| HOME |
| The Mission |
| The Refuge |
| Field Prep |
| Analysis |
| Data |
| The Crew |
Dendrochronology: Some excellent off-site discussions... A look at tree rings - A little more on dendrochronology Diversity Index: A diversity index is a good way to compare diversity between habitats or compare within one habitat over time. The Shannon Index (H) is one of MANY diversity indices. There is no "good" Shannon number - it is just a way to compare between habitats. For example, in 2008 the VT crew sampled two Mason Neck habitats.They found H = 1.1 in aVirginia pine stand, and 1.5 in an "old growth" stand. This indicates the old growth is more diverse than the Virginia pine stand. A related value - the evenness (E) value - can indicate if any one species dominates a plot. In the Virginia pine stand, the VT crew found E = 0.7 (Virginia pine and sweetgum dominated, with other species present in low numbers). In the old growth, E = 0.9, indicating that species present were present in almost the same numbers. Diversity does not always mean more species. In this sample, nearly the same numbers of species were found in both habitats! The big difference is that a few species dominated in the Virginia pine stand. The formula for the Shannon index is: YIKES! Actually, the calculation is not too bad. Perhaps the best way to look at this formula is with an example (calm down). An example: Let's consider a sample plot from a young stand. In this plot we found 4 species of trees:
Step 1: First calculate p (the proportion of each tree), which equals the number of trees for each species divided by the total number of trees (for ACRU: 26 / 138 = .188). Step 2: Next, take the natural log of this number (for ACRU ln(.188406) = -1.66916). Step 3: Multiply these two numbers together (for ACRU 0.188406 x -1.66916 = -0.31448). Step 4: Sum the numbers for step 3 for all species and drop the "-" to get the Shannon Index (H). The higher the H, the more diverse the sample.
Step 5: To calculate the Evenness (E) of the species in this plot, divide H by the ln(species count). In this case 1.10264/ln(4) = .795386. BTW, if there were 35 of each species in your sample, the evenness would be 1... or if there were 5 of each species in your sample the evenness would be 1! Basal Area: Foresters often use the term "basal area" to talk about how much wood is on a forested site. If you tell Ranger Nick that your site has 14 square meters of basal area per hectare, he would look at you funny. However, if you do the quick math in your head (multiply by 4.305564) and tell Nick you have 60 square feet per acre, he would picture a very sparse site with few trees. If you tell him you have 220 square feet per acre (51 square meters per hectare), he would picture a dark and dense stand.
If you know your sampled tree diameters, you can calculate basal area. This is more math... don't panic. For each tree in your sample, figure out the area of wood at DBH (area of a circle = Π x radius2). Sum all of your areas to get total basal area per plot. If you sampled a 1/20 acre plot, multiply by 20 to get total basal area per acre.
Making a Scientific Poster: A scientific poster is a common method of presenting experimental results. A poster is very different from a scientific paper. Think of it as if you were walking by two televisions. One television has scrolling text with the main point buried somewhere in all that text. The other is colorful, with lots of pictures and (hopefully) a good point. Which would you stop to look at? Learn how to make an effective poster (courtesy NC State). See examples of environmental posters presented at the VT Dean's Forum on the Environment.
Download an image from your plot Some Actual VT DATA: Some numbers from the baseline data collected by VT Fall 2008, 8 plots per location...
Old Field Virginia Pine Stand Species (early succession):
Never Farmed Woodlot Species (later succession):
Which site had the higher Evenness (E) value?
Diameter Distribution:
|
| Last Updated: October 2009 | Contact Us |