Draft 1 Why Do Leaves Leave the Tree?

This topic submitted by Ryan May and Maria Martinez ( mayrc@muohio.edu, martinsa@muohio.edu ) on 10/7/04 .

Natural Systems 1 Syllabus---Western Program---Miami University

Introduction
Every year during autumn a marvelous phenomena occurs. For a few weeks, entire forests look ablaze as its deciduous trees turn all different shades of orange, red, yellow. Though the timing may differ between when color starts to change and when a plant actually loses its leaves for all species and even populations, it is something deciduous trees share nonetheless. But what are the actual factors that contribute to how quickly a tree loses its leaves? Is it precipitation? Day length? Is wind alone the most influential factor, or do certain species have a tendency to lose their leaves quicker than others? These are the questions we are setting out to answer in our semester lab, exactly what environmental factors facilitate leaf loss.

We predict that it is the wind that will play the largest part in how quickly trees lose their leaves due to the fact that it has physical force behind it. Other aspects such as temperature, humidity, UV and day length are also being measured, but we predict it will be unlikely that we find that on days when, for instance, the UV factor was really high that more leaves would be lost that day. In addition to wind being a key factor, we also predict that species, location, and relative height of the tree will play a part as well. Because the woods has somewhat of a canopy layer, we predict that these leaves will be the first to be lost because of their exposure to the elements. Species affects if a tree can even make it to this height, let alone whether or not it may have a different tendency to lose leaves quicker simply because of its genetic makeup. Thus we predict that the maples will lose their leaves the quickest out of the other species in the woods, not only because of their great numbers but because they dominate the canopy. In order to accommodate a somewhat diverse area of study, we have chosen a hillside next to a creek. In this case, we predict that tree location on the hill will also affect how quickly a tree sheds its leaves. We predict that there will be a tendency for trees toward the top of the hill to lose their leaves quicker than at the bottom, mostly due to the fact that at the top of the hill they will be susceptible to higher wind speeds. Thus in short, our hypothesis is that the older maples toward the top of the hill will lose their leaves quicker than others, and the largest factor that will attribute to the leaf loss will be the maximum wind velocity for each day.

Background
In order to help the reader understand the process of leaf fall, we have done an extensive background research on the process. Leaves are designed to be discarded sooner or later. If they weren't gotten rid of, the relatively thin and unprotected surface of most leaves would seriously threaten the tree in wintertime, because it would be exposing too much of the tree's surface to evaporation when there isn't much water present (Levitt 1980). Some evergreen needles are lost as well, despite a waxy layer of protection from winter water loss, they eventually become too inefficient at photosynthesis and have to be gotten rid of (Page 1983).
Each leaf serves as an endpoint to a transport network that extends continuously to the roots (Birdsey, Hom, Mickler 2000) . Therefore, a tree must shed its leaves without exposing vascular tissue to harmful bacteria or fungi or opening it to water loss.

All during the spring and summer, a trees leaves are used to make glucose through the process of photosynthesis. Chlorophyll is the actual molecule in which the light, water and hydrogen reactions take place and is found in two forms, a and b (Mooney 2001). These forms differ in the composition of their molecules, but they function nearly the same. Chlorophyll molecules are found in tiny, green, saucer-shaped bodies called chloroplasts, where they are organized in complex layers (Mohr, Schopfer 1993). The chloroplasts are most heavily concentrated in leaf blades, but they are also found in other green tissues like stalks and fruits (Blankenship 2002). Depending on the overall nutrition of the tree, the production of chlorophyll may rise or fall, and new chlorophyll is always being produced to take place of worn-out molecules.

In addition to chloroplasts, there are also chromoplasts in the leaf which contain non-photosynthetic pigments (Gregory 1989). These are carotenoids, the carotenes and xanthophylls that color squash, carrots and corn, and that give a leaf its yellow coloration when it loses chlorophyll (Kelley 2004). A plant grown in the dark can make no chlorophyll, and that is why poorly-lighted houseplants turn yellow due to the carotenoids. Also, there is another class of pigments found in tree leaves which are the anthocyanids, which produce the reds and purples seen in apples and grapes (McGuire 1998). These are not found in chromoplasts however. They are dissolved in the cell sap of the spongy tissues deep within a leaf blade. They form each fall by chemical reactions between accumulating sugars and organic compounds called anthocyanidins. Light is necessary for their development, and low temperatures above freezing and dry weather enhance their production.

Among the most important of the hormones found in leaves is auxin, which forms in growing leaves. Auxin is a key ingredient in the formation of buds and is also an overall growth hormone and is found throughout the leaf (Auxin 1997). Starting in early fall when leaves start to get old, their ability to make auxin is reduced and then triggers the movement of nutrients out of the leaf (McLaughlin 2002). Soon the balance of auxin and the growth-inhibiting hormone abscisin becomes unfavorable, and a layer of corky cells develops where the leaf's stalk joins the twig (Kozlowski 1989). The leaf is now walled off from the tree, and cannot transfer the sugars that now thicken its sap (Marchand 1989).

