Procedure for Exercise C

  1. Twig description, lake a walk outside, and collect a sample of twigs from various deciduous trees. Alter you have 5 or 6 twigs from different species, lay them out and observe their characteristics. What features do thev have in common? What distinctive characters are found on some of the twigs bur not others? Select one of the twigs and describe it as thoroughly as you can. At this point, use your own words and don't worry about the precise terminology. Your job is to describe the twig so accurately and precisely that someone else can identify it from among your collection. Complete the first half of worksheet 15-1 at the end of this laboratory topic by writing a detailed description of your twig.
  2. Description critique. F.xchangc descriptions with one of your classmates. Read his or her description and try to identify the twig described from among the collection of 5 or 6 twigs. Did he or she describe it well? Was there one character that easily gave it away? Were there any unique characters, such as a wound or blemish, that you would not expect to be on other twigs of the same spccics? What features do you sec that your classmate did not? Record your comments on your classmate's copy of worksheet 15-1 and return it to him or her.
  3. Identification of twig structures. Now examine anodier of your twigs. Try to locate all of the following structures then find and label them on worksheet 15-2.
  • Terminal bud: This is the grow ing tip loeated at die tip of the branch. The apical mcristcm is in die ccnter of the bud. Normally when this bud grows, the entire branch is extended. What type of growth would you expect, primary growth or secondary growth?
  • Axillary bud: At the axis of each leaf is another bud. If no leaves arc present, the axillary bud is immediately above die scar left on the twig when the leaf dropped. In many trees, the growth of an axillary bud is inhibited by hor mones produced in the terminal bud. What do you suppose would happen if the terminal buds on a live branch were removed?
  • Bud scalcs: Covering either of the buds there may be one to manv scale-like leaves that protect the bud. Note the texture and color of the bud scalcs.
  • Leaf scar: If you collected your twig in late autumn, winter, or before new growth in the spring, you will noticc scars left on the twig where a leaf was oncc attached. If you have a twig with leaves, carefully remove a leaf. What is the shape of the scar? How big is the scar com pared to the diameter of the twig? How arc the scars (or leaves) arranged on the twigs? Arc the scars opposite each other, whorlcd, or alternating down the twig?
  • Vascular bundle scars: Within the leaf scar you may see small "spots." These spots corrc spond to the vascular bundles veins) that ran from the stem into the leaves. What is the pat tern of the vascular bundle scars? How many do you see?
  • Hud scale scars: These arc thin scars left behind from the bud scales of terminal buds of the previous years1 growth. Usually they form a cluster encircling the stem. Many trees produce a terminal bud at die end of the growing season that remains dormant through the winter. When the tree expands in spring, the terminal bud scales are dropped, leaving the scars behind. You can determine how far the twig grew during the last year by measuring the distance between clusters of bud scales.
  • Lcnticels: On the bark of the twig, you may see raised, rounded or linear slitlike markings. These are lcnticels, raised areas of loose cork that provide regions for gas exchange. They function the same as stomatcs in younger tissue. Does your twig have lcnticels? What is the shape of the lcnticels?
  • Nodes and intemodes: The region of the twig where leaves are attached is a node. The region of the twig between two nodes is callcd the intcrnodc. If w e were to make a thin cross section of the stem at the node and compare it with the intcrnodc, wc would notice that the internal arrangement of tissues, especially vascular bundles, is different. This is because in die node some of the vascular bundles travel out to the leaves. Compare the lengths of the intemodes along one growing season. Was growth greater at the beginning, middle, or end of the growing season? Docs it look consistent from vcar to year?
  • Thorns, spines, and prickles: All these structures are sharp projections attached to the twigs. Thorns arc modified stems. Spines are either modified leaves or stipules (extensions at the base of leaves). Prickles are merely epidermal or cortical outgrowths, as in roses. Does your twig have any of these structures?

EXERCISE D: What Is The Name Of That Tree

The objective of this exercise is to learn the characteristics of trees while using a dichotomous key. Laboratory Topic 4 provides a dichotomous key using the leaves of trees. In this exercise, we will concentrate on the characteristics of trees during the winter. You should use bodi key s to identify the trees on your campus and in your community.

Materials Needed for Exercise D

Trees around campus

Dichotomous keys in this laboratory and in Laboratory Topic 4

Procedure for Exercise D

Take another walk around campus or around your com munity. Now that you know what features to look for and the terms used by botanists, use the dichotomous key in table 15.1 to identify the trees vou sec.

