Density is the number of counting units per unit area and is measured in quadrats of varying sizes. Density techniques are most effective when measuring either species recruitment or mortality. This method is less sensitive to vigor-related changes. It is also not as effective when dealing with plants that are long-lived, and respond to stress with reduced biomass or cover instead of mortality, or for plants that fluctuate in numbers from year to year (e.g., annual species). Density has been particularly useful in monitoring threatened and endangered species or other special status plants because it samples the number of individuals per unit area.
The type of vegetation and distribution will dictate the technique used to obtain the density measurements. In particular size and shape of the quadrat is an important consideration when designing density studies. For example, belt transects provide a good way to measure the presence of invasive plants or woody seedlings and provides a good means of monitoring brush or shrub encroachment. For species that are hard to see, such as seedlings and small annuals, it would be better to use quadrats placed at regular intervals along the line. See Measuring and Monitoring Plant Populations (Elzinga et al. 2001) for more information on quadrat design.
Since density is reported as a per unit area measure, it enables comparison between sites that might have a different size or shape of quadrat. For other methods, such as frequency measurements, this is not the case. There is an exception however. Comparison can be difficult between different plot shapes and sizes because the density estimates can vary depending on how the observer deals with boundary plants (plants that are half in and half out of the quadrat). Because the most typical way of dealing with boundary plants is to include them, plots that are small or long and narrow (i.e., they have a high perimeter to area ratio) usually have higher estimates than those recorded in larger or square quadrats.
It is also often hard to identify individuals of plants that are capable of vegetative reproduction (e.g., rhizomatous plants or clonal species like aspen). Therefore the counting unit must be clearly defined in order to be recognized by all observers and to ensure consistency in data collection.
If the individuals can be identified (when the targets counted are few and can be easily recognized), density measurements are repeatable over time and observer error is low. However, error increases when small, cryptic species are located in the quadrats or when there are numerous plants in the quadrat. When this is the case, taking the extra time to make sure these species are recognized and recorded makes this method more time consuming.
See Measuring and Monitoring Plant Populations (Elzinga et al. 2001) for more information on quadrat design.