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Part 9: Collecting Monitoring Data
Density Method
Density is a measure of the number of plants (or members of a
single species) per unit area. The method described here is suitable
for all rangeland plant forms, although it is most often used to
estimate density of larger plants (trees and shrubs) and perennial
forbs.
Density for perennials is generally not as affected by variables
such as precipitation, but the technique is sensitive to changes
over extended periods of time. Monitoring density can provide valuable
information about seedling emergence and survival, as well as information
about how vegetation responds to management practices.
This method does present some challenges. It can be difficult
to differentiate between individual plants when they are rhizomatous
or sod forming. Sampling may be time-consuming in areas of thick
plant growth. In order to collect useful data using one standard
(40 cm X 40 cm) quadrat size, sampling is best limited to a few
key species.
Criteria
- Units to express density should be selected so that actual
plant numbers are easy to visualize. For example, it is more
meaningful to express an infrequently occurring shrub as 50 shrubs/hectare
rather than 0.005 shrubs/square meter, whereas 15 seedlings/square
meter is easier to imagine than 150,000 seedlings/hectare for
the density of an abundant annual grass. Density measurements
are sometimes unsuitable for the herbaceous layer, especially
when there are numerous plants to count or when identification
of individuals is difficult. Nonetheless, density is regularly
used to evaluate seedling emergence and survival in a rangeland
reseeding program.
- Species composition can be determined from density studies
by calculating the contribution of each species to the total
plant count of the sample.
- Since density data remains fairly stable from year to year,
it can be a useful indicator for effects of management decisions.
- Determine before hand which plants will be included in the
study and how you will determine whether or not plants are counted
as inside the quadrat.
Equipment
Conducting the study and study sites
- Density studies can be set up using a baseline,
linear or block plot design.
- Sites should be adequately marked and documented.
- Establish photo plots for general view and close-up photos.
Collecting data
- Establish a transect by laying out a tape and marking the
area with a stake.
- Count the number of individuals of each plant species of interest
having at least 50% of their base in the quadrat and record on
the Density form.
- Determine beforehand whether or not plants that fall on the
edge of the quadrat will be counted as inside or outside.
- Collect data from randomly placed quadrats along the transect;
quadrats are usually placed at every pace.
- You may need to alter your quadrat size to ensure that most
contain more than one key species.
Data Analysis
- Calculate the estimated average density for each key species
per quadrat.
- Extrapolate this calculation to an appropriate plot size
that makes sense (such as plants per hectare or per
square meter; see above discussion).
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