Details of density

What is Density?

The density of an object refers to the quantity of that object present within a container of given size, whether that size is defined as a length, area, or volume. For example, the 'density" of a liquid is often defined as mass/volume (e.g., lbs/gallon). However, in natural resource measurements, "density" is often 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 an amount of spaces (i.e., plants/m² or trees/acre).

An example: Number of people living in cities

Population Persons/square mile	Abundance	Density
New York City	8.1 million	26,402
Los Angeles	3.8 million	7,877
Idaho	1.5 million	17
Source: US 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 another.

The measure of density is often applied when we are seeking 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. In particular, density measures can reveal the increase or loss of seedlings within management areas.

Density is sometimes mistakenly thought to be an estimate plant cover or biomass. However, plants can vary substantially in size which leads 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 communities. 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 Vegetation)

Advantages of Measuring Density

1. Density is an easy measure to conceptualize - it is very straightforward and both easy top grasp and explain to others

2. 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.

3. As density is a count of plants and it is not highly affected by seasonal or yearly variation due to weather fluctuations or other factors.  This property makes inter-year comparisons relatively easy to obtain.  By contrast, measurements of cover or biomass can vary quite substantially within or between years.

4. 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 measure.

Some Challenges in Measuring Density

1. 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.

2. It is often difficult to determine whether you have one individual or many. For example, how do you record the density of aspen or sod-forming grasses?

3. It is very difficult to use density to describe fungi, mosses, or lichen.

4. 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.

5. 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).