Principles of Vegetation Measurement & Assessment
and Ecological Monitoring & Analysis

 

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Veg Sampling
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Line Intercept Techniques to estimate cover

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measuring cover

Basics for Line Measurements

A common and objective way to assess cover is by laying down a transect and tallying up how much of a plant intersects the transect.  This is called the Line Intercept Method first described by Canfield in 1941. (Canfield, R.H. 1941. Application of the line interception method in sampling range vegetation. J. Forestry 39:388-394.)

A full description of how to conduct a line intercept in the field is written in the  Sampling Vegetation Handbook

  • The line intercept method is fairly objective.  All the observer must decide is what the plant species is and where the edge begins and ends. 
     
  • The beginning and end point for each time a plant intercepts the line can be recorded and the total distance of intercept can be calculated back in the lab.
     
  • If sampling objectives can be met without keeping track of the exact place where a plant intercepted the line, then all that needs to be recorded is the distance of intercept along the line.
     
  • Lines should be as thin as possible in theory, a line has no width at all. When reading distances along a transect created with a measuring tape, read only along one edge of the tape. It is also important to ensure that the tape is not inadvertently moved to include or exclude certain plants.
     
  • Usually lines are used to estimate basal and canopy cover, but can be used for foliar cover for species that have large leaves and dense canopies.

Cover is calculated as:  

Data Example

Below is an example of Line Intercept data presented in the Sampling Vegetation Handbook

  • Notice that 4-letter codes are used for species (like BOCU, or BOGR2). The species that the codes represent can be found in the USDA Plants Database (www.plants.usda.gov).
  • Notice that distances are measured and recorded in 10ths of feet. This may seem a bit odd because there are 12 inches in a foot. It therefore, takes a special transect tape measure to do this one that divides feet into 10 segments. But, it makes sense for calculating distances. As in this example, when distances (recorded in 10ths of feet) are totaled for a 100 foot tape, the total distance = % cover.
  • It turns out that a 100 foot (or 30 meter) distance is often used in grassland and shrubland studies. This distance is about right generally long enough to get a good representative sample, but short enough that several lines per site, pasture, or landscape can be read.

Issues of Overlap and Gaps

The line intercept method is simple, just measure the distance of first contact to the last contact of the species along the line.  But, what should be done when plants overlap on the line? And what if there is a gap in intercept for a single plant?

A few rules to deal with issues of overlap and gaps in the field:

  • Ignore overlap of individuals of same species.
     
  • Disregard overlap of differing species. In other words, count the total distance of a species regardless if it overlaps with another species.
     
  • It may be difficult to tell where the canopy starts or ends. Create a set of rules to use in the field to clearly describe how you determined canopy dimensions.
     
  • Decide what to do with a broken canopy within a shrub or tree. For example, if there is a gap less than 2 inches you could ignore it but count gaps greater than 2 inches. In other words, don't count the distance of the gap (if > 2 inches) in total distance of intercept by the plant you are measuring.

Summary

In the line intercept method, the line is the experimental unit. Therefore, determine % cover for each line then average lines together to estimate cover for the total landscape, site, or pasture.

Remember that because each plant species is examined separately and overlap between species is allowed, the total cover along a transect determined by distances of vegetation cover can exceed 100%.

It might seem intuitive that to get total cover for plant type (i.e., grasses, forbs, shrubs, or trees) along a transect you could simply add the cover for all plants of each plant type on a line to get the total % cover of by type. For example, you could add all distances covered by a grass to get total grass cover. But, this is not appropriate. Think about how overlapping vegetation is handled along the line. If there was a lot of overlap between species of the same plant type, then the total cover by plant type would be inflated.

  • It is possible to use the line intercept method to ascertain the total cover by grasses, forbs, shrubs, or trees, but these distances must be determined in the field.
     
  • If the beginning end ending point of vegetation intercept is recorded for each species, it would be possible to look at data back in the lab and determine the total distance of intercept by plant type. It would be a bit tedious, but measures of overlap between different species within a plant type could be determined and removed from the total distance calculation.

Summary Questions

  1. How should overlap among plants be handled for line intercept on a transect?

  2. In the example below (for a 100 foot line with measurements in 10ths of feet), what is the cover of Desert Ceanothus (CEGR) and Juniper (JUNIP) on this line?

**Click here for answer

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