Assessing an Ecosystem's Plant Biodiversity

This lab exercise allows students to explore an ecosystem's plant diversity and to mathematically determine the dominant vegetation, and to estimate the relative presence of the invasive species in the study area.

Materials: measuring tape, compass, surveyor's tape, tree and plant identification guide, map of site, DBH measuring tape (for accurate measurements of trunk diameter, available from arborists and tree services), and binoculars.

Methods:

The Plot Sampling and Percent Cover method will be used for vegetation analysis.  Small sample plots will be selected inside the larger study area.  Frequency, density, and dominance of mature canopy trees will be used to calculate relative importance of each canopy species.  Under story trees will be counted to determine density (number of trees per unit area).  Other plants are small and numerous, and therefore difficult to count each stem.  Percent of the ground covered by this species will be used to estimate the population.  Sample plots will be located using the Stratified Sampling method.  The class will be divided into 4 groups. Each group will run their own transect line and establish and evaluate 5 quadrats or sample plots.  Class data will be compiled if time permits.

1.  Using a compass, run a transect line from an assigned start point using the stratified sampling method.  Note compass direction.  Because of the narrow boundaries of the forest community, the compass direction of the transect line may change during the study.  Use surveyors tape to mark the transect line every 50 feet at the corner of each new quadrat by tying tape around the nearest tree at eye height.

2.  At the start point of the transect line, establish quadrat #1 to the LEFT side of the transect line. Create a 15 m X 15 m (50 ft. X 50 ft.) quadrat with right angles at the four corners.  Mark the four corners with surveyors tape.

3.  Within the quadrat, identify and record the common name of each canopy tree.

4.  Determine the diameter-at-breast height (DBH) for each canopy tree using the DBH tape.  Take the measurement 4.5 ft. from the base of the tree/ground.

5. To determine the understory community within each quadrat, establish a nested 20 ft. X 20 ft. quadrat within the 50' X 50' plot, starting at the lower corner of the transect line.  Record the common name of each understory tree.

6.  For ground cover plants, estimate and record the percent of the plot area covered by the plants.

7.  From the far corner of quadrat #1, extend the transect line 50 ft., and establish quadrat #2 to the RIGHT of the transect line, as above.  Continue to move down the transect line, establishing quadrats # 3, 4 and 5 every 50 ft. with odd numbered quadrats located to the left side of the transect line and even numbered quadrats to the right side of the transect line.  Place the base of each quadrat on the transect line with the quadrat axis projecting into the forest away from the transect line, with right angled corners.

8.  Repeat the data collection on canopy and understory trees and on the English ivy groundcover.

9.  To make generalizations about density, relative importance and percent cover in the larger study area, compile the results for your sample plots with the data from one other group to get a total of 10 data sets.  Enter the data for each plot in a spreadsheet entitled Table I Raw Data.

10.  Set up a separate data table for the understory calculations.  Calculate the average density for each understory tree species using the formula below and graph the data.

Average species density =

(density in plot 1) + (density in plot 2) + (density in plot X)
total # of plots

11.  Set up another data table for the canopy calculations.  Create graphs for relative frequency, relative density, relative dominance, and importance values of the canopy trees.

a.  Determine the frequency or the percentage of quadrats occupied by a given species.  Frequency measures how often a species occurs in a community.  In general, the higher the frequency, the more important the plant is in the community.

           Formula: (# of plots in which species occurs/total # of plots) X 100

b.  Determine relative frequency by comparing the frequency of occurrence of the species with the frequency of occurrence of all species present.  Relative frequency can indicate the distribution of species in a community.  If the value for a species is between 0% - 30% than the species tends to occur in clumps.  If the value is between 31% - 80%, the species tends to be randomly distributed.  If the value is between 81% - 100%, the species tends to be uniformly distributed throughout the community.

          Formula: (frequency of a species/total frequency of all species) X 100

c.  Determine the density or the number of a certain species per unit area.

          Formula: (# of a certain species/total area sampled) X 100

d.  Use the density values above to determine the relative density.  Relative density is the density of a given species in relation to the total density of all species.  This determines which species is the most abundant.

         Formula: (density of a species/total density of all species) X 100

e.  Determine the dominance or the proportion of the total area occupied by a species.  Using the DBH measurement (diameter), convert to area to calculate dominance.

         Formula: total area covered by a species/total area sampled

f.  Relative dominance, like relative frequency and relative density, gives a better indication of the importance of a species than does the absolute value.

        Formula: (dominance for a species/total dominance for all species) X 100

g.  Determine the importance value for each species, by adding the relative frequency, relative density, and relative dominance for that species.  This value gives a good overall estimate of the importance of the species in the community.

12.  Graph the percent cover for the ground cover plants in all quadrats.

Discussion: Describe the purpose of the study and a description of your transect line.  Describe what the calculations mean and what they tell about the community.  How extensive was the coverage of any invasive plant species?  What impact might this species have on native ground cover and on canopy trees in the park?
 

This lab can be used as a part of an AP Environmental Science Class.

Contributed by: Paula Wang, Sidwell Friends School