Harvesting to Determine Biomass

Direct harvesting of vegetation from quadrats of a known size is the most straightforward approach to determine biomass at a site. A wide variety of mechanized clippers, lawn mowers, vacuum collectors, etc, have been invented in attempts to making the task less onerous, but plant material is usually painstakingly gathered using clippers. Data are usually collected from many quadrats located along a transect, so that the transect is the sample unit. Therefore, data must be collected from several transects to determine the precision of the sample, for statistical analysis of biomass data.

When samples are carefully collected, harvesting is regarded as the most accurate method to determine biomass. However, clear ground rules are needed to ensure consistency between observers, including

  1. Boundary decisions - are especially important because sampling should incorporate quadrat volume, leading to potential errors in judgement regarding its vertical boundaries.
  2. Clipping height - clipping at ground level is recommended for best repeatability, but clipping at a grazed-height gives a more pertinent measure of forage biomass. Clipping height is most sensitive when a greater proportion of the plant biomass occurs close to the ground such as in herbs or prostrate shrub species.
  3. Separation of dead and live material - depending on the biomass property to sample, biomass is removed from most samples before weighing.
  4. Species groups - identification and separation of species, whether occurring in the field or in the laboratory, needs to be conscientiously and consistently performed between observers.

Because each quadrat represents only a very small area of the entire site, sample variance is generally high, and many quadrats must be clipped to obtain a sample size that adequately represents the amount of biomass on the site. Therefore, clipping is very time consuming and not practical for inventory or monitoring purposes over extensive areas. Likewise, the method is destructive and not suited to permanent quadrats. In these situations, estimation approaches to determine biomass or indirect methods to determine biomass are often adopted.

References and Further Reading

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Culley, M., Campbell, R.S., and R.H. Canfield. 1933. Values and limitations of clipped quadrats. Ecology 14:35-39.

Brummer, J.E., Nichols, J.T., Engel, R.K., and K.M. Eskridge. 1994. Efficiency of different quadrat sizes and shapes for sampling standing crop. Journal of Range Management 47:84-89. (pdf)

Bonham, C.D. 1989. Measurements of terrestrial vegetation. John Wiley Sons, New York, NY. pp 201-202.

Glatzle, A., Mechel, A., and M.E. Vaz Lourenco. 1993. Botanical components of annual Mediterranean grassland as determined by point-intercept and clipping methods. Journal of Range Management. 46:271-274. (pdf)

Heady, H.F., and G.M. Van Dyne. 1965. Prediction of weight composition from point samples on clipped herbage. Journal of Range Management 18:144-148. (pdf)

Johnson, M.K. 1986. Estimating ratios of live and dead plant material in clipped plots. Journal of Range Management 39:90. (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 452-454.

Singh, J.S., Lauenroth, W.K., and R.K. Steinhorst. 1975. Review and assessment of various techniques for estimating net aerial primary production in grasslands from harvest data. Botanical Review 41:181-232.