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Part 9: Collecting Monitoring Data
Line Intercept Method
Canopy cover, or the vertical projection of
the vegetative part of the plant over the ground, is measured using
the Line Intercept Method. This method evaluates shrubs, trees,
grasses, and forbs. It is based on establishing a transect and
then recording the plant crowns that overlap or intercept the tape.
When using this method, it is important to establish a photo plot
and to take both general view and close-up photos.
This method is best suited to plants with boundaries that are
easily distinguished such as with shrubs and matted plants. It
is less suited for use with grasses and some thinly leafed forbs.
Criteria
- You may decide to consider small gaps as closed canopy until
they reach a predetermined width.
- This criterion may change depending on the type of vegetation
being evaluated.
- For overhead vegetation, a pole can be used to determine canopy
cover.
- It is best to conduct this type of study on a windless or
low wind day.
Equipment
Conducting the study and study sites
- Sites should be located within a single plant community and
within a single ecological site
- The length of transects is determined by the similarity and
density of the vegetation. Longer transects are called for if
vegetation is sparse. A rule-of-thumb is to establish transects
that are between 100-200 feet in length.
- Baseline technique is the preferred layout for Line Intercept
Method
- Place study stake at the beginning of the baseline.
- Determine the bearing of your study and place a stake at
the end (or use a natural marker such as a hill or rock)
- Run transect at random perpendicular to the baseline
- Be sure to permanently mark the study site.
- Be sure
to adequately document information on the Study Location and
Documentation Data form.
Collecting Data
-
Proceed down the tape stretched along the transect line and
measure the horizontal linear length of each plant (for canopy
or basal cover) that intercepts the line.
- Measure grasses at ground level.
- For forbs, shrubs, and trees, measure the vertical projections
of the foliar cover intercepting one side of the tape.
- Record data on the Line
Intercept Form.
Study Layout
-
A tape is extended to create a transect across the site.
- There should be several transects created at each study
site.
- Transects should run about 15 meters for dense cover to
50 meters or more for areas of sparse vegetation.
- Since variation of cover can be high, it is important to
take as many readings along the transect as is possible.
- Proceed along the line-transect and identify plants intercepted
by the tape.
- Intercept distance is recorded.
- Measurements should be taken from the same 100-foot tape placed
along the transects at each cluster (star-shaped layout).
- The distance between where the canopy begins on the tape
and where it ends is recorded for each species.
- As you proceed down the tape laid out for the transect,
measure the horizontal linear length of each plant that intercepts
the line.
- Measure grasses and grass-like plants at ground level.
- Measure the vertical projection of the foliar cover intercepting
one side of the tape for forbs and shrubs.
- The total distance for each species is then divided by the
distance of the tape for the percent cover for that species.
Gaps less than 2 inches in the canopy of an individual species
are not considered. Canopies of multiple species that overlapped
one another were measured separately, potentially resulting in
total percent shrub cover for the entire transect to exceed 100%.
Standing dead canopies should not be recorded.
- Cover is calculated by adding all intercept distances and
expressing this total as a proportion of the tape length.
Analyzing Data
From data recorded on the Line
Intercept Form
- Cover : Calculate the percent cover of each
plant species by totaling the intercept measurements
for all individuals of that species along the transect line
and convert this total to a percent of total cover. Calculate total
cover measured by adding the cover percentages
for all plant species.
- You can also calculate species composition using
this method by totaling the percent cover for each species.
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