Primer on the Use
of Methods of Quantitative Vegetation Measurement
The various methods discussed here are tools for measuring different aspects of plant communities quantitatively. Some examples of why quantitative vegetation measurement may need to be used are as follows:
- How many sheep can prudently be grazed on this pasture?
- Is there sufficient ground cover on this site to control erosion?
- Are shrubs sufficiently spaced on site to support sage grouse nesting?
- What are the long-term trends of diffuse knapweed abundance on site?
Before choosing an appropriate tool for the job, one must determine what kind of numeric representation (eg. percentage or lbs/acre) will directly address the question/s.
Tool Bench
Density
Density refers to the number of plants per unit area. It is most often
measured by direct count of "individuals" of each species within
the boundary of a conveniently sized plot of known area. When young, apparently
separate plant entities are fairly likely to be real individual plants.
Past the first year (or even later in the first year), the identity of
truly separate individuals is often problematic. For example, vegetative
expansion may allow one individual to appear to be several individuals.
When the distance between new ramets produced by vegetative reproduction
is small, observer perception directly influences whether one individual
or multiple individuals are indentified. In this circumstance, density
data usefulness may be seriously reduced. Units of measure may often include
individuals per mē, individual per acre, or individuals per hectare.
Production
Production refers to
various means of approximating standing crop biomass. It usually involves
the removal of above ground biomass for herbaceous species or the last
year's wood for shrubs and trees. It may be done by species, by lifeform
or by total material within the plot. Removed plant material is typically
separated as necessary and dried to consistent weight (usually at 105°C)
and weighed. Plot size will vary, determined by the size necessary to
encompass sufficient area and vegetation to keep variability within prescribed
statistical limits. Units of measure may often include gm per mē, pounds
per acre, or kg per hectare.
Frequency
Frequency refers to the proportion of randomly or uniformly placed plots
within which a given species or lifeform occurs, without any reference
to its physical abundance, rather only to its physical presence. It is
expressed as a percent. Frequency data are useful in assessing the distribution
of a plant in a community, but are often only the most indirect suggestions
of its abundance. For less common plants, whose physical abundance is
at or below the lower limit of detection, frequency data can be useful
in monitoring to assess subtle variations in those species which though
never likely to participate as a dominant may show fluctuations suggestive
and informative of slight environmental changes.
Cover
Cover referes to the amount of the ground surface in vertical projection
that is covered by a given species, lifeform or all plants.