STUDENT- GENERATED LAB
GAP EFFECTS AND THE EFFECT OF CANOPIES ON PLANT VARIATION
Question:
How does light intensity affect plant diversity?
What is the effect of a canopy on plant diversity in Western woods?
Introduction:
Gap effects have been researched for a variety of reasons. Some reasons include finding species at risk and managing the reduction of those risks. There are also various environmental concerns connected to the preservation of tree canopies.
In order to complete this lab, we should understand the basic concepts of the importance of canopies. There are three main levels within a forest, whether tropical or temperate. The first is the canopy itself, consisting of mainly trees; the canopy is defined as “woody perennial plants with one main stem or trunk which develops many branches, usually at some height above the ground” (Websters). The second level of the forest is the under story, which receives less than ten percent of light that strikes the initial canopy. This level usually consists of shrubbery, “a low woody plant with several permanent stems instead of a single trunk like that of trees” (Websters). The lowest level is that of non-woody herbaceous plants, which receive less than five percent of the actual sunlight. This consists of “any various plants of the grass family” (Websters).
Gaps occurring naturally (i.e. one tree falling) generally have minimal impact on their ecosystem. However, “in the late 20th century, we are coming to realize that biological resources have limits, and that we are exceeding those limits and thereby reducing biological diversity” (World Resources Institute). Biological diversity is essential to our survival, for as we place limitations on our resources, more limited become our resources to respond to societal changes. Forests also soak up carbon dioxide, emitting oxygen; the loss in tree numbers creates a shortage of oxygen.
If our hypothesis: “Plant diversity is greater under a canopied environment than one that is not canopied” is correct, then serious issues of the importance of the preservation of canopies must be addressed. Most frequently broadcasted is the issue of rain forest depletion. The rain forests are home to the largest quantities of plant diversity on the planet. This destruction carries with it serious environmental issues (greenhouse effect, global warming, potential soil erosion). It also eliminates possibilities for future medical research within the rain forests, which in turn affects agriculture and industry. Because we do not yet have the technology to recreate species, there is no hope for recovering extinct species.
On a smaller scale, this applies to Western woods. As mentioned, gaps created naturally in the woods have minimal impact. However, planned (or unplanned) clearings of the forest have far-reaching consequences- large patches of high-intensity light may create an environment unsuitable for biodiversity.
Hypotheses:
Because light intensity affects plant diversity, plant diversity is greater under a canopied environment than one that is not canopied.
Materials/Resources:
Our resource will be selected portions of Western Woods. We will rope off the areas that will be sampled. We will also have maps available to classmates so that they can make references as to where their allotted sampling areas are in comparison to the entirety of Western Woods. All students will also have:
1. Light intensity meter
2. Data sheet compiled by our group. It will be similar to the attached page.
Methods:
1. Students will work in groups. A measured plot of canopy and non-canopy in Western Woods will be provided for each group.
2. Each group will randomly sample plant species within each measured plot.
3. In order to obtain a random sample, we will use a tennis ball and tape measure. Students will stand within their selected area. Then, a tennis ball will be thrown into the woods. From the point that it lands, the group will measure 5 feet on each side. In this way, we will have a 100 foot squared area in every sample.
*
In this way, our data will be more comparable because all samples will be taken from be of the same area (100 ft. squared).
4. Record in the data sheet the following:
5. Steps 2 and 3 will be repeated until each group has at least 4 samples. In this way we will have 16 samplings from canopied and un-canopied environments.
(Background information)
Students should understand that the light meter should be placed close to the ground when taking a reading, and the distance from the ground should be consistent in all measurements. It is also important to note that canopies can contain different levels. We will refer to three of these: a canopy, a sub-canopy, and no canopy. Be certain to fill this category out on the data sheet, as the different levels can say a lot about the results obtained. Because we are not distributing species diversity indices to all students, it will be impossible to make exact calculations when determining species diversity. However, we are making the assumption that students have a general knowledge of plant diversity. If students need help in differentiating, our group members will be available to help.
Analysis
Species diversity indices are formulas which give a quantitative way of analyzing the information on our data sheet. The diversity index we chose (Simpson’s Diversity Index) is fairly simple. Because it is more generalized than some of the other indices, we felt that it would be appropriate for our needs since we are not dealing with specificities of plant diversity. The formula:
D= 1/(sum (pi2 ))
means this:
D is the diversity, pi is the proportion of the certain species relative to all species. For example, if we use our data sheet as an example, we may find that:
Species number
1. 50
2. 100
3. 75
4. 200
5. 25
Since the total amount of species sampled is 450 (50+100+75+200+25), then you divide these numbers by 450. Your answers would be:
50/450= .11
100/450= .22
75/450= .16
200/450= .44
25/450= .05
These numbers must then be squared: (.0121+.0484+.0256+.1936+.0025)= .2822
Then you divide 1 by this number, which equals (1/.2822) 3.54
In this way, the D values can be compared to calculate different diversities. This, of course, was only a sample; the numbers will not always be so nicely divisible.
THE GRAPHS:
The following are the graphs that we made. The first is the species diversity index readings, and the second is the lux measurement of light intensity. The light intensity proves that there was considerably less light in the canopied environment than the non canopied environment.
*THE ABOVE IS THE GRAPH OF SPECIES DIVERSITY INDICES PER SECTION
*
THE ABOVE IS THE LIGHT INTENSITY GRAPH
*THIS IS THE DATA SHEET THAT WE USED TO FORMULATE CALCULATIONS
*THIS IS THE MAP OF WESTERN WOODS USED TO DETERMINE WHICH SECTION
OF THE WOODS WE SAMPLED
INTERPRETATION OF RESULTS:
(see analysis section above for explanation of how the data was analyzed)
Our interpretation of the data was that canopies have little effect on the plant variation in western woods. We decided that there must be a balance of high and low light intensity. Because western woods does not have a high intensity light, which might exist by more equatorial regions, western woods is affected very minimally by canopies. The averages of the species diversity index readings for canopied environments was: 2.66 and the species diversity index readings for uncanopied environments was: 2.77. Therefore, there was actually more diversity in the uncanopied environments than in the canopied environments. However, the difference in the indexes was so minimal that it is not fair to assume that uncanopied environments foster more plant diversity either.
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