contributed by Karen Colson and Jason Karl
Description and Uses
The Dry-weight Rank method (DWR) is used to determine species composition. Assessment of range standing crop typically requires large numbers of samples to account for high variability in the plant communities. The Dry-weight Rank method was developed to be a rapid means of collecting species composition data by eliminating the need for clipping and hand-sorting plant samples to determine relative proportions on a dry weight basis. This method consists of observing various quadrats and ranking the three species which contribute the most weight in the quadrat.
The observer decides which three species in the quadrat have the greatest yield of current year’s growth on a dry matter basis. The species with the highest yield is given a rank of 1, the next 2, and the third highest a 3. All other species present are ignored. If there are not three species present in the quadrat, a multiple rank is assigned.
The Dry-weight Rank method assumes that a rank of 1 corresponds to 70% composition, rank 2 to 20%, and rank 3 to 10%. If only one species is found in a quadrat, it would be ranked 1, 2 and 3 (100%). If two species are found, one may be given ranks of 1 and 2 (90%), ranks 1 and 3 (80%), or ranks 2 and 3 (30%), depending on the relative weight for the two species.
As with most methods, sample design and plot size are important considerations when using this technique. Sufficient training is also necessary to perform this method.
Advantages and Limitations
The Dry-weight Rank method is suitable for grassland and small shrubs types or understory communities of large shrub or tree communities, however it does not work well on large shrubs and trees.
An advantage of this method is that a large number of samples can be collected quickly. It is useful because it deals with estimates of production, which allows for better interpretation of the data to make management decisions. It is also easier to rank the top three species in a quadrat and easier to apply them similarly by individual observers, resulting in less observer bias.
However, the disadvantage with this technique is that, by itself, it will not give a reliable estimate of plant standing crop, and it assumes there are few empty quadrats. In many large shrub or sparse desert communities, a high percentage of quadrats are empty or have only one species present. The quadrat size required to address these concerns is often impractical.
Sufficient training for evaluators performing this method is required.
Sampling Vegetation Attributes 1734-4. http://www.blm.gov/nstc/library/pdf/samplveg.pdf.
Technical and Application References
- Despain, D.W., P.R. Ogden, and E.L. Smith. 1991. Plant frequency sampling for monitoring rangelands. In: G.B. Ruyle, ed. Some Methods for Monitoring Rangelands and other Natural Area Vegetation. Extension Report 9043, University of Arizona, College of Agriculture, Tucson, AZ.
- Dowhower, S.L., W.R. Teague, R.J. Ansley, and W.E. Pinchak. 2001. Dry-weight-rank method assessment in heterogenous communities. Journal of Range Management. Volume 54:71-76.
- Lt Mannetje, K. P. Haydock. The Dry-Weight-Rank Method for the Botanical Analysis of Pasture. Grass and Forage Science. Volume 18, Issue 4 , Pages268 – 275.
In Sampling Vegetation Attributes it states that data can also be collected and recorded for each quadrat in conjunction with frequency and canopy cover methods, and is particularly powerful when combined with the comparative yield method.