on. Set the unit to "mm." The scale of the image should now be displayed.
8. You will see a series of dots on your image. Scientist place single dots to represent each decade, two dots to mark every 50 years, and three marks at the century mark. You will begin your data collection at the ten-year or century mark on the left side of your image. Note: Be sure to omit the first couple of rings of the core sample (all the way left). These rings are often distorted. Zoom in the first section of your core on the left side of the core sample. Since the rings are narrow, it will make measuring easier if you work on just 10 to 20 years/rings at a time.
9. Go to Analyze/Set Measurements and remove area or any other measurements listed besides "Length".
10. Click on the straight-line tool. Draw a line across the first tree ring in your sample. One full tree ring includes the earlywood (formed in the growing season, typically lighter in color), and the latewood (formed at the end of the growing season, typically darker in color). Make sure the line is perpendicular to both sides of the ring. See illustration at the right. Hit "Command M" (or Control M on a PC). The width of the ring (in mm) will be displayed in a results box.
11. Measure each tree ring width in your sample in succession from the inner most ring (left side of the image) toward the outermost one just inside the bark (right side of the image). Re-position the straight line measuring tool each time to measure with a line perpendicular to both sides of the ring. You do NOT have to follow the line of marked dots. You will need to click on the hand tool and re-position your image as needed to complete your measurements. Leave your last measurement on the screen when you do this so you can return to the next ring in the sequence.
12. Note: Any ring marked with a red circle is a missing ring. Do not include the measurement in your data. To add these missing rings to your sequence, double click your cursor in the image so the previous measured line disappears leaving the "line" icon still highlighted. Then hit Command (or Control) M. Check and see if measurement is now zero. Proceed to the next visible ring.
13. When you are finished, download your data table to your desktop.
14. The dots representing the decades and centuries on your core samples were derived by carefully cross-datingis a technique that ensures each individual tree ring is assigned its exact year of formation by matching patterns of wide and narrow rings between cores from the same tree, and between trees from different locations samples from the same site.
15. Look at the image of your core sample. The first group of three dots found in the core samples (left side near the center) represents the year 1900, the last group of three dots near the right edge represents the year 2000. Count backward and forward from this point to place actual dates on your table instead of 1 - X. Save your new table to your desktop.
16. For Tech person: You should have two data tables delivered by your data crunchers. Import both into Excel or other similar program. Create graphs of each core with "year" on the x-axis and "width (mm)" on the y-axis. Save the graphs to your desktop for inclusion into your group's report.
17. Group Work
The data you derived was raw--it hadn't been processed to removed growth patterns that are related to how a tree grows over time, called age-trendsa pattern of differing tree ring widths that result from early tree rings tending to be wider than later growth tree rings. The growth ring of a very young tree is spread over the small diameter of the tree and is proportionally larger than the ring you observe in trees that are much older that have the growth ring spread over a larger diameter tree. You may have noticed how large the rings in the first ten years of growth in your sample were compared to the last ten years. Scientist use conservative statistical methods, called detrending the process of removing the age effects of in the data by dividing the actual measurements by those predicted from a statistically derived equation that relates tree growth over time to tree age , to remove the age trends in samples, while preserving climate information. We wont be removing age-trends in this lab, and therefore, when you look at the graph of your data, you have to consider that some of the patterns are related to this age trend.
18. Look at the big picture: Compare your tree-ring data with the master chronology for your site.
To see how your core sample data fits into a longer term record for the region, compare your two tree-ring graphs to the master tree-ring chronology for your group's study site shown below.
19. The three sites chosen for your study are close to the San Juan River. To see if your tree-ring analysis correlates to the stream flow of the San Juan River, compare your data to the San Juan River data below. Look for narrow rings on your tree-ring plot and see if they match up with times of low-flow volume of the San Juan River. Note any matches or discrepancies between the two datasets.
20. Open the interactive time series of Colorado River Flow at Lee's Ferry. Adjust the # of years on the x-axis to match the range of years of your tree-ring graph.
21. Look for years with narrow rings in your tree-ring plot and see if they match up with times of low flow volume of the Colorado River. Note any matches or discrepancies between the two datasets.
Stop and Think
Your group will produce a PowerPoint that you will share with other groups in your class. Include the following in your presentation:
Tree-Ring Research Colloquium
1. The three research sites are fairly close to each other in New Mexico but you probably noticed that differences in the types of the trees and the environment they are growing in exist. Your group will present your research results to your class and then will discuss what your research says about the future of water use in the Colorado River Basin.
2. Research groups from the three study sites (J9, Four Wheel Drive, Pueblita Canyon) should present their Powerpoint research summaries in turn. Allow a few minutes at the end of your presentations to answer questions from class members. After all the presentations have been heard, discuss the following questions with your instructor: