8.2 Point- and Line-based Cover Methods

Video Presentation

Learning Guide

Introduction

The simplicity and ease of using point- and line-based sampling methods make them some of the most common approaches to estimating cover. Additionally, measuring cover by points is recognized as the most objective of the 3 main cover measures; point-based, line-intercept and ocular. With point methods, a number of points are placed within an area and cover is determined by calculating the proportion of points that intercept vegetation or other materials.  With line-based methods, cover is calculated as the percentage of the line that is intercepted by plant canopy.

What is a “point”?

A point is the smallest possible “plot” that is measured to determine what plant or soil attribute occurs at that exact location. Points can be created from a variety of items including long metal pins that are sharpened to create a point. Points can also be created from crosshairs or laser sights. It is important to consider and fully document the following when designing a point-based sampling method:

Size of Points – Point sampling is based on the assumption that cover is assessed from a dimensionless point. In practice, however, most items that we use as points have some dimension, (i.e. wire pin, pin flag, ball point pen, etc.). The larger the size of the point, the less accurate we are in estimating cover. For example, a thin wire will slide through leaves of vegetation resulting in fewer hits than something like a wooden dowel. The crosshairs of sighting instruments are the most accurate way to create a point.

Angle of Points – The angle to the ground at which the point is viewed can affect the size and the location of the point (Figure 1). Most often, points are placed vertically to the ground; however, points inclined at a specific angle may provide more accurate information for the cover of narrow-leaved species such as grasses. When an angle of projection is used, it should be appropriately documented to allow repetition.

Figure 1. Diagram of a 5-point frame that can be adjusted for various angle projections.

Arrangement of Points – How points are arranged in space dictates the time and precision of the cover estimates. Observations based on single points, such as the step point method, tend to take less time to record, and give a more precise estimate of cover. Grouped points, either along lines or a cluster in a point frame method, can be easier to locate than individual points, providing time advantages if individual points are widely spaced. Randomly located points are most desirable statistically, but may not be practical in the field because of location time.

Ground cover, basal cover, foliar cover, and canopy cover can all be measured by point intercept, depending on the rules established to guide decisions of what constitutes a “hit”. It is generally easier to determine if a point hit the base of plant (i.e., basal cover) or a leaf (i.e., foliar cover) than if a point is in the canopy of a plant. Therefore, points are seldom used to estimate canopy cover, though it is possible. Ground cover is most often and easily measured with a point technique. Total cover can also be calculated by determining the proportion of points that hit vegetation, relative to the total number of points sampled in an area. By recording the species of plant encountered at each hit, species composition of the site can also be determined.

Step-Point Method

One common technique used on grasslands and shrublands for estimating ground cover is called the “Step-Point” method. For this method, the “point” is defined by a mark placed on the tip of the observer’s boot, (often a pin or a notch). A transect bearing is determined and a prominent landmark is selected for the transect bearing point. Instead of using a transect tape, the observer paces the transect with the points occurring at specified intervals, usually 5 paces to ensure independence of consecutive observations. At each point, the observer lowers a pin to the ground at the mark on the boot toe. Using a pin instead of the entire toe of the boot as a point prevents the boot from bending the vegetation and the smaller point provides more accurate estimates of cover.

Observers may find it difficult to pace a straight line, particularly when trees and large shrubs are present. When an observer encounters an obstacle, they should sidestep 90° and continue in a straight line until the obstacle is past and they can sidestep back to the original transect.

The step-point technique can be used to measure ground cover, basal cover, or foliar cover. The transect is always the sampling unit when using the step-point method to assess cover.

Advantages of the Step-Point Method

The step point method is:

• simple to perform
• easy to learn
• requires little equipment
• can be assessed fairly rapidly, especially in uniform vegetation
• allows for a great number of samples to be collected covering a large area in a short amount of time

Limitations of the Step-Point Method

The greatest limitations of the method are:

• rare plants may be underestimated on point transects
• it requires multiple transects to be examined before statistical comparisons can be made among sites or years, as there is typically a lot of variation between observers
• bias is easily introduced to this method through point size or subjective foot placement
• it is difficult for observers to maintain a consistent pace in broken terrain or in shrubby or cactus-dominated areas
• pacing in a straight line is challenging with trees, shrubs, and in hilly or broken terrain

Line-Point Intercept Method

Points can also be assessed along an established transect to eliminate some of the subjectivity associated with the step-point method. Pins or sighting devices with crosshairs are used to delineate points, and sighting tubes, plumb-bobs or frames are often used to ensure vertical placement of the points through the tape (Figure 2).

Figure 2. Point intercept method for measuring cover. Reprinted from Elzinga, et al 1998, Figure 8.10, pg 182.

The line-point intercept method can be used to assess total cover, ground cover, or foliar cover. It is best suited to vegetation less than 1.5m in height as sighting devices and point frames require the observer be looking down on the vegetation from a vertical line above the ground.

It is important to thoroughly document the specific methods employed when using line-point intercept as there can be differences in the angle of the point intercept, the size of the pin, and whether cover is measured for single or multiple layers of vegetation (see ground rules for vegetation layers).

Advantages and Limitations of Line-Point Intercept Methods

The line-point intercept is the least biased and most objective method to measure cover, making it highly repeatable and very precise.  It is also easy to learn and fairly rapid to assess in the field.

