How to Calculate Absolute Shrub Density

Choose the dimensions and shape of your sampling plot. This depends partly on the shape of the shrubs you’re inventorying: A larger shrub will be better surveyed in a larger plot. In his 1989 book “Measurements for Terrestrial Vegetation,” Charles D. Bonham gives a typical plot size for shrub surveying at about 53 square feet. As a general rule, set the plot larger than the average size of the shrub and, in dense shrublands or woods, larger than the average distance between shrubs. Common plot shapes are squares, rectangles and circles, and consider which one best captures the diversity of ecological landscapes in your study area.

Decide on criteria for counting plants within a plot. Shrubs can be difficult to define, and this will likely play a role in a survey. Many are multi-stemmed, meaning you may decide to define a clump as an individual. Shrubs also may manifest themselves in tangled thickets, making it difficult to discern a single plant. Establish a rule for those shrubs that border on one of your plots; perhaps count a plant as belonging to that plot if half or more of its trunk or crown lies within the bounds.

Distinguish shrubs from trees using some standard -- whether it's your own standard or an established standard. Many plants occupy a gray area. Hawthorns, for example, often grow as low, twisted shrubs, but under the right conditions some species may exceed 50 feet in height. Shrubs commonly are defined as having multiple stems, while trees usually have just one stem. However, depending on the purpose of your study, you may want to simplify matters and describe a shrub as a plant within a certain size range.

Count the shrubs within each plot. If you’re simply looking for general shrub density -- and are unconcerned with specific species and instead are treating all shrubs as an ecological guild – you can divide this number by the size of the plot to determine an absolute density. If you are investigating a particular shrub, only include the total number of individuals of that species.