The objective of rangeland vegetation trend monitoring is to document changes over time in vegetation or other rangeland resources. Common methods often used together throughout Arizona and the west include Point Ground Cover, Pace Frequency, Dry-Weight Rank, and Comparative Yield. Further details regarding these methods and ground rules can be found in Sampling Vegetation Attributes (Interagency Technical Manual, 1996), Guide to Rangeland Monitoring and Assessment (Smith et al., 2012), or Southeastern Arizona Monitoring Program: Methods and Ground Rules (McReynolds and Brischke, 2015). This suite of rangeland monitoring methods can be conducted using a single piece of equipment, a monitoring frame.
Selection of proper frame size, or quadrat size, is the most important consideration in using frequency sampling. Frequency of a plant species cannot be evaluated over time or areas unless the same frame size is used (Smith et al., 2012)
There are costs and benefits to frame size. Larger frames increase the probability of a species occurring in the quadrat, but if they are too large they can become unwieldly to use in the field. Smaller frame sizes may miss some of the more uncommon plants in the area. For example, Figure 1 shows that by increasing frame size from 0.5 m2 to 1.0 m2 will often not increase frequency values significantly and the smaller frame sizes encounter fewer species in the same area (Despain et al., 1997).
Each species has an optimum frame size, but it would be impractical to use many different frame sizes in the same area. However, if one frame size will not provide desired frequency values for all species of interest, two or more frame sizes may be used in a nested fashion. This situation is typically encountered where one or two species are very abundant in relation to others (Smith et al., 2012).
Often, rangeland monitoring programs use a .16 m2 frame size (40 cm x 40 cm) for the methods listed above. If a nested frame size is required, a .01 m2 (10 cm x 10 cm) frame size is used within the .16 m2 frame (see Option 1). Points or screws are fixed to the frame to measure Point Ground Cover.