Cover - Quantitative
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.
This method is best used on species that have dense canopies (e.g., shrubs) as opposed to species with narrow canopies (e.g., grasses). It is also difficult with any species (shrub, grass or forb) with lacy canopies because of the amount of small intercepts that would need to be recorded.
Cover is one of the most commonly used measures when conducting community monitoring. However, there are limitations. One limitation of the canopy gap technique is when comparing results of canopy gap to other projects, the protocol for determining what a gap is may vary. Because most plant canopies have gaps, which can be formed by dead centers, matting of plants, gaps between grass blades and shrub branches etc., a protocol for dealing with these gaps must be developed. For example, some variations call for the observer to identify the canopy as closed until the gap exceeds a predetermined width. Even within a project, if this protocol is not clearly documented it can lead to inconsistencies among observers. If the sighting line is not perpendicular to the tape or plumb, observer bias can also be a problem. Wind can also influence the angle of the plant, resulting in a larger surface than would be present under normal conditions. Some disadvantages with cover measurements in general is that they can change dramatically over the growing season, making timing of sampling very important. In addition, cover measures are sensitive to changes in number (mortality and recruitment) and vigor (annual biomass production) making it difficult to interpret cover trends.
Video from the USDA-ARS Jornada Experimental Range
Basal Gap Intercept measures the proportion of the line covered by large gaps between plant bases and is important as an indicator of runoff and water erosion. 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 Method to help determine whether observed erosion changes are due to loss of cover, changes in spatial distribution of vegetation or reduced soil stability.
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