contributed by Jason Karl and Karen Colson
Cover - Quantitative
Line-Point Intercept is a rapid and accurate method for quantifying soil cover, which in addition to vegetation, includes cover by litter, rocks and biological soil crusts. With this method, cover is measured along a linear transect line and is based on the number of “hits” on a target species out of the total number of points measured along that line. It is used when precise, repeatable measurements are required.
There are three main techniques for measuring cover: Ocular or Visual Estimates, Point Intercept, or Line Intercept. Point intercept is considered to be the least biased of all three. Line-point intercept in particular can be used to generate more indicators than virtually any other monitoring method. For example, height measurements or additional information on vegetation structure can be added into this technique. See Monitoring Manual for Grasslands, Shrublands, and Savanna Ecosystems VII (Herrick et al. 2009) http://jornada.nmsu.edu/sites/jornada.nmsu.edu/files/Quick_Start.pdf for a comparison of the different variations of this technique.
There can be slight differences in the way this method is executed. For example, pins or optical sighting devices can be used, the angle of the point intercept and the size of the pin used can vary, and cover can be measured for a single layer or multiple layers of vegetation. Therefore, the monitoring methodology should always be very specific about the approached used.
The Line-Point Intercept method is the least biased and most objective of the three basic cover methods described above. It is also a fairly rapid technique. One of its limitations however is that species with very low cover values, such as rare plant species, are often not intersected by the points and therefore are not adequately sampled. It is also difficult to detect small changes (which is a common disadvantage of many other techniques as well). Therefore, sample design is extremely important when using this method (e.g., determining how many points are sampled and the number and placement of transects in the sample area).
Line-point intercept is a variation of a more general point-intercept method. When conducting other point-intercept methods, the sampling unit depends on the arrangement of points. Points can be measured in frames (the frame being the sampling unit), as a single randomly located point (with each point being the sampling unit), or as points along a transect (which allows for either the points or the transect be used as the sampling unit). Line-point intercept is the mostly commonly used variation of point intercept and perhaps the most efficient. For information on the other variations of this method refer to Elzinga et al. 2001.
Line-point intercept is a method for acquiring information on vegetation cover. Other similar methods include Step Point (a quick derivation of line-point intercept that is more rapid because no tape is required, but also more subjective), Daubenmire plots or other ocular estimates, and Photo plots.
Video from the USDA-ARS Jornada Experimental Range
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