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There are two major methods of sampling flora:
using sample units (plots), or
using plotless sampling methods
Sample
units (plots)
Sample units or plots comprise a subset of the total population
from which measurements are taken during sampling. Sample units
are distinct, non-overlapping entities, such as quadrats or transects,
individual plants, branches within a plant, etc.
- Quadrat: a 2-D shape (e.g. square or rectangle, or other
shape) used as a sampling unit. The choice of dimensions and shape
of the quadrat will affect the precision and accuracy of the parameter
estimates subsequently computed (see below). Sometimes a tape
is laid on the ground, more often a frame is used to define the
quadrat boundaries.
- Transect: Long, thin quadrats that are used to sample
along narrow ecotones or to sample across ecotones to get better
averages.
Line transects feature only a length dimension, usually defined
by a tape stretched across the area to be sampled.
Belt transects have a width as well as length.
Pace-transects are established when the observer strides
along an imaginary line across the sample site, and uses their
foot placement to determine specific sampling points.
Transect sampling in areas with broad-scale vegetation patterns
increases the likelihood of encountering species that occur at
a very low density (Krebs 1989, Caughley, 1977).
Line intersects can be used in aerial surveys. In difficult terrain,
line transects allow estimates without having to locate quadrats.
In open terrain, line transects allow sampling over large areas
to provide more precise estimates.
Assumptions
1. Individual vegetation patches are randomly oriented with no
preferred orientation of the major axis of the patch.
2. Sample lines are randomly oriented across the study area.
- Gradsect: This is the deliberate selection of transects
which contain the steepest environmental gradients present in
an area to ensure sampling of the greatest range of vegetation
(Gillison & Brewer, 1985). This is a cost-effective method for
sampling large areas (Austin & Heylinger 1991).
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weblinks
http://www.ffp.csiro.au/nfm/mdp/
bbproj/survdes.htm
The gradsect method was used in this study at Bateman's Bay
to describe the floristic variation of the forest environment.
Climatic, lithological and topographic characteristics were
used.
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5.2 (i)
Please describe why and when a gradsect would be used.
Give an example of the application of this method, either
from your reading or work experience.
What are the limitations of this method?
We will discuss this topic in our next online tutorial.
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Plotless
sampling methods
Due to the fixed nature of plants, plotless sampling methods can
be used to estimate density, species composition, growth and environmental
factors. Plotless sampling methods are based on the random selection
of points within a particular survey area and can be summarised
into four main techniques (from Goldsmith et al. 1986):
- closest individual method
Distance is measured from each random point to the nearest individual.
- nearest neighbour method
Distance is measured from an individual to its nearest neighbour.
- random pairs method
Distance is measured from one individual to another on the opposite
side of the sample point.
- point-centred quarter (PCQ) method
Distance is measured from the sampling point to the nearest individual
in each quadrat.
- commonly used in conducting forest inventories in moderate
to dense stands of woody plants that are shrub size or larger;
also applied in bunch-style grassland
- will underestimate density when plants are aggregated.
From the mean value of each of these measurements the mean area
and thus the density of the species can be calculated: Density =
square root (mean area/2)
Sample plants are identified, and some measure of dominance (e.g.
basal area) is taken for each, providing estimates of total and
relative dominance and frequency, by species, for the sample area.
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5.2 (ii)
Please describe when and why a plotless (or distance) method
of sampling would be used. Describe an example and the use
of this method, either from your reading or work experience.
What are the limitations of this method?
We will discuss this during out next online tutorial.
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back to sampling concepts
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