The purpose of this laboratory experience was to understand the effects of trees on plant growth. Trees are a crucial element of our ecology for many reasons: one, they are large producers of oxygen for humans to breathe; two, they provide us protection from the sun; three, they are a habitat and potential source of food for many species of birds, insects, mammals, and others; and four, their shade allows plants that can’t take too much sun to still survive.
Simpson’s Diversity Index has been defined more than one way, but the intuitive way involves counting the total number of all species (N) as well as the number of each individual species (n), dividing n by N, then squaring it. Do this for all species then add them together. To “standardize” this number, subtract it from 1. This will give a number between 0 and 1 such that the closer to 1 the number gets, the more diverse the area is.
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To summarize:
D = ? (n/N)2 then 1-D = Standard Diversity Index
For this lab, the transect method was used to examine a 5m area around an elm tree (probably an American elm) by quadrants. was examined, the number of species and the number of individuals of each species were counted, and the height of the grass was measured at intervals from the tree with a measuring tape and a ruler.
The following species were identified and counted: 2 types of mushrooms, mock strawberry, wild garlic, nutgrass, and crabgrass. Farther from the tree (about 4 m) grass began to grow in relatively large patches and when 5 m was reached, almost full sunshine was obtained and the grass was thick, but still containing weeds such as crabgrass and wild garlic. Total number of species: 6. The following table outlines the species findings:
? (n/N)2 = 0.31 1-D = 0.69
Since Simpson’s Diversity Index is 0.69, we can say that the area around the tree is fairly diverse.
Measurements of grass height showed that grass was very short, brown, or nonexistent until just past the 3 m point. Thin, small areas of grass began to appear but some looked yellow. Viable, thicker patches of grass grew about 4 m away from the tree, and by 5 m the grass was thick and in good health, although there were many weeds still. Thus, this experiment determined that, although trees give too much shade for plants that prefer strong sunlight (such as most non-weedy grasses), there can still be a diverse environment under the tree.
