Plant Dimensions

Many different attributes describing plant dimensions can be used to determine plant biomass. Measurements on plant height, plant basal area, twig length and diameter, trunk diameter, canopy cover, or canopy volume, are all used as indirect methods to estimate biomass.

The method involves initially developing a regression relationship, by recording the appropriate dimensions and harvesting a small number of individuals that are chosen to encompass the range in plant size which will be encountered in the population. This step should be completed before further sampling commences, to ensure that the selected dimensions provide a good prediction of biomass (r2 > 0.70, for example). The main sampling then proceeds by taking only dimension measurements, which are converted to biomass values using the regression equation.

Although a combination of plant dimensions is generally a better predictor of biomass than a single variable, the objective of designing a simpler sampling strategy can sometimes be lost. For example, although a strong relationship is found between biomass and the multiple factors of stem length, leaf length, and number of stems, the effort required to collect such data would be impractical in most rangeland inventory or monitoring situations!

This approach is routinely adopted in many rangeland inventory or monitoring situations. It is especially suited to shrubs and trees, which are difficult to estimate by harvesting to determine biomass. For meaningful interpretation, the data must be converted from a biomass per plant to a biomass per area basis, requiring an additional estimate for density. The regression relationship is also specific to the population where the data is collected, and caution must be taken when attempting to apply it to other years or other sites.

References and Further Reading

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Andariese, S.W., and W.W. Covington. 1986. Biomass estimation for four common grass species in northern Arizona ponderosa-pine. Journal of Range Management 39:472-473. (pdf)

Blankenship, J.O., and D.R. Smith. 1966. Indirect estimation of standing crop. Journal of Range Management 19:74-77. (pdf)

Bonham, C.D. 1989. Measurements for terrestrial vegetation. John Wiley Sons, New York, NY. pp 208-209, 210-212, 229-255.

Bryant, F.C., and M.M. Kothmann. 1979. Variability in predicting edible browse from crown volume. Journal of Range Management 32:144-146. (pdf)

Clark, I. 1945. Variability in growth characteristics of forage plants on summer range in central Utah. Journal of Forestry 43:273-283.

Ludwig, J.A., Reynolds, J.F., and P.D. Whitson. 1975. Size-biomass relationships of several Chihuahuan desert shrubs. American Midland Naturalist 94:451-461.

Karl, M.G., and R.A. Nicholson. 1987. Evaluation of the forage-disk method in mixed-grass rangelands of Kansas. Journal of Range Management 40:467-471. (pdf)

Mitchell, J.E., Elderkin, R., and J.K. Lewis. 1993. Seasonal height-weight dynamics of western wheatgrass. Journal of Range Management 46:147-151. (pdf)

Murray, R.B., and M.Q. Jacobson. 1982. An evaluation of dimension analysis for predicting shrub biomass. Journal of Range Management 35:451-454. (pdf)

Pieper, R.D. 1988. Rangeland vegetation productivity and biomass. In: P.T. Tueller. (ed). Vegetation science applications for rangeland analysis and management. Handbook of Vegetation Science, Volume 14. Kluwer Academic Publishers, Dordrecht. pp 458-460.

Ruyle, G.B., Bowns, J.E., and A.F. Schlundt. 1983. Estimating snowberry (Symphoricarpos oreophilus) utilization by sheep from twig diameter-weight relations. Journal of Range Management 36:472-474. (pdf)

Schuster, J.L. 1965. Estimating browse from twig and stem measurements. Journal of Range Management 18:220-223. (pdf)