Details of density
What is Density?
The density of an item is quantity of that item per unit
measure, especially per unit length, area, or volume. In natural resource measurements, "density"
is usually used to refer to the number of items per unit area. For example,
plants/m2 or elk/hectare).
The term “abundance” is often used as synonymous with density.
But, density is unique because it is specifically related to a specified amount of
space or area (i.e., plants/m2 or trees/acre).
An example: Number of people living in cities
|Population Persons/Square Mile
|New York City
|| 8.1 million
Census Bureau (http://quickfacts.census.gov)
Density in vegetation measurement refers to the number
of individuals per unit area (for example plants/m2). The
term consequently refers to the closeness of individual plants to one
The measure of density is often applied when we want to monitor changes
in a given vegetation species over long periods. As with other measurements,
the measure of density can be useful in detecting the response of plants to
a given management action. For example, density estimates can reveal the
increase or loss of seedlings within management areas.
Density is sometimes mistakenly thought to be an estimate of plant cover or
biomass. However, plants can vary substantially in size which can lead to
weak relationships between density and cover. For example, density of
juniper trees per hectare could include little trees that are just a few
meters tall and very large trees that are more than 5 meters tall covering
several square meters. Thus, knowing the density of juniper trees may tell
you little about the cover of juniper on the landscape.
The only case in which density might relate to cover is in the estimate of
plants that are all roughly the same size. For example, if density of camas (a
single-stemmed native forb) is greater on one site than another, then it is
likely that camas has greater cover on the site with greater density.
Importantly, the term density
can be used to describe characteristics of plant
However, the caveat is that comparisons can only be based on similar life-form and size. This
is why density is rarely used as a measurement by itself when describing
plant communities. For example, the importance of a particular species to a
community is very different if there are 1,000 annual plants per acre versus
1,000 shrubs per acre. It should be pointed out that density was synonymous
with cover in the earlier literature. (Interagency Handbook - Sampling
Advantages of Measuring Density
Density is an easy measure to conceptualize; it is very straightforward and
both easy to grasp and explain to others
Once a quadrat size is chosen, the measure of density is very
fast to obtain. This is because the measure is mainly just a count of a species rather
than any more extensive measurements.
Density is a count of plants and is not highly affected by seasonal or
yearly variation due to weather
fluctuations or other factors. This property makes inter-year
comparisons relatively easy. By contrast, measurements of
cover or biomass can vary quite substantially within or between years.
Density is easy to measure in xeric ecosystems or for specific life forms
such as trees, shrubs, and bunch grasses.
For example, in the picture below, the
density of shrubs in the Salt Desert Shrub region are fairly easy to
Some Challenges in Measuring Density
Density is only a count of the number of plants present. It does not tell
you much about plant health,
forage, or productivity. For example, 4 healthy individuals per unit area will produce
the same data as 2 healthy and 2 unhealthy individuals per unit area.
It is often difficult to determine whether you have one individual or many.
For example, how do you record the density of
It is very difficult to use density to describe fungi, mosses, or lichen.
Density is a scale-dependent measure. Importantly, it
is dependent on both the size of the quadrat (or other sampling frame you
may use) and the scale over which the species typically occupies. For
example, if the species has a sparse population then you might need a very
large sampling frame to capture any individuals. Alternatively, if the
species occurs in patches, then depending on the size of your sampling frame
you may record a high density where in other areas you may record none.
Determining density can be very time consuming in dense or complex
environments. For example, imagine measuring the density of shrubs in the
two figures below. (Difficult on the left and easy on the right).