Meanwhile, the destruction of chlorophyll in the leaves is outpacing its production, then the overall manufacture of chlorophyll ceases altogether, finally exposing the carotenes and xanthophylls. Also, sugars trapped in the leaf sap react with anthocyanidins.

It is now that the leaf is ready to fall. The leaf will remain on the tree for a few days until the corky layer is thick enough, further weakening its attachment to the tree until it falls under its own weight, or is aided by an environmental factor.

Materials: Measuring tape String
Boxing tape Cardboard
Box cutter Camera
Composition Book Computer
Garden staples Scissors
Marker

Methods: In order to observe the effects of a difference of location on leaf fall, we sectioned off a large rectangle of land in the woods to study. Using the measuring tape, we marked off a 27X47 meter section of tree growth and marked the four corner trees of this are with bright pieces of string. This piece of land was selected for its variation in elevation. The selected rectangle is located on a hill, so that half of the observed trees (on the top of the hill) are more elevated those on the bottom of the hill. It is necessary to create this distinction so that we may prove our hypothesis that those trees that are elevated will lose their leaves faster due to a higher exposure to wind and other factors having to do with weather. With that in mind, we then sectioned our rectangle into quadrants. There are two sections on both elevations to ensure that if a higher section does have greater intensity of leaf fall, it is not by coincidence. If both of the sections on the two elevations reveal a similar proportion of leaf fall, it will help us to prove that height does have an impact on the rate of leaf loss. In addition, we made sure that this section was populated by a variety of tree species. Each of the quadrants has at least four of the same type of tree. This way, we can verify if the species of the trees determines the rate of leaf fall by monitoring how many leaves of each species are collected. The variation of tree species also will help us to understand if leaf color change is specific to a certain species.

Because this lab is designed to monitor leaf fall in various areas of a forest, it was necessary to create a contraption with which leaf collection is possible. We cut out 16 pieces of 2 ¸ X 2 ¸ foot cardboard squares. To make them into leaf collectors, we needed to give these squares sides that so when the leaves fall into them, they are not moved by the wind or other outside forces. To do this, we cut out 2X2 inch squares on each corner of the larger squares with the box cutter. We then folded the sides and taped them up using the boxing tape. After constructing the collectors, it is time to place them outside to do their job. We took four boxes to each quadrant and evenly dispersed them. In each quadrant, there is a rectangle of leaf collectors of 5X4 meters (one collector on each corner). This way, there is a greater chance that there will not only be more variety in the types of leaves that fall, but there will also be a number from which we can calculate an average of leaves that fall. Also, to make sure that the boxes themselves are not moved by outside forces, we used garden staples to secure them to the ground. Finally, the boxes were labeled with a marker for each quadrant, (1A, 2A, 3A, 4AÉ.and the same for quadrants B, C, and D) so that we know exactly what was in which collector on any given day.

After all of this was set up, it was time to begin collecting data. Everyday, one of us goes out to the woods to record what we find in each of the collectors. We observe and record how many of what species and of which color is in each collector. We record any significant leaf coloration. If there are any leafs that are any other color than the usual brown, we make a note of it in our handy dandy composition books. The other part in the collection of data is the monitoring the change of the leaf density in each quadrant. We chose the largest tree in each quadrant to serve as examples of the rate of leaf fall and discoloration. To record the changes in leaf color and concentration, we simply take pictures of the example tree every other day. By comparing the pictures, it will be possible to see the physical change in each tree from day to day. The tree we used in each quadrant to take pictures of is also surrounded by the four leaf collectors. This way, we can be sure that some of it's leafs will end up in them and we will be able to take notes on its rate of leaf fall color change not only through looking at the pictures (qualitative data), but by counting the actual leaves that fall from the tree (quantitative data). We will then be able to compare the changes in the leaves to the weather extremes of that day and see if there is any correlation between the two. Our prediction is that we should see a significant amount of wind on the days where it looks like there has been an obvious change in the density of the leave on the tree. We also hope to find that if the UV and temperature levels have dropped a notable amount, the change in leaf color will increase.

With all of this information that we are receiving, it was necessary to create a sheet in order to record all of this data in an organized manner. It occurs as follows:

1 2
A B C D A B C D

3 4
A B C D A B C D

In each lettered section, we mark the number of each type of species found, as well as the color, using abbreviations for each color.The observation and recording of the leaf fall and weather change began at the end of September. We plan on taking measurements until the leaves have stopped falling. This should happen around the end of November which will give us enough time to process and analyze the data that we have collected before the project is due. So overall, the experiment will run for approximately two months.

Research Design: During this experiment, we will be observing different factors that may play a part in a tree's leaf coloration and rate of its fall. Every day, we will be observing the different weather factors supplied by the weather website online. Measuring the temperature, wind, and UV will possibly help us to identify a logical pattern to the rate of leaf fall. If our predictions are correct, those days with less sunlight and increased wind will result in an increase in leaf precipitation. Because of decreased sunlight, the leaves photosynthetic capabilities are lowered, and additional wind increases the chances that the leaves will be blown off of the branches. Elevation also plays a large part in the amount of wind a tree is exposed to. By observing the trees in the elevated quadrants, there will be one other way of determining the effect of wind on leaf fall.
We will also be monitoring the color of the leaves that fall into the collection boxes. This will help us to know which species of tree reveals which colors in their leaves before they fall. Noting the color every leaf in the collector will allow us to find if there are any correlations between the leaf species and its color.

