A cautionary note on the minimum crown cover criterion in forest definitions

Forest area and its changes are understood as an important and, supposedly, easily measurable indicator for sustainable management of natural resources in larger areas. The observation and estimation of forest area must be based upon a clear definition. The minimum crown cover percentage is, in many forest definitions, a central element. This paper illustrates that any definition of a minimum cover percentage must be complemented by a definition of the sampling unit, which is used as a reference area on which the percent cover is to be determined. Otherwise, the results are not unique. A simple theoretical example and an aerial photograph are analyzed to illustrate these relations. The examples underline that, for the same minimum crown cover, the forest cover estimates vary considerably when the size of the sampling unit is changed. In general, for small values of the minimum crown cover as they are commonly used in forest definitions (0.1 to 0.3, say), the expected value of the cover estimate increases consistently with increasing size of the sampling unit on which the cover measurement is done. This effect is the more pronounced the more fragmented the forest cover and the more open the forest formations are.