The key consists of pairs of choices (each pair has the same line number). Compare your plant specimen w ith the description at step 1, and choosc which direction to go. After you make a choicc, your next decision step is at the end of the dotted line to the right. If you reach a step that identifies your specimen, the name of the tree is given. For some trees, this name is the species. For oth crs, it is the name of the genus that combines related species. If you want to know more about these trees, consult additional resources.

The key conccnt rates on some of the common coniferous and deciduous trees of North America. Keep in mind that you may encounter horticultural varieties or non-native trees that are not included in this key.

EXERCISE E: Dendrochronology-Reading The Record Of Time

The objective of this exercise is to use the pattern of growth rings to understand the history of a tree. The process of dendrochronology, or tree-ring daring, uses the perennial grow th of woody plants as integrators of all the factors (such as rainfall, fire, and competition) that affect growth. You will use the same techniques used by researchers to accurately date archacological buildings or to understand prehistoric cycles of drought.

For this cxercisc, you w ill compare the grow th of a stand of trees with meteorological data. Each member of the class w ill analyze the tree rings of separate trees. Then the data of the class will be pooled to look for overall trends in growth for a region or grove of trees. Finally, you will connect the tree-ring analysis with independent meteorological records to dctcct possible explanations for the growth patterns.

Materials Needed for Exercise E

Graph paper square mm)

Hand lens or dissecting microscopes

Increment tree borer

Meteorological data

Metric rulers (at least mm rule)

Plastic drinking straws

Razor blades

Rubber bands

Slotted supporting blocks for mounting cores

Trees

Wood glue

Procedure for Exercise E

1. Coring a tree. Use an increment borer to extract a core from the center of a tree. Avoid anv areas of

the tree where the growth rings may be distorted, such as near branches or on die uphill and downhill sides of the trunk. Direct the borer level and toward the center of the tree. Firmly press the borer against the bark at right angles to the axis of the trunk and turn the handles clockwise. Oncc the borer tip is firmly anchored in the wood, pressure is needed only on the handles.

You may encounter pockets of pitch or decayed areas. If die borer enters a pitch pocket, it will become harder to turn. If this happens, remove the borer immediately or else die borer will become cloggcd and hard to clcan. If you encounter a decayed region, die borer w ill suddenly turn much more easily. Again, it is best to remove the borer.

Re careful with the tip of the borer. It needs to be kept sharp to work effectively. Also, try nor to damage the tip, sincc it is difficult to sharpen correctly.

  1. Removing the core. Turn the borer into the tree to a depth of penetration sufficient to include the pith, or as far as reasonable given the size of your borer. Insert the extractor spoon that conies with the borer into the borer from the handle end. When the extractor is inserted to its full length, give the borer a lull turn counterclockwise to break the core free. The extractor spoon can be removed, hopcfiilly with the core intact. Remove the borer by turning counterclockwise.
  2. Handling the core. The core is fragile and needs to he handled carefully. Placc the core in a plastic drinking straw. Sometimes it helps to slit the straw along one side in advance and then fold the straw around the core. Corrugated cardboard can also be used to secure the core until you return to the lab.

Be sure to jot down notes about the tree sampled, location, slope, altitude, soil, associated species, and any physical characteristics of the tree worth noting.

  1. Mounting the core. If die w ood is wet, let it dry at room temperature for a few days. Once dry, carc-tullv mount the core to a supporting block of wood with a groove notchcd down the ccntcr. Orient the core so you can look into the cells. Glue the core to the slotted block. Wrap rubber bands around the core and block while the glue dries. Put the most rcccnt growth rings on the right this is the end with the bark), and mark the area with a marker. Common convention in tree-ring analysis is to place the most rcccnt wood on the right.
  2. Surfacing the corc. Oncc the glue is set, remove the rubber bands. Cut a smooth surfacc on the core with a razor blade held at a low angle. You want to shave off the top surface so it is easier to see the grow th rings. If the corc was mounted at the correct orientation, you should now be cutting the vessels and tracheids in cross section. If the corc is too difficult to cut. you can use sandpaper to prepare a smooth surfacc.
  3. Tree ring measurement and recording data— Skeleton Plot Method. The simplest and most common method for recording patterns in the grow di rings is called the skeleton plot. It has the advantage of being relatively fast, but it docs require a little practice to be consistent in recording the results.

l ake a 2 x 10 -inch strip of graph paper (1 square mm grid). Placc the strip of paper lengthw ise in the horizontal position. Write zero on the extreme left end of the rule. Placc a hash mark every 10 squares to the right. Each vertical line corresponds to one year, or one growth ring. Every hash mark is a decade. The innermost ring on the corc is plotted at zero, and the plotting progresses from this point to the right (or to the outside of the corc >.

