Summarizing values to Describe Plant communities

Assessing Composition

Composition, like diversity and similarity, is a variable that is not measured in the field, it is one that is calculated attribute based on plant measurements.  Composition is defined as the proportions (%) of various plant species in relation to the total on a given area.   Composition is also known as "species composition" or "botanical composition."

Why estimate composition?

• Traditional rangeland guides for proper stocking rates and range condition are based on plant composition and the classic range succession theory.

• Composition has been used extensively to describe ecological sites and to evaluate rangeland condition.

• Measurements of composition over time can be used to characterize trend or changes in rangeland condition.

• Allows comparison of dominance of individual plants across plant communities. For example, two sites my be very different but they could both have about 50% mesquite by weight.

• Composition can be calculated based on individual species or groups such as % of noxious weeds, or % forbs, grasses, and shrubs.

• Species composition is expressed as % of total community; this is easy to understand. Composition is an attribute can therefore be more easily visualized by those unfamiliar with rangelands or the range of that area.

Calculating Composition

Species composition is generally expressed as a percent, so that all species components add up to 100%. Composition can be calculated with measures of cover, density, weight or biomass.  It is not appropriate to estimate composition based on frequency.

• Using Frequency Data

Frequency is regarded as an unsuitable basis for the description of species composition, because simply recording the presence of a species does not indicate its absolute amount.

• Using Density Data
  
   
• Using Biomass Data
  
   
• Using Cover Data
  

For example, the composition based on density of shrubs on a Clay Loam site South Texas from a previous lesson is: 

What is the composition of a few shrubs on the Sandy Loam site?

Click here for Answers

Value of Calculating and Comparing Cover

• Allows for "relative" comparison of individual species across sites or times that vary significantly.
• The composition reflects the relative contribution of a species to a community and reflects dominance of a specific species on a site.
• Many management objectives are focused on the assessment or manipulation of species composition. For example, a land manager may want to:
  » minimize the composition of noxious weeds in a community.
  » increase the relative abundance of desirable forage species in a pasture.
  » alter the relative contribution of various species that provide shelter or food for wildlife.

Summary Questions

1. Why is frequency not an appropriate variable to use when calculating composition?

2. What variables can be used to calculate composition?

3. How could composition be valuable in determining the success of a restoration practice?