written by Karen Colson and Jason Karl
Description and Uses
Plant production can be determined using a variety of methods or combination of methods including estimating, harvesting, or by estimating and harvesting (double-sampling). When using the Estimating (by weight units) Method the relationship of weight to volume is not constant; therefore, production and composition determinations are based on weight estimates, not on comparison of relative volumes and does not require clipping of vegetation.
This method is based on a several factors: 1. A weight unit is established for each plant species occurring at the site, 2. A weight unit can consist of part of a plant, an entire plant, or a group of plants, and 3. The size and weight of a unit will vary according to the type of plant.
Advantages and Limitations
The Estimating Method is the most rapid of the three production methods listed above since it takes a lot less time to estimate the weight in a quadrat than it does to clip a plot. However, it is not as accurate as the harvesting method or the double sampling method due to the personal error caused by estimation of utilization levels. Extensive training is required to perform this method, which involves weighing representative units of a plant and training the observers “eye” for the weight categories look like.
Similar to the other two methods, plants grow at different times of the year so when the sampling is conducted is important since sampling at different times of the year can change the results. When measuring annual production, current year’s growth can be hard to separate from previous year’s growth. Likewise, seasonal and annual climatic fluctuations affect biomass, and therefore production is not a suitable measure for long-term trend studies that compare data taken in different years. It also requires adjusting for material that has not been produced yet or has been removed which is difficult.
- NRCS National Range and Pasture Handbook, Chapter 4, Inventory and Monitoring Grazing Land Resources, pages 4-3 through 4-13 (USDA-NRCS 1997).
Technical and Application References
- Plant Production – Harvesting
- Plant Production – Double-Sampling