For the skeleton plot, the narrow rings arc of most interest. Mark a line on your plot corresponding to the narrow years. The height of your line should correspond to the narrowness of the growth ring compared to its neighbors (fig. 15.8;. In other words, the narrowest growth ring gets the highest line on your skeleton plot. By convention, the narrowest ring gets a line 2 cm in height. Rings of average width arc not marked. Rings with widths between the narrowest and the average arc given a height rel ative to their degree of narrowness. Wide rings are marked by a "B."

7. Creating a composite skeleton plot. After the skeleton plots of cadi of die samples in a group arc completed, compare all the plots at one time. Since all the cores in this experiment were taken from live-trees, the most recent year (on the right) is the same for all the samples. Line up all the plots vertically on the same date. If you were using corcs from dead-wood, you would not know the date and would need to look for similar trends to identify the likely correspondence of dates. Once the plots arc lined up, place another strip of graph paper under all the plots and graph a composite of all the skeleton plots. Since the heights of the lines w ere made using a degree of judgment, the comj>ositc plot helps to even out differences between individuals. The composite skeleton plot also helps identity anomalies such as missing rings or double rings. Try to match as many of the rings as possible to review common trends or patterns.

After the composite skeleton plot is completed, vou can date the corc. Sincc vou took vour core from . . .

a live tree, the growth ring to the right is the current year. You can work toward the left to sec when each specimen was a sapling. Remember that even if you were luck>' to pass through the pith of die stem, the age of the innermost ring corresponds only to the year the tree reached the height at which you bored your hole. It Is not the year the seed germinated, bccausc it takes any plant some time to reach a given height. What would you expect if you took your core from a higher position on the trunk? How would the corc of a major branch compare with a core from the trunk?

If you have a specimen core taken from deadw ood, such as a log from an archaeological site, you can date your specimen by comparing your composite skeleton plot with a master chronology. Several master chronologies are available via the Internet or tree-ring analysis laboratories (sec Additional Resources at the end of this laboratory topic . Master chronologies were creatcd by combining data from several samples and overlapping the results. Dendrochronologists have ere ated master chronologies that go back nearly 9,000 years in some locations. Tree-ring analysis is more accu rate than using HC lor dating wooden archaeological specimens, such as a beam used in the cliff dwellings of Mesa Verde.

8. Connecting tree-ring data to weather patterns.

Get access to meteorological data for your region for the past century. Most colleges or cities maintain accurate records for temperature and precipitation. For this comparison, you arc mostly interested in the values for the entire year rather than daily or monthly values. Examine your composite skeleton plot and look for clusters of narrow rings or

Dendrochronology Exercise

FIGURE 15.8 SKELETON PLOT FOR TREE-RING ANALYSIS. EACH VERTICAL LINE ON THE GRAPH PAPER CORRESPONDS TO ONE GROWTH RING THE HEIGHT OF THE LINE CORRESPONDS TO THE NARROWNESS OF THE RING COMPARED TO OTHERS. (THE INNERMOST RING IS ON THE LEFT.)

The Beauty of Wood 209

1850 1860 1870 1880 1890 1900 1910 1920 1930 1940

A beam from a new house

A sample from a living tree

A beam from an o*d house clusters of broad widths. Now examine die meteorological data for those same years. Can you sec any possible correlations? What weather factors seem to affect tree growth more than other factors? For now, merely look for possible interactions of environmental conditions and growth. You could test any hypothesis by a more exhaustive statistical analysis if you desire.

TERMS

*

pidi 200

bark 199

prickles 207

bud scales 206

primary mcristcms 197

bud scale scars 206

radial section

cork 199

or cut 201

cork cambium 197

resin ducts 200

dcndrochonology 207

ring porous 201

determinate growth 197

sapwood 205

diffuse porous 201

secondary

carlywood

mcristcms 197

(springwood» 200

secondary phloem 199

figure 205

secondary xylcm 199

hardwoods 199

skeleton plot 208

heartwood 205

softwoods 199

indeterminate

spines 207

growth 197

tangential section

intcrnodc 206

or cut 201

latcwood (summer-

terminal bud 206

wood) 200

thorns 207

leaf scar 206

transverse section

lenricels 206

or cut 201

mcristcms 197

tyloses 200

node 206

vascular bundle scars 206

periderm 199

vascular cambium 197

phellodcrm 199

wood 200

point of an engineer. What arc the vector forces wood can experience?

5. What saw cut of wood would make the best plank for a staircasc with significant foot traffic?

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