contributed by Karen Colson and Jason Karl
Method Type
Cover – Quantitative
Other Names
None known
Description and Uses
Basal Gap Intercept measures the proportion of the line covered by large gaps between plant bases. It is important as an indicator of runoff and water erosion. Basal cover is measured along a line intercept transect by recording the beginning and end of each gap between plant bases (therefore measures only the proportion of the plant that extends into the soil).
Advantages and Limitations
Cover is one of the most commonly used measures when conducting community monitoring. It can be used to measure a variety of life-forms (i.e., moss, annual forbs, shrubs, trees), it is strongly related to biomass and ecosystem processes, it does not require determining the number of individuals within a species, and it can easily be used to measure plants, mosses, or lichens at the ground surface. Unlike most measures of cover, basal gap does not vary greatly depending on climatic conditions and is not affected by utilization of animals. However, this method can be difficult to measure for plants with a single, small stem.
Manuals/Instructions
- Monitoring Manual for Grasslands, Shrublands, and Savanna Ecosystems (Herrick et al. 2009).
- Measuring and Monitoring Plant Populations (Elzinga et al. 2001).
Video Instructions/Demonstration
Similar Approaches
Canopy Gap Intercept measurements represent the proportion of a line covered by large gaps between plant canopies and is an important indicator of the potential for wind erosion and weed invasion. Canopy cover is measured along a line intercept transect by documenting the point along the tape at which the canopy begins and ends. Herrick et al. 2005 suggest using both the canopy and gap intercept methods together in combination with the cover indicators from the line point intercept and the soil stability to help determine whether observed erosion changes are due to loss of cover, changes in spatial distribution of vegetation or reduced soil stability.
Data Forms
Printable data forms for the Basal Gap Intercept method can be downloaded from
Electronic data forms can be found at the following sites
- Basal gap intercept data can be recorded directly into the Rangeland Database in the field.
- An Excel version of the basal gap Intercept method data form that automatically calculates cover percentages can be found at the Jornada Experimental Range Monitoring and Assessment Site.
- An version of the basal gap Intercept Excel form that is compatible with electronic PDAs can also be downloaded from the Jornada Experimental Range Monitoring and Assessment Site