Press release 14.12.2006
The objective in a study carried out as a cooperation between the Finnish Forest Research Institute (Metla) and the University of Joensuu was to evaluate assessment and measuring methods of the canopy cover. The methods studied were point-based sampling, using a convex spherical densiometer, digital photographs, and ocular estimation.
The international definition of a forest, as defined by the United Nations Food and Agricultural Organization (FAO), is based on tree height and canopy cover. An area is considered to be forest if mature-stand tree height is at least 5 metres and potential canopy cover is 10 percent. Additionally, the area must be at least 0.5 hectares in size and at least 20 metres wide. The conventional way to determine whether a land area is a forest is based on the site’s timber productivity: an area is considered forest land if the average annual tree growth during the rotation period is at least 1 m3/ha, and scrub land if the annual tree growth is less than 1 but at least 0.1 m3/ha. For the needs of international forest statistics, it is clear that forests in Finland should also be classified based on tree canopy cover.
Accurate estimates of canopy cover can only be obtained by detailed examination of individual points to determine whether the point is covered by canopy or not. Since at least 100 grid points are required for measuring the canopy cover of even a relatively small test plot, accurate measuring is usually not possible during forest inventories in practice.
Reducing the number of sampling points by using a densiometer did not produce reliable estimates of canopy cover. Digital photographs can be recommended only if a sufficient number of photographs can be taken with a narrow angle of view. In this study, the five photographs taken on a rather small plot did not produce sufficiently accurate results. A skilled and experienced forest surveyor can make a more accurate ocular estimation of canopy cover than can be obtained by using the inaccurate measuring methods described above. If the inventory needs to be carried out rapidly, ocular evaluation can be recommended, provided that the surveyors can be trained for the task using carefully measured test plot materials.
The study will be continued by investigating the relationship between canopy cover and stand variables that are easier to measure. The study was financed by the Marjatta and Eino Kolli Foundation, the University of Helsinki and the Finnish Forest Research Institute.
Publication: Korhonen, L., Korhonen, K. T., Rautiainen, M. & Stenberg, P. 2006. Estimation of forest canopy cover: a comparison of field measurement techniques. Silva Fennica 40(4): 577–588.