Lab 2. How to Read a Tree

Size: px
Start display at page:

Download "Lab 2. How to Read a Tree"

Transcription

1 Lab 2. How to Read a Tree (Reminder: bring a calculator to lab if you have one and you will be having your first Pre-lab quiz on the material in this lab manual.) Goals: To infer environmental conditions from close observation of twigs and wood, and in the process learn about the structure of woody stems. You ll also learn how to test for differences in growth conditions among years using a simple statistical test. Part 1. Twigs. Over the winter, deciduous trees are dormant. When the leaf buds swell in the spring, the bud scales fall off leaving a ring of scars around the twig. In summer, deciduous trees produce new terminal leaf buds ready for next year's growth. In winter, the growth of a twig over the past year can be determined by measuring the distance between the base of the new terminal bud and the old bud scale scar. See the diagram for an example. Twigs of different species grow at different rates. Some species are genetically programmed to grow faster than others. The rate of growth of any plant can also be influenced by environmental conditions, such as light, temperature, and moisture. In this activity, you will use a key to identify winter twigs collected from deciduous trees. You will also measure and compare growth rates of tree species. A. What is this twig? Choose a couple of twigs and from the collection. Carefully inspect them and locate the buds, leaf scars, terminal bud scars, lenticels and any other distinctive characteristics. Identify your twigs to species using the keys provided (see below). Using a dichotomous key: An efficient way to identify an unknown plant is to use a "dichotomous key" if one is available. The word dichotomous means "separating into two groups", which describes the kind of choices that must be made in using such a key. Starting at the top of the key you must choose between 1A and 1B. The choices are opposites. The tree you are trying to identify MUST fit into one group or the other. Compare your sample to the two descriptions and follow the path that 1

2 matches your sample most closely. It is important to read both alternatives carefully before making your choice and try to match your sample to every detail in the description. Eventually you should end up with the name of a tree species (or genus). Once you have a tentative identification, check with your TA or with a picture in one of the tree guides to confirm your ID. B. How could we possibly be able to tell how the tree has been growing using twigs!! Each table should choose a species of twig. For that species each person should measure 5 individuals as it describes below. For each twig, measure its growth for as many years as you can by measuring the distance between terminal bud scars. Measure from the base of the terminal bud to the base of the next bud scar. (See the diagram above). Record your measurement to the nearest 0.1 cm. Repeat for at least 10 individuals. You may notice that there are differences in growth rate between species of trees or between different branches of the same tree. For example, the apical or "leader" branch often grows faster than the lateral branches. Because of this, when possible, measure only the leader branch. Species: Sample number Table s Average Widths Within your table, average the growth of all the twigs for each year. You may have used some of the same twigs, but that s ok. Fill in the final column of the table above with this information. What is your species smallest year of growth? You will use these data with the data you will gather in Part II of this lab to write a short lab report due next week. 2

3 Part 2. Woodpile botany. We have brought in several slices of wood from firewood logs. What can you conclude about the species of tree it came from and its pattern of growth in the last few years? The samples are labeled with a number representing the year the firewood was cut, and a letter representing the particular log. Make sure you choose a sample from a different log than your neighbor. We'll assume all of the logs are independent samples, though it is Softwood with resin ducts Diffuse porous hardwood Ring-porous hardwood possible that some of the logs may have come from the same tree. C. What species of wood do you have? It is possible to discover a lot about a tree from the microscopic characteristics of a piece of wood. Under a dissecting scope, locate the early wood and late wood. Is your sample a hardwood or softwood? (i.e. can you see resin ducts or pores?) Are the xylem vessels all of similar size (diffuse porous) or is there a mix of large and small vessels (ring porous) Carefully draw what you see. 3

4 Make sure you examine at least one hardwood (Deciduous: Ash, Maple, Beech ect.) and one softwood species (Coniferous: Pine, Spruce, Fir). What are some obvious differences? In which type of tree is it easier to see the rings? D. Reading past environments using Tree Rings!! AKA Dendrochronology Dendrochronologists are scientists that use tree rings ( Dendro = Tree, chronology = Time) to study past environments. The key to being able to use tree rings is a special meristem tissue found in trees called cambium. Cambium is found right under the inner bark of trees and is what allows trees to lay down wood around their entire trunk every year (This is what forms a tree ring ). The width of tree rings can tell us a lot about what the environment was like for a tree in the past. Because trees can live a very long time, this allows us to use tree rings to estimate climate before there were instruments to do so. We call records like this Proxy Records. There are other proxy records that scientists have used to study past climate such as ice cores and lake sediments, but we won t talk about those today. What sets tree rings apart, however, is their ability to reflect environmental information on an annual resolution. This works because if it is a bad growing season for a tree for one reason or another (it could be too cold, too hot, too dry, too wet, or the tree could not grow well for non-climatic reasons), the tree won t be able to afford to use too much energy to make new wood cells and the ring for that year will be small. If it s a good growing season for the tree, there will be lots of energy available for making wood cells and the ring will be very large. This is what makes tree rings a very valuable tool for looking at recent climate change over time and why you will hear about it in the news from time to time. If you re lucky (and there s time), your TA might show some examples of treerings in the news. The alternation of the seasons in the temperate zone produces the visible growth rings since early in the summer, trees make large, light colored cells, but as the summer winds down, trees start making smaller, darker cells. Discussion Question: Can tree rings be used to study past climate all over the world? Now let s look at your piece of wood. Identify the type of tree (hardwood or softwood) and the species of your sample using the key provided by your TA. How can we estimate the age of this tree? What was the year do you think your tree started growing? What is a potential problem with estimating tree age this way? 4