The observation and recording of the leaf fall and weather change began at the end of September. We plan on taking measurements until the leaves have stopped falling. This should happen around the end of November which will give us enough time to process and analyze the data that we have collected before the project is due. So overall, the experiment will run for approximately two months.

Annotated Bibliography
Birdsey, Richard, Hom, John ad Mickler, Robert. Responses of Northern U.S. Forests to Environmental Change. New York: Springer-Verlag, 2000.
-This book gave us reference the structure mechanisms of transport inside the leaf, as well as the information regarding sap build-up in a leaf about to fall.

Blankenship, Roger. Molecular Mechanisms of Photosynthesis. 1st ed. Williston: Blackwell Publishing, 2002
-This book gave us a background in photosynthesis and the molecules involved, as well as refreshed our memory of the different types of chlorophyll.

Braun, Lucy. The Woody Plants of Ohio. Ohio: The Ohio State University Press, 1961
-We used this reference as a final check in the process of confirming the species of trees in the woods being studied. Because it gave physical descriptions of the trees, we were able to double-check our results and confirm our findings.

Gregory, R.P.F. Biochemistry of Photosynthesis. 3rd ed. London: Bath Press, 1989
-This book gave us our information on chromoplasts, as well as further information regarding chlorophyll and the photo systems.

Hora, Baynard. The Oxford Encyclopedia of Trees of the World. Oxford: Oxford University Press, 1981
-This illustrated encyclopedia helped us identify leaves from trees that we weren't sure of based on their color. It is more specific than Autumn Leaves, so it helped us fill the gaps.

Kelly, Raina. Fall Foliage. Newsweek; October 4, 2004. Vol. 154, Issue 13. http://search.epnet.com/login.aspx?direct=true&AuthType=cookie,ip,url,uid&db=aph&an=14534025
-Fall Foliage is where we learned of the carotenes and xanthophylls which color the leaf after the chlorophyll has left.

Kozlowski, Theodore. Tree Growth and Environmental Stresses. Seattle: University of Washington Press, 1979.
-This book is from where we furthered our knowledge about the roles of auxin and its involvement in the growth of and maintenance of leaves.

Lanner, Ronald M. Autumn Leaves. 1st ed. Minocqua: Northwood Press, 1990
-This book was virtually our bible when trying to identify the species of tree in the woods where we are studying. Because the leaf samples we collected had changed color, this book was most useful in identifying a tree based on its foliage.

Levitt, J. Responses of Plants to Environmental Stresses. 2nd ed. Volume 1. New York: 1980.
-This book goes into detail about the tree's mechanism for detecting when light is becoming less and less (photoperiodism). We used at as further reference regarding this aspect of leaf color change.

Marchand, Peter J. The Fall of the Leaf. Natural History; Nov. 99, Vol. 108. Issue 9. http://search.epnet.com/login.aspx?direct=true&AuthType=cookie,ip,url,uid&db=aph&an=2464129
-This article goes into detail about the changes that occur at the base of the leaf stem when it is about to fall, and contains more information about the compounds that form color.

McGuire, Esther. Peek, Jim. Foliage Afire. New York State Conservationalist; October 1998. Vol. 53, Issue 2. http://search.epnet.com/login.aspx?direct=true&AuthType=cookie,ip,url,uid&db=aph&an=1228122
-This article goes into the question of why leaves change color, as well as the various chemical processes involved in color change.

McLaughlin, Lisa. Fall Colors Falling. Time. 10/7/2002, Vol. 160, Issue 15. P. 105. http://search.epnet.com/login.aspx?direct=true&AuthType=cookie,ip,url,uid&db=aph&an=7426220
-This reference contained information regarding the actual process of how and why leaves fall, as well as some of the preparation processes that take place beforehand.

Mohr, Hans and Schopfer, Peter. Plant Physiology. 2nd ed. Berlin: Springer-Verlag, 1995
- From this book, we learned about the structure of the leaf, as well as the transport systems inside the tree.

Mooney, Harold. Roy, Jacques. Saugier, Bernard. Terrestrial Global Productivity. San Diego: Academic Press, 2001
-This book goes into the processes of the leaf, including its functions as a producer of glucose and respiration of gasses.

Page, Jake. Planet Earth: Forest. New Jersey: Time-Life Books, 1983
-This book goes into why leaves fall, and from it we learned that evergreens lose some of their needles as well.

Thomas, Brian and Vince-Prue, Daphne. Photoperiodism in plants. 2nd ed. Volume 1. London: Academic Press, 1997
-Photoperiodism is where we learned about auxin, a key hormone which triggers the falling of the leaf, as well as the changing of the leaf's colors.

Next Article
Previous Article
Return to the Topic Menu