Quadrat and Point Frame
Grasslands in Hong Kong are mainly found
in the east and west parts of the New Territories, ranging from single specie
dominant grassland to high biodiversity grassland. There are many factors that
can affect the distribution, types, abundance and the growth of grasses, include
soil pH, soil moisture, soil types and textures, temperature, organic and water
content, light intensity, human impacts and etc.
The commonly use methods to measure the vegetation cover in grassland are
quadrats, of two types: square quadrat and point frame quadrat.
Square Quadrat
Square quadrat is widely used in determining the distribution of plant
communities (composition of plant species). Quadrat is a sampling unit of a
known area. Square quadrat is basically a big square frame which consisted of
many equal smaller squares. Large quadrats can be laid out using string and pegs
as in the case of sampling grassland and woodland communities. More commonly
used are the 1 m2 and 0.25 m2 frame quadrats (see diagram
below).
What
size of quadrat should we used?
A small quadrat has generally been found to be more efficient than a large one when the dispersal of the population is aggregated. Since many small quadrats cover a wider range of habitat than a few large ones, the sample will be more representative.
How
many quadrats in each sampling area?
Many people believe that the more measurements you make, the more accurate
results you have. However, sorting and counting all the species in a very large
sample can be tedious and time consuming. In addition, when you plot the number
of species found against the number of quadrats used, eventually a point is
reached when all the common species have been identified and the increase of
quadrats sampling will not increase the number of species found (see graph
below).
There is not a rule saying that this size of area has to do this number of
sampling. However, for your convenient, here is an example. If the sampling site
is about the size of a football pitch, using a 0.25 m2 quadrat, 25
sampling points may be suitable to give representative results, and so on.
How
to carry out sampling using square quadrat?
Using square quadrat for sampling is easy and the followings are the suggested procedures: (based on 20 samplings)
1.
identify the sampling site(s)
2. set up two transect lines (x and y axis at the edge of the site)
3.
divide the two transect lines into 99 equal distance points each
4.
Use a random table (see table) to determine the x and y coordination
number. For example, the numbers are 02, 06. Find the point 02 on the x axis and
06 on the y axis, the point where these two lines meet is the point for
sampling.
5.
After the sampling point is identified, place the bottom left corner of
the square quadrat onto the sampling point.
6.
There are 25 small squares on the quadrat and therefore each whole small
square represent 4% of the population. Now find the species present in the
quadrat and count the number of squares covered by each specie. If the specie
covers the whole small square, counts as 4% but if the specie only covers part
of the small square (no matter how big part it is), count this part square as
2%. At the end, sum up the total coverage of that specie within the big quadrat.
For example, the specie Common grass has covered 4 whole small squares and 4
part small squares, then the total coverage is 4 * 4% + 4 * 2% = 24%. [Common
grass has 24% coverage on this quadrat].
7.
After identifying all the species and their coverage within the quadrat,
use the random table to choose another set of coordinates and carry out the
sampling again, until the total sampling quadrat number is 20.
8.
For the coverage % of the whole area, average the results of each
species. Identify the number of species found in this area (biodiversity) and
their abundance.
9.
These types of surveys are good at measuring the plant biodiversity and
abundance between different sites, or compare the vegetation cover at different
seasons. It is often to be used in conjunction with measuring other parameters
stated at the beginning.
Point Quadrat
The point quadrat was developed to give an objective assessment of cover and
gives reliable results for low growing vegetation. The apparatus consists of a
free standing frame with a row of ten sliding pins, the points of which can be
lowered down onto the vegetation. As each point is lowered to the ground, a
record is kept of the number of times out of ten (10 pins per frame) that each
species of plant is touched, i.e. the number of “hits”. The number of point
quadrats used is dependent upon the same considerations taken into account when
using square quadrats. The most convenient number per sampling area is ten, thus
giving the number of units per 100 points.
Percentage
cover = Hits
´
100
Hits + Misses
For example, we used 15 point quadrats per sampling area and the number of hits
for one particular species is 20. The total number of hits and misses will be
150 (10 points per quadrat).
Therefore % cover = 20 /
150 * 100 = 13.3%.
The placing of point quadrats within the sampling area
may be done by using random coordinates as for square quadrats. If the
percentage cover values for the different species recorded are added together,
the total often exceeds 100%. This is because a pin may well touch more than one
species on the way down. If the same species is hit more than once by the same
pin it only counts as one hit.
Example
At Sha Lo Tung, a vegetation survey using point quadrat was carried out in May
and the species found and their coverage were recorded, as follows:
|
Number
of quadrat used |
Vegetation
identified |
Number
of hits per quadrat |
|
1 |
Common narrow-leaved
grass Common board-leaved
grass Dandelion |
6 8 2 |
|
2 |
Common narrow-leaved
grass Common board-leaved
grass |
7 2 |
|
3 |
Dandelion Common board-leaved
grass |
1 10 |
|
4 |
Common narrow-leaved
grass Common board-leaved
grass Daisy |
3 8 3 |
|
5 |
Mimosa Dandelion Daisy |
3 4 2 |
|
6 |
Common board-leaved
grass Common narrow
board-leaved grass Golden rod Dandelion |
5 7 1 2 |
|
7 |
Mimosa Common board-leaved
grass |
3 8 |
|
8 |
Common narrow-leaved
grass Common board-leaved
grass Daisy |
7 5 2 |
|
9 |
Common narrow-leaved
grass |
10 |
|
10 |
Common narrow-leaved
grass |
10 |
Find out the coverage of all the species listed in the table for the whole
sampling site, thus determining which is the most dominant specie and which is
the least common specie.
