Principles of Vegetation Measurement & Assessment and Ecological Monitoring & Analysis

## What is Density?

1. This lesson provides an introduction to what the term density means as it is applied to natural resources. Brown text highlights key words.
2. Read through the Text and work through the Summary Questions at the end of the text.

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 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 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 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)

1. Density is an easy measure to conceptualize; it is very straightforward and both easy to 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. 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.

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 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).

Summary Questions

1. If we have a quadrat of size 12 inches by 12 inches what is the density in square feet if we record (a) 20 individuals, (b) 200 individuals, (c) 4000 individuals and (d) 50,000 individuals?

2. How would this answer change if we consider the measurement per square mile?