contributors: Jason Karl
Description and Uses
The Paired-plot Method is a technique for estimating utilization or destruction of plant biomass over time by the comparison of pairs of plots – one of the pair being subjected to grazing/foraging/use while the other one is not. The plots are set up before the use occurs and after the use has occurred, the vegetation in each plot is clipped and the vegetation weights compared. The differences in weights between the pairs are averaged over all plots to estimate the average utilization.
The Paired-plot Method is implemented as follows (please refer to the technical reference as it contains much more detailed instructions on implementing this method). First, a pre-determined number (ideally determined through analysis of the variability of some pilot sampling data) of locations of plot pairs are selected at random from within the sampling area. A cage is anchored over one plot of the pair (refer to the technical references for design considerations). The other plot of the pair is left open to foraging/use (again, consult the technical references for considerations in locating/marking open plots). Data collection consists of clipping and weighing current year’s growth on key species from the protected and unprotected plots. Percent utilization per plot pair is calculated as the difference between total protected and unprotected weights divided by the total protected weight.
Advantages and Limitations
This method has several key advantages. First, it is a conceptually straightforward method and a direct measure of utilization. Second, the little training that is required to implement (aside from plant identification) this method can be given quickly, and this method is generally very accurate.
There are several disadvantages to consider as well: First, this method can be very time consuming and requires that the plot pairs be established beforehand. Also, results can be influenced by plot placement (e.g., protected plots placed in long-term exclosures may not be representative of ungrazed conditions outside of the exclosure), so careful consideration must be given to where plots are located. Second, new plots must be established every time sampling is conducted as once the sampling has been completed, the plots cannot be used again because all the vegetation has been removed. Third, the Paired-plot Method doesn’t account for regrowth (and potential re-browsing) of forage plants (i.e., cumulative plant growth). This could be problematic when the time-span is long between when the plots are established and when the clipping takes place.
Utilization Studies and Residual Measurement: Interagency Technical Reference (BLM 1999). This manual includes field forms for the Paired Plot Method (see below).
Technical and Application References
- Frischknecht, N.C. and P.W. Conrad. 1965. Adaptable, transportable utilization cages. Journal of Range Management 18:33-34.
- Bureau of Land Management. 1984. Rangeland Monitoring – Utilization Studies. TR 4400-3.
Related utilization and residue measurement methods include:
- Cole Browse Method
- Extensive Browse Method
- Stubble Height
- Utilization – Grazed Class Method
- Herbaceous Removal-Height-Weight Method
- Herbaceous Removal-Actual Weight Method
- Herbaceous Removal-Ocular Method
- The class notes from Range 357 at University of Idaho have a good, succinct comparison of different techniques for measuring utilization. http://www.cnr.uidaho.edu/range357/notes/Utilization.pdf.