5 Now let s try to Skeleton Plot using your sample. Skeleton plotting is a method used to try to identify climatic variables responsible for growth of your particular tree. (This is a slightly modified way of skeleton plotting than what most dendrochronologists would do just so you re aware.) 1) Draw a straight line WITH A PENCIL from the bark of your wood section to the pith. 2) Identify the rings representing 2010, 2000, and 1970 if present. In each of these rings, put a small pencil mark so you can easily know what year ring you are looking at like in the picture below: o Now identify your trees narrowest rings. What years do these correspond to? What about the widest rings? 3) On your skeleton plot paper below, use vertical lines to indicate the relative size of each tree ring like this: 4) Now connect the tops of the lines in your skeleton plot to show the trend in ring width across years. 5

6 Skeleton Plot for sample # What kinds of climatic factors might be affecting the growth rate of your tree? List as many non-climatic factors that could affect tree growth as you can. 5) Compare the shape of your skeleton plot to the shape of the graphs showing the temperature and precipitation records from Burlington Airport over the past 50 years and answer the following questions. Do you think your tree is responding more closely to one of these climate variables over the other? What does this mean about where might have been growing? Do you think your tree is instead, responding to non-climatic forces? If so, what forces do you think are most likely? Why? Does the data from the twig segment lengths support or refute the trends you see in the treerings? 6

7 Assignment: Due Next Week at the Beginning of Lab (20 points) Your assignment is to write two sections of a lab report. These are the first two sections you should write of any lab report even though in terms of order, they are the last two sections of a full lab report. The first section will be the Results section containing three figures: 1) your skeleton plot (cut out and taped neatly into your typed document), and Excel line plots from Burlington Airport showing 2) Temperature and 3) Precipitation data. The Excel plots should be inserted into your paper electronically (Set up a time with your TA if you need help with Excel. Don t wait until the night before the lab is due). You should also include a table with the information you gathered on twig segment lengths in Part I. Accompanying each table and figure should be a descriptive caption and a few sentences describing what trends or patterns exist in the data. No analysis should be talked about in this section however, save that for the next section. See TA handouts for further writing instructions. You will also be writing a Discussion section that explains what conclusions you ve drawn from your results. You should use the questions found above on this page to guide your discussion. Each of those questions should be addressed. One additional question you should answer in your discussion has to do with small rings that do not correspond to extreme climatic events. If you had rings like this in your sample, you should try to explain them by doing a Google search to see if there were particular events (floods, ice storms, ect.) that could have impacted your tree but that wouldn t necessarily show up in annual records. Remember, events that occur in the winter, will influence tree growth the following year (ex. An ice storm in December of 1987 will reduce the growth in 1988). Your Results and Discussion combined should be no longer than three pages of text, doublespaced. This means you will have to be concise in your analyses. A grading rubric is also posted on Blackboard. Refer to it both before and after you complete your results and discussion to be sure you addressed all the key points. 7

8 KEY TO SELECTED NORTHEASTERN DECIDUOUS WINTER TWIGS (from SUNY Oswego) 1A. Leaf scars opposite (2 scars per node).go to 2 1B. Leaf scars alternate (1 scar per mode).go to 4 2A. Terminal bud large (more than 2 cm),dark and sticky.horse Chestnut 2B. Terminal bud not as above.go to 3 3A. Leaf scar with crescent-shaped bundle scar.white Ash 3B. Leaf scar with several separate bundle scars.maple (Sugar, Red and Silver Maple are most common) 4A. Stem "knobby" with large nodes Cottonwood 4B. Stem not knobby; nodes not greatly enlarged go to 5 5A. Stem yellowish; axillary buds present and flattened to the stem 5B. Stem not yellow; axillary buds, if present, not flattened to the stem Weeping Willow go to 6 6A. Buds long (1-1.5 cm), orange and pointed American Beech 6B. Buds not long and pointed, less than 1 cm.go to 7 7A. Buds clustered at end of twig.oak (White, Red and Black Oak are most common) 7B. Buds single, not clustered, at ends.go to 8 8A. Twigs bright green Sassafras 8B. Twigs not bright green.go to 9 9A. Buds and twigs red or greenish red.go to 10 9B. Buds and twigs brown, not red go to 11 10A. Buds lopsided American Basswood 10B. Buds rounded, not lopsided.black Cherry 11A. Leaf scars entirely surround stem.sycamore 11B. Leaf scars not as above American Elm 8