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Point Intercept (PO) Sampling Method

Point Intercept (PO) Sampling Method

The FIREMON Point Intercept (PO) method is used to assess changes in plant species cover or ground cover for a macroplot. This method uses a narrow diameter sampling pole or sampling pins, placed at systematic intervals along line transects to sample within plot variation and quantify statistically valid changes in plant species cover and height over time. Plant species or ground cover classes that touch the pin are recorded as “hits” along a transect. Percent cover is calculated by dividing the number of hits for each plant species or ground cover class by the total number of points along a transect. This method is primarily suited for vegetation types less than 3 ft (1 m) in height and is particularly useful for recording ground cover.

The Point Intercept (PO) method is designed to sample within-plot variation and quantify changes in plant species cover and height, and/or ground cover over time. This method uses transects located within the macroplot. First, a baseline is established from which to orient the transects, then transects are placed randomly along the baseline. Characteristics—such as transect length, number of transects, and number of points per transect—are recorded about the general sample design. A sampling pole or sampling pins are systematically lowered along each transect and “hits” are tallied when contact is made with a plant species or ground cover class. Percent cover is calculated as the number of hits for each plant species or ground cover class divided by the total number of points per transect. Height is also recorded for each plant species along the transect.

This method is primarily used when managers want to monitor changes in plant species cover and height or ground cover and is best suited for sampling ground cover and grasses, forbs, and shrubs less than 3 ft (1 m) in height. The Point Intercept method works well for fine textured herbaceous communities, fine leaved plant species, and species with open canopies (pastures, dense grasslands, and wet meadows), which can be more difficult to estimate with the Line Intercept (LI) method. It provides a more objective estimate of cover than the ocular estimates used in the Cover/Frequency (CF) sampling method. It can be difficult to detect rare plants with the PO method unless many points are used for sampling. Point intercept sampling requires many points to sample rare species (200 points to sample at 0.5 percent cover). Quadrats sample more area and have a greater chance of detecting rare species. If rare plant species are of interest the CF or RS methods are preferred because it is more effective to sample rare
species using quadrats or by marking individual plants, than with points or lines. We suggest you use the PO method if you are primarily interested in monitoring changes in ground cover. The PO method may be used in conjunction with the CF method to sample ground cover by using the CF sampling quadrat as a point frame.

The Point Intercept method is considered one of the most objective ways to sample cover (Bonham 1989). The observer needs to decide only whether a point intercepts a plant species or the ground. No cover estimates are required. Points offer quick and efficient data collection and can be used to estimate cover values with minimal bias and error. However, errors can be caused by plants moving in the wind or sampling poles lowered incorrectly. The points themselves have dimensions and can be considered small quadrats. In theory, if you sampled an infinite number of points in an area, you could measure the exact cover for each plant species. Points are either the end of the sampling pole or the intersection of cross- hairs in a sampling frame.