Lab 8
Plant Community Analysis
Assessing plant community parameters presents different challenges from those
pertaining to animal communities. Plants do not move around, and are usually not
cryptic making them easier to count than most animal populations. On the other hand, the
concept of ‘individuals’ may be more complicated when dealing with plants (e.g. is the
concept of individuals really relevant when dealing with an expanse of grass?)
The simplest form of sampling for sessile communities is Quadrat analysis;
essentially a sampling frame of fixed dimensions is placed at multiple, random sites
throughout the community and all the individuals within each quadrat are counted.
However, the quadrat method runs into difficulties when the physical sizes of plant
species varies widely (e.g. a 1 square meter quadrat frame might be adequate to an
analysis of spring wild flowers, but would be useless in a mature oak forest).
In this lab, we will compare the results of two alternative methods of quantifying
vegetation.
Photographs of characteristic CSS shrubs are on-line at:
http://bio.lmu.edu/socal_nat_hist/plants/comms/coas_sc.htm
Line Intercept Analysis.
LIA yields density, areal cover and frequency data. You will lay out a 30 m tape
in a randomly chosen direction, and record the identity of each plant that physically
touches (“intercepts”) the line, the distance along the line that the individual extends, and
the maximum width of the individual perpendicular to the line. The data required is:
L = total transect length (30 m)
N= total number of individuals of each species.
Sum I = total of all the intercept lengths for each species
S = number of transect segments in which species x occurred
Sum 1/M = total of reciprocals of maximum plant widths
From these data:
Density = (Sum1/M)/L
Relative Density = (density of species x / density of all species) *100
Dominance = (Sum I/L) * 100
Relative Dominance = (dominance of species x / total dominance of all species) x 100
Weighted Frequency = ((Sum 1/M)/ n )* S where n is the number of M values.
Relative frequency = (Weighted frequency of species x/weighted frequency of all
species)*100
Point Quarter Analysis
In this method, a random point in the community is selected, and two
perpendicular lines are laid out on the ground dividing the area into four quarters. Within
each quarter, the nearest plant is located and identified, and the distance from the starting
point to the center of the individual is measured. The area (or width) of the individual is
also recorded. The data is:
d = point to individual distance
D = average point-to-individual distance for all members of all species.
Density of all species = 1/D2
relative density = (individuals of a species /total individuals of all species )* 100
density = (relative density of a species /100)*total density of all species
average dominance for a species = sum of basal area of a species / number of individuals
of a species
dominance = density of a species * average dominance of a species
relative dominance = (dominance for a species/ total dominance for all species)*100
frequency= number of points at which a species occurs/total number of points sampled
relative frequency = (frequency for a species/total of frequency values for all species
)*100
Using data for Coastal sage scrub plants from the Bernard Field Station,
compare the values obtained using the two methods. Do they differ? If so, how do
they differ? What does this suggest about the relative effectiveness of the two
techniques? Are the sources of error the same in both methods?