One limitation is that species with low cover values are often under-estimated, and it can be difficult to detect small changes in plant communities using the line point-intercept method. A large number of points are required to estimate uncommon or rare species, especially when vegetation is sparse.

Ground Rules for Vegetation Layers

Measuring vegetation would be much easier (and more boring) if it occurred in just one horizontal plane.  But, this is seldom the case. The layers of vegetation that occur in the real world create a problem and an opportunity when conducting point measurements to estimate cover.  There are basically two solutions:

1) Ignore layers – Make a rule to ignore anything except the top or bottom layer of cover. Making a rule like this is okay as long as it meets your objectives, is consistently followed, and stated in the results and methods. For example:

• When conducting a ground cover survey, the vegetation hanging over the ground is ignored.
• When conducting a study of foliar cover or canopy cover only the top layer of vegetation is counted. Any vegetation below the top layer or on the ground would be ignored.

2) Include Layers – Record hits of vegetation encountered as the point is lowered. This point-hit method is much more time consuming but gives a richer 3-dimensional view of the site or landscape. For example, in the following four points (Figure 3), hits would be recorded as such:

Figure 3.  Adapted from Sampling Vegetation Attributes, Illustration 14, page 77.

Table 1. Datasheet with recorded points from Figure 3.

Data summary:

Ground cover (total of 4 hits) = 25% bare ground, 50% perennial vegetation and 25% litter
Vegetative cover (total of 8 hits) = 38% wheatgrass (3 hits), 25% Pine (2 hits), 12% Bitterbrush, 12% Sagebrush, 12% Rabbitbrush.

Continuous Line Intercept Method

Another common way to assess cover is by establishing a transect and tallying up how much of a plant intersects the transect.  This is called the Continuous Line Intercept Method and was first described by Canfield in 1941. Canopy cover is measured by recording the point along a transect tape at which the canopy begins and ends (Figure 4). This method is often conducted by species and is most appropriate for species with dense canopies, (i.e. shrubs), as opposed to sparse, net-like canopies with frequent gaps (i.e. grasses). Line intercept can be used to measure basal cover of distinct plants like bunchgrasses. Line-intercept is used to measure foliar and basal cover and composition by species.

Figure 4. Line intercept method for determining canopy cover of a single shrub species. Reprinted from Elzinga et al. 1998, Figure 8.9, page 181.

• The line intercept method is fairly objective.  All the observers 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.
• A 100 foot (or 30 meter) transect 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.
• 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.

Advantages and Limitations of Line Intercepts for Cover

The line-intercept method is easy to learn and simple to use.  It also provides an accurate, objective measure of cover.

Still, line intercept is not well suited for plants with single, small stems or net-like canopies because of the complexity in determining intercepts and gaps. Bias is introduced if the sighting line is not perpendicular to the tape

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.

Ground Rules for Assessing Cover via Continuous Line Intercept

Before starting a cover assessment, it is important to establish ground rules for cover estimation to ensure consistency among observers and sampling points. Specific questions that need to be addressed are:

• How do you deal with overlap of plant canopies?
• Do you ignore or include layers in a point cover assessment?
• Should cover be assessed on plants rooted in the quadrat, or hanging over?
• Will you assess cover by species or by life-form, or some other designation?
• How do you deal with dead centers in bunchgrasses?

There is not a set answer for these questions, however, they should be addressed in the study design and notes to ensure consistency from year to year and between observers.

How to Calculate Cover from Point and Line Methods

The calculations for cover from point and line intercepts are simple and intuitive.

Calculating Cover from Point Measurements

The formula to calculate cover is simply the proportion (%) of hits for a species of vegetation type:

You can calculate the following cover attributes from point data:

• Percent Cover by Species: You can obtain the percent cover by species by totaling all the “hits” for that species along all of the transects, dividing by the total number of points, and multiplying by 100. Repeat the steps for ground cover categories.
• Cover for Multiple Transects:  Calculate the total percent cover for the species in the sampled area by adding together all of the cover values for the species and dividing by the number of transects. Do the same for ground cover.
• Total Vegetation Cover: Total vegetation cover is calculated by summing the number of “hits” in the top canopy layer only, dividing by the total number of points, and multiplying by 100.  Including sub-canopy hits would artificially inflate the estimate for total vegetation cover.
• Species Composition: Species composition is calculated by dividing the percent cover for each plant species by the total cover for all plant species present.

Data Example

Table 2 shows data collected from a point-step cover assessment. For each point, only one layer is recorded, so the total cover adds up to 100%. The observer recorded 150 points as vegetation, litter, gravel, bare soil or rock. The degree to which the cover categories are broken down depends on the observer and the goals of the monitoring project.

Table 2. Point-step cover assessment data summary sheet.

Calculating Cover from Line Intercepts

Line Intercept Cover is calculated as:

In the line intercept method, the line is the sampling unit. Therefore, determine percent cover for each line, then average the cover values 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, and cannot be calculated retroactively, after data collection.
If the beginning and 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.

Data Example

Figure 5. An example of a data sheet for recording line intercept data.

In a line intercept datasheet (Figure 5), the distance of the intercept is recorded for each species.

Then the total length of intercept for each species is divided by the total length of the transect, and multiplied by 100 to get percent cover.

The total cover for the line is found by summing all the percent cover values for each species.

The percent composition is determined by dividing the percent cover of each species by the total cover of all species.