Methods in EEC (BIO 221B)
Dr. Jim Baxter
Dept. of Biological Sciences
Vegetation Sampling Using the Quadrat Method
A quadrat is a frame that is laid down to mark out a specific area of the community to be sampled.
Within the quadrat frame, the occurrence of plants is recorded using an appropriate measure of
abundance. Quadrats may be square, rectangular or circular and they may be of any appropriate size.
The quadrat method can be used in virtually any vegetation type to quantify the plant community.
However, some vegetation types are best sampled using other techniques (e.g., a point‐frame for
grasslands, or point‐quarter method for forests).
Because a single quadrat cannot be expected to sample a community adequately, repeated quadrat
samples are taken. Typically, the community is divided up into sub‐areas dependent on topography,
aspect, other physical features – and apparent floristic differences – and these are sampled separately;
within sub‐areas, quadrats are located randomly. This type of sampling approach ensures a
representative sample of the different physical and floristic features of the community. This type of
sampling is called stratified random sampling. Once collected, the sample data from all quadrats are
added together and are considered to constitute an adequate sample of the community.
When sampling vegetation using quadrats, different measures of abundance can be quantified to
assess the influence or “importance” of each species in that quadrat. For example:
Counts – a simple tally of the number of individuals of a species
Cover – the percent (%) area of the quadrat occupied by a plant species.
Density – estimated by quantifying the number of individuals of a species per unit area.
Frequency – the proportion of quadrats sampled in which the species is represented.
Overall cover, density and frequency estimates are then calculated for each species from the entire
data set by combining all of the quadrats together, as indicated on the left side of the table below. To
determine the proportional representation of each species relative to the entire plant community,
relative cover, relative density and relative frequency values can be computed (right side of table
below). For example, relative cover is the proportional cover of an individual species as a percentage of
total plant cover; hence, it is expressed as a percentage, ranging from 0 – 100%.
“Importance” is a measure of overall influence of a plant species in the community. An Importance
Value (IV) for each species is derived from the combined contribution of the relative cover, relative
density and relative frequency of each species in the community. Because it combines relative cover,
density and frequency, importance values range from 0 – 300. The table below summarizes the key
quantitative community measures derived from the quadrat method.
Abundance (Ai) =
Cover (Ci) =
Density (Di) =
total number of individuals of species i
Total % cover of species i
Relative cover (RCi) =
Ai
Relative density (RDi) =
Area
1
Cover of species i
Total plant cover
Di
Total plant density
Methods in EEC (BIO 221B)
Frequency (Fi) =
Dr. Jim Baxter
Dept. of Biological Sciences
# of quadrats with species i
Relative frequency (RFi) =
Total # of quadrats sampled
Importance value (IV) = RCi + RDi + RFi
Fi
Total plant frequency
IV ranges between 0 ‐ 300
Quadrat size and shape
The size of the quadrat that is appropriate depends on the layer or type of vegetation being
sampled. For example:
Vegetation
Dimensions
Area (m2)
Mosses
10 x 10 cm
0.01
Herbaceous
31.6 x 31.6 cm
0.1
Forest floor herb
1x1m
1
Shrub
3.16 x 3.16 m
10
Forest/tree
10 x 10 m
100
Of course, these may be varied for convenience of measuring, though always being sure to use the
same size quadrats for any comparisons.
The shape of a quadrat can be square, rectangular or circular. Each one has advantages and
disadvantages. Two main considerations must be taken into account when deciding on which shape to
use. The first has to do with edge effect, which occurs when researchers must make subjective
decisions on when a species is considered “in” or “out” of the quadrat. This bias reduces the accuracy of
the sample. Circular quadrats have the least edge to interior ratio and so have the least bias. They are
also easy to define in the field. However, this shape may not be advantageous in dense plant
communities. Square and rectangular quadrats are sometimes easier to define, since tape measures can
be strung through dense vegetation stands. Rectangular quadrats are considered a good compromise
because they have a lower perimeter to interior area than a square and also can capture more linear
distance along the ground. This distance property can more effectively capture environmental variation
than square quadrats.
Determining the number of quadrats to sample
Using too few quadrats will result in an incomplete or inaccurate representation of all the species.
Using too many will be a waste of time and effort. A species‐area curve (discussed in lecture) can be
used to determine an adequate number of samples. To do this, keep a count of the cumulative number
of species added with each additional quadrat sampled. Where the curve levels off, the number of
samples is adequate.
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Methods in EEC (BIO 221B)
Dr. Jim Baxter
Dept. of Biological Sciences
Example calculations
Area sampled: 5 m2; No. of quadrats = 50; Size of quadrats = 0.1 m2
Species
A
B
C
D
E
F
G
H
Totals
# Individuals
139
78
55
3
3
1
1
1
281
% Cover
15.5
10.1
4.3
1.0
0.6
0.2
0.2
0.1
32.0
# Quadrats
31
21
15
2
3
1
1
1
75
All Species
Total density
281/5 = 56.2 plants/m2
Total cover
32.0
Total frequency
75/50 = 1.5 plant
species/quadrat
Calculations for Species A
Mean cover = 15.5/50 = 0.32
Relative cover = 15.5/32.0 x 100 = 48.4
Density = 139/5 = 27.8
Relative density = 139/281 x 100 = 49.5
Frequency = 31/50 = 0.62
Relative frequency = 31/75 x 100 = 41.3
Importance value = 48.4 + 49.5.0 + 41.3 = 139.2
Cover classes
Because plant cover is quite heterogeneous and estimations of cover made by different researchers
can be biased, different cover classes have been developed to translate cover estimates into classes.
Several cover class methods are given in Table 9‐2 of the Barbour et al. (1999) Ch. 9 handout. One of
these, Daubenmire cover classes, is given below.
Cover Class
Range of Coverage
Midpoint of Range
1
0 ‐ 5%
2.5%
2
5 ‐ 25%
15.0%
3
25 ‐ 50%
37.5%
4
50 ‐ 75%
62.5%
5
75 ‐ 95%
85.0%
6
95 ‐ 100%
97.5%
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