An assessment of methods for estimating plant cover

Plant cover estimates are used in a wide variety of disciplines and for a broad range of purposes.  Estimates of plant cover have been defined and collected variably.  Resulting calculations of species composition often homogenize differences in definitions and methods and produce data that cannot easily be compared between studies.  This problem is magnified by the increasing reliance on photographic collection of cover data.  We review some of the different ways plant cover has been defined over time and within cohort periods of publication.  As an example of the differences, we used the collection of cover data using points or pins.  Several common practices – only counting the first plant touched as the point is lowered, counting all the plants touched until the pin touches mineral soil, and counting all the species encounter until the pin touches mineral soil – result in widely different species composition estimates for both absolute and relative composition.  When there are three species occupying 100% of three different structural layers in the herbaceous canopy, one could justify a species composition from 33 to 300%.  Each method has a data layer comparable to photographic data collection but data recording must be improved to allow comparison between studies with different methods.