Field Trip to Asbury Woods & the James Preserve
This week we will sample the forest at Asbury Woods and the James Preserve, a tract of
property owned by Mercyhurst that borders the Asbury Woods Nature Center property. We will
be studying two forest communities there to determine whether tree diversity is higher in oldgrowth vs. secondary forest. We will take data to characterize the tree community in these two
community types and analyzing data on abundance and cover of the different species. Analyzing
the diversity of plants in a community can tell you a lot about its history, its ecological health,
and what kind of other organisms (e.g. animals & fungi) live in that community. Which type of
forest would you expect to have higher diversity? Why?
There are several good methods of measuring the structure and diversity of a forest
community. The most thorough method, of course, would be to count and measure every tree and
shrub in the community, but this is not practical. Instead, sampling methods are usually used.
Another method called the belt transect samples the forest in long, skinny plots. Today we will
use one of the quickest methods, called the Point-Quarter method.
Advantages and disadvantages: The point-quarter method is simple and rapid (once
you can identify the trees), and works quite well. The underlying assumption is that individuals
of all species are randomly dispersed. Although this assumption may not be true, it does not
seem to produce significant error, except where a deviation from overall randomness is quite
obvious. In spite of this, we can find a good estimate of relative density and relative dominance
of the trees within each forest community.
Methods:
1. Divide into groups of 3-4; instructor will tell you where to begin sampling the forest
2. Choose a start point by randomly choosing a number
between 1 and 10, and then sample at 10 m
intervals along a line transect passing
through the stand.
3. At each sample point, mark a point in the
ground.
4. Divide the working area into four
quarters or quadrants by visualizing a
gridline predetermined by compass bearing,
and a line crossing it at right angles, both
passing through the point (Figure 1).
5. Select the tree in each quarter that is
closest to the point. Record its distance
from the point, diameter at breast height
(DBH), and species. The tally sheet will
thus contain data for four trees at each point,
one from each quarter.
6. Tally at least 5 such points.
Location:
Old Growth Forest
Point
Tree Species
1
1
1
1
2
2
2
2
3
3
3
3
4
4
4
4
5
5
5
5
Distance from Point
(m)
DBH (cm)
Location:
Secondary Forest
Point
Tree Species
1
1
1
1
2
2
2
2
3
3
3
3
4
4
4
4
5
5
5
5
Distance from Point
(m)
DBH (cm)
Calculations (you will be analyzing the entire class data set next week in lab)
1. Add all distances in the samples and divide the total distance by the number of distances to
obtain the mean distance of point-to-tree.
Mean distance=
Total distance
Number of distances
2. Square the mean distance to obtain the mean area covered on the ground per tree.
3. Calculate total density of trees per hectare:
Total density of trees per hectare = one tree
Mean area (m2)
covered per tree
X
10,000 m2
1 hectare
4. Calculate relative density for each species:
Relative density =
Number of individuals of species A
Total individuals of all species
X
100
5. Calculate density for each species:
Density = Relative density of species A
100
X
Total density of trees per hectare
6. Calculate relative cover for each species:
Relative Cover =
Total basal area of species A
Total basal area of all species
X
100
7. Calculate the Shannon Index (H’) and the exponent of the index (eH’). See the attached sheets
for description and examples.
Evaluation of results – Asbury Woods Forest
Summarize your results here:
1. Forest density
– Old Growth Forest _________________________________
-- Secondary Forest _______________________________
2. Species Present and Densities
Old Growth Forest
Species
Rel. Density (freq)
Total number of species_____
Density
Rel. Cover
Secondary Forest
Species
Rel. Density (freq)
Density
Rel. Cover
Total number of species_____
3. Calculate the Shannon index (H´ and eH´) for each forest type. An explanation of the
Shannon Index and the method for calculating it is described in the attached handout.
Old Growth Forest:
H´ =
eH´ =
Secondary Forest:
H´ =
eH´ =
SHANNON INDEX: Biodiversity and evenness calculated using the Shannon index (H´), one of the most
popular ways of calculating diversity.
Shannon’s index measures both richness (the number of species) and evenness, or how evenly individuals are
distributed among species. High values of H´ denote high biodiversity. Shannon’s index is advantageous over
simply counting the total number of different species, because species count is greatly affected by how big your
sample size is. The greater the sample, the more rare species you find. H´ is superior because it is calculated
from proportions (or frequencies), and rare species contribute very little. Therefore, this index is relatively
insensitive to the random inclusion or omission of rare species that happens with any sampling effort. The
equation for Shannon’s index is:
OR
where the p’s are the frequencies for each species category, and S is the total number of species.
NOTE: Therefore, p= the number of times that species was found divided by the total number of trees
counted. (In other words, p= that species’ relative density from the whole class data, expressed as a
decimal, not as a percent.)
Shannon’s index is unitless and has no true biological meaning. However, if we take the exponent of this index,
or eH´, we have an “equivalent number of equally common species.” In other words, eH´ is a type of weighted
number of species present in your sample for which very common species contribute much more than do rare
species to the numerical “diversity” estimate.
Here are two examples of how to calculate the Shannon Index:
Example 1: Let’s say there were only five different species found in the forest (whole class data). And let’s say
that each species was found in the frequencies listed on the table below. (Note that the first species, in our
example Red Maple, accounts for 70% of the observations.)
Species
Red
Maple
Red
Oak
Sassafras
Ash
American
Elm
Frequency
0.70
0.12
0.06
0.10
0.02
H’ = -[(0.7) × ln(0.7) + (0.12) × ln(0.12) + (0.06) × ln(0.06) + (0.10) × ln(0.10) + (0.02) × ln(0.02)]
H’ = -[ (-0.2497) + (-0.2544) + (-0.1688) + (-0.2303) + (-0.0782)] = 0.9814
eH’ = 2.67 equivalent species
*So this is not a very diverse forest
Example 2
In contrast, in Example 2 below, note that all 5 species are equally abundant; consequently eH´ equals 5 species.
Thus, this is a high diversity community – as high as it can get for a 5 species community. In fact, the maximum
value of eH´ will always be S, the number of species, when all species are equally abundant, and the actual value
of eH´ should always be compared against S.
Species
Red
Maple
Red
Oak
Sassafras
Ash
American
Elm
Frequency
0.20
0.20
0.20
0.20
0.20
H’ = -[ 5 X (0.2) × ln(0.2)]
H’ = -[ 5 X (-0.3219)] = 1.61
eH’ = 5.00 equivalent species