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Home arrow Experiments arrow The Leaves on the Trees
The Leaves on the Trees Print E-mail
Written by William Finney   
Sunday, 19 August 2007

Fall Oak Leaves

I was taking a walk in the neighborhood this weekend, enjoying the cool breeze and thinking about the first experiment I would talk about on this site.  I was looking up and saw a vast sea of green above my head, the leaves on the trees lining the street, thinking about how the trees would look in a month or two and then it struck me, LITERALY, a leaf falling from a tree hit me in the face!.  This startled me for a moment, but it gave me an idea.  Just how many of these leaves are up there?  How much do all those leaves weigh? Are there any interesting patterns to how the leaves fall off the trees in the fall?  How does this change with the size of the tree?  This experiment is good for teaching about averages, sample sizes, and data collection.

As a note:  Many of the experiments that I will write up are open ended, and may not have a definitive answer.  I feel it is actually more important to acquire a strong ability to look at the world around us, ask good questions and learn skills that are applicable to a wide variety of scientific subject areas as it is to learn a particular fact.  Learning science isn't just learning facts, but gaining a new way to look at the world!

Expected Time Requirement

This experiment could take about 30 minutes a day, several times a week, for several weeks to months (depending on how persistent the leaves are at clinging to the trees). 

Materials You Might Need 

  • One cooperative tree.  This could be a tree that is out in the open with no other trees around it, or a tree with leaves that are very different than any other trees leaves nearby, or maybe when its leaves change color they are very different than others around it, (they turn red when the others are all yellow) otherwise you won't be able to easily tell what tree the leaves came from.
  • A kitchen scale, postal scale or some other way to weigh light objects with some accuracy.
  • A bathroom scale.
  • Several feet of string or a long soft measuring tape (optional).
  • A large plastic bag.
  • Your lab notebook and a pen or pencil.

Our Initial Observation

Every good scientist looks hard at the world around him or her before they ask a question.  I was looking at the trees and my observation struck me in the face!  It is getting close to the time of year that the trees shed their leaves, which gives us some excellent experimental opportunities.  We can use the leaves on the trees and don't have to worry about damaging the trees!

What questions do we want to answer? 

Sometimes this is the hardest part of being an experimentalist, knowing what we need to ask!  In this case I had a few questions when I looked up at the trees:

  • How many leaves are on a tree?
  • How much do the leaves on a tree weigh in total?
  • Is there any pattern to the rate at which the leaves fall off of the tree?

 

Method

If we simply went out, anytime we liked and picked up and counted and weighed the leaves without recording when we did it we could answer our first two questions but not the third.  Why?  Well, we wouldn't know how long any of the leaves had been there, so we couldn't tell if those leaves had fallen off all at once overnight or over several days.  We need to establish a baseline (no leaves around the tree) and an interval or observation period over which we will let the leaves fall and then count them (maybe 24 hours).  We can use this in two different ways:

  • We can see how the number of leaves that fall during our observation period changes, does it go up or down with time?  We can track this with a graph.
  • We can take measurements every few days, instead of every day, as long as we clean up all the leaves that have fallen before we start  and use  data we collect to predict the number of leaves that fall on during the times we don't collect data.  This is called interpolation.  


What this means is, we don't have to count every leaf that falls off the tree to know how many were there, we can calculate it knowing how many leaves fall over any given period of time and how the number of leaves that fall during an observation period changes over time.

Suggestion for small trees:  If you have a small tree, it might be easier to count all of the leaves than to weight all the leaves.  To determine the weight of all the leaves you would weigh, using a postal or kitchen scale, several small groups of leaves several times.  In this way you can calculate the average weight of an average leaf and multiply that by the total number of leaves collected to get the total weight.

Suggestion for large trees:  It may be difficult to count the large number of leaves that fall off of a large tree in even an eight or twelve hour period of time, a better way to do this would be to fill a bag with leaves until you have maybe 5 pounds of leaves in the bag.  To do this you will need to have someone stand on the bathroom scale with the empty bag and then with the full bag later on, because most bathroom scales are not very accurate for small weights.  Then you can count the leaves in the bag and know how many leaves are in a pound of leaves.  Later on you can simply collect all the leaves that fall in a given observation period and weigh them, and multiply that weight by the number of leaves in one pound to calculate how many leaves fell in that period of time.

Other suggestions:

  • Use the string to measure the circumference of the tree, so you have some measure of the size of the tree. 
  • Record how old the tree is, if you don't know, ask around!  Someone else may know.  
  • You could use the string to divide the area under the tree in half or quarters if you have more than one child working on this, then they can compare their numbers together.  Don't forget when making your calculations to take into account what fraction of area under the tree you are working with.
  • Identify the tree, or keep a leaf in your notebook so that you can identify it later if you can't now.

End Results

  • Make a graph plotting the number of leaves that fell off the tree each given observation period starting with when leaves start to fall from the trees until the tree is bare.
  • Calculate the total number of leaves that were on the tree.  Calculate how many observation periods fit in the time period from when the leaves start to fall from your tree until the tree is bare.  Calculate the average number of leaves collected in each observation period.  Multiply the two numbers together to get the total number of leaves.
  • Calculate the total weight of the leaves that were on the tree.  Now that you know how many leaves were on the tree, calculate the average weight of one leaf and multiply the total number of leaves by the weight of one leaf. 

Following Up

 Sometimes our data collection can lead us to ask other interesting questions:

  • How do the number of leaves change with the size of the tree, collect data from larger and smaller trees of the same type, don't forget to include a way to measure the size of the trees.
  • Are there changes that occur as a tree gets older, try to follow the same trees for a few years and record the changes.
  • Do different kinds of trees have different numbers and weights of leaves?  Try to look at more than one kind of tree.
  • What might introduce an error in our measurements?  Were the leaves wet sometimes when we weighed them?  Could some of them have blown away?


Trees are powerful and impressive features in our landscapes.  By calculating the numbers of leaves that fall off the trees in the fall and how much they weigh you can learn good data collection skills, practice calculating averages, learn about sampling, and explore your data with graphs.  We can also become more aware of the different types of trees in our environment and how our environment changes with the seasons.

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