Fall Leaf Drop Rate (Denise Randol)
Research Question and Hypothesis
Does the overnight low temperature increase the rate of fall leaf drop in a mature, deciduous Mulberry Tree in North Hollywood, CA?
The photo at right is a microscopic view of the abscission zone. The tree has activated a layer of cells near the base of the leaf and it is secreting enzymes called cellulases and pectinases. The cellulose of the cell wall breaks down and the leaf separates from the tree.
Every fall, I observe a huge leaf drop from my backyard Mulberry tree. I usually coincides with the end of October, cooler night time temperatures and the daylight savings time change. Prior to the big drop there is a period of light leaf fall. When the huge leaf drop occurs, it lasts for about two weeks. Although, there is a specific chemical action by the tree that sets off the leaf drop, I am interested to see if overnight temperature variations affect this 'light leaf drop phase'.
*Hypothesis of 6th grade class: To get a sense of misconceptions that middle school students might have regarding the causes of leaves falling from trees, I surveyed 28 6th graders at 2nd Street Elementary School. I asked them to give a written opinion to the following question: "Why do leaves fall off trees in Autumn?" The overwhelming number of students (21) believed that the colder nights causes the leaves to fall. Other opinions were: trees are on a timer (2), wind knocks the leaves off (4) and photosynthesis "does something" (1).
Standards
HS-LS2 Ecosystems: Interactions, Energy, and Dynamics
Experimental Design
Quantity of leaves are tracked over a three week period. Data is recorded by smart phone photos and internet temperature reports. Instead of counting each leaf, quadrants are established and an estimated, quantitative mean is calculated. This is a similar method used when officials are determining estimated attendance at large outdoor gatherings.
Independent variable
Time
Dependent variables
Number of leaves
Series
Study the number of leaves (dependent variable) as a function of time (independent variable) at various overnight, low temperatures (series).
Wind speed, humidity and shortening daylight variable
Constants and Controls
Constant: Time of data collection (48 hr cycle)
Control: Reset yard by raking after data collection
Materials
Digital camera (smart phone ap)
Computer, i photo or similar application
Weather web site: http://www.wunderground.com/weather-forecast/91607
Procedures
Every two days, at approximately 4:00 pm, photograph the ground area near the mulberry tree.
Photograph from same position each time.
Record daily, overnight low temperature.
Rake ground area, removing all leaves.
Repeat photo documentation & raking every 48 hour period for three weeks.
Upload photos into i photo.
Superimpose a grid onto photo and date.
Estimate number of leaves.
Use double line graph to compare temperature & leaf quantity
Analysis & Conclusions
The graph on the right shows a small temperature range compared to relatively large fluxuation in leaf quantities. The high spike in leaves on Oct 30-31 correlates to the lowest temp reading of the period. There does not appear to be any other relationship indicated between the number of leaves and the temperature.
Therefore, the 6th grade class' hypothesis is disproven- temperature is not a direct determinant to the quantity of leaves dropped overnight. Except if the tree signals the leaves, how does the tree know? Lack of production from the leaves--due to colder temperatures.... students can continue to explore these ideas......
Research regarding why leaves fall, supports this conclusion. The data provides an opportunity for students to test their misconception. Since the evidence does not show a matching pattern of temperature and leaf quantity, students will have to revise their conception and consider alternative explanations.
On a Personal Note:
The mulberry tree in my yard has not yet sent a signal to its leaves that its time to 'close up shop.' Although there has been a steady, constant rate of leaf drop over the 3 week period of study, the large drop of leaves that I have witnessed every fall has yet to happen. Historically, I have great amounts of leaves dropping in one overnight event, that is then followed the very next night (after raking).
Photos & Movies
References
Robert Krulwich On Science contributor to NPR radio, Oct. 30, 2009
Why Do Leaves Really Fall Off Trees?
This is a great explanation about leaf drop of deciduous trees--and it dispels misconceptions people generally have (cold temperature and wind).
For a more detailed explanation of botany/photosynthesis concepts at work:
Why do Trees Lose Their Leaves?
To track your daily weather, including temp, wind, humidity, dew points, etc. etc