Dry-Weight Rank Method

The dry-weight rank method is specifically designed to determine species composition by providing a measure of the relative contribution of various species to the total biomass (based on dry matter content) for a site.

Dry-weight rank results are expressed only as percentage values, and do not quantify the actual biomass for each species. For example, dry-weight rank sampling may indicate that black grama (Bouteloua eriopoda) makes up 11% of herbaceous biomass and burroweed (Isocoma tenuisectus) makes up another 43% of the total biomass, but we do not have the information to convert these values to quantify the actual biomass (kg/ha or lb/acre) for either species. However, this problem can be circumvented by also determining the total biomass for the site, which is then proportioned to various species according to the percentage values derived from the dry-weight rank method. The comparative yield method complements procedures followed for the dry-weight rank method, and is usually employed concurrently to estimate total biomass.

To follow this technique, in each quadrat the observer identifies the first, second, and third most abundant species (on a dry weight basis), to which the ranks of 1, 2, and 3, are respectively assigned. When only 2 species occur in the quadrat, one of them should be given two ranks. For example, in a quadrat dominated by Lehmann lovegrass (Eragrostis lehmanianna) but including a few small croton (Croton spp.), the first and second rank may be assigned to Lehmann lovegrass, while croton is allocated the third rank. If only one species is found, it receives all three ranks for that quadrat.

At the end of sampling, ranks are tallied for each species, and weighted by a set of multipliers, usually 0.7 for Rank 1, 0.2 for Rank 2 and 0.1 for Rank 3. The weighted values of the three ranks are then added together for each species, and the result represents species composition. For many observers, the seemingly arbitrary multipliers are a source of bemusement, because it effectively assumes the highest ranked species within the quadrat contributes 70% of the biomass, the second contributes 20%, and the third ranked species 10%, while other less conspicuous species are disregarded. However, these multipliers have been tested across a wide variety of vegetation types in USA, Australia and Southern Africa, and found to provide reasonably accurate and precise results.

The main advantage of the dry-weight rank method is that little preliminary training is required, except to correctly identify species and to rank dry matter content of species with different phenological status or growth habits. Because the technique is based on ranking rather than quantification, data can be quickly collected in the field. In fact, rapid evaluation of each quadrat is the key to success for this technique, since a large sample is less sensitive to an occasional incorrect ranking. A sample size of 50-200 records is recommended for accurate results.

The main shortcoming of the dry-weight rank method rests on an underlying assumption that there is no relationship between a species occurring in the quadrat and its biomass. In reality, this assumption is often breached, because species that are characterized by large individuals tend to be found in high-biomass quadrats, whereas small species tend to be associated with low-biomass quadrats. The outcome is that species composition of regularly dispersed small plants, such as annual ground covers, will be overestimated. This problem can be solved by collecting dry-weight rank and biomass as paired data, so that the allocation of biomass to individual species can be computed on a quadrat by quadrat basis.

References and Further Reading

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Barnes, D.L., Odendall, J.J. and B.H. Beukes. 1982. Use of the dry-weight rank method of botanical analysis in the eastern Transvaal highveld. Proceedings of the Grassland Society of Southern Africa 17:79-82.

Bureau of Land Management. 1996. Sampling vegetation attributes. Interagency Technical Reference, BLM/RS/ST-96/002+1730. pp. 50-54.

Friedel, M.H., Chewings, V.H., and G.N. Bastin. 1988. The use of comparative yield and dry-weight rank techniques for monitoring arid rangelands. Journal of Range Management 41:430-434. (pdf)

Gillen, R.L., and E.L. Smith. 1986. Evaluation of the dry-weight-rank method for determining species composition in tall grass vegetation. Journal of Range Management 39:283-285. (pdf)

Jones, R.M., and J.N.G. Hargreaves. 1979. Improvements to the dry-weight-rank method for measuring botanical composition. Grass and Forage Science 34:181-189.

Mazaika, R., and P.R. Krausman. 1991. Use of dry-weight rank multipliers for desert vegetation. Journal of Range Management 44:409-411. (pdf)

Reese, G.A., Bayn, R.L., and N.E. West. 1980. Evaluation of double sampling estimators of subalpine herbage production. Journal of Range Management 33:300-306. (pdf)

Smith, E.L., and D.W. Despain. 1991. The dry-weight rank method of estimating plant species composition. In: G.B. Ruyle (ed). Some methods for monitoring rangelands and other natural area vegetation. University of Arizona, College of Agriculture, Extension Report 9043. pp. 27-47.

t'Mannetje, L.H., and K.P. Haydock. 1963. The dry-weight-rank method for the botanical analysis of pasture. Journal of the British Grassland Society 18:286-275.