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National Forest Inventory of Finland (NFI) has evolved from a line survey to a plot-based inventory (Sect. 1.3). Simulated sampling was introduced as a tool for planning the design during NFI8 and it was fully adopted in NFI9 (Sect. 2.1). The main assessment units in the field work are stands and individual trees, although some of the variables are assessed on circular plots with fixed radius (Sect. 2.2). Section 2.3 explains how the centre points of the sample plots were located in the field.

In boreal forests, especially in mature stands with a closed canopy, mosses are an important component of the forest ecosystem in terms of total biomass, annual biomass production and consumption of nutrients (Malkonen 1974, Skre and Oechel 1979, Weber and Van Cleve 1984). In deciduous forests, on the other hand, the biomass of forest floor mosses is often of little significance (Zavitkovski 1976). In boreal coniferous forests, where the climate is humid and the soil relatively nutrient-poor, the moss carpet forms 55–94% of the total biomass of the understorey vegetation (Makipaa 1994). The total biomass of mosses is highest on nutrient-poor sites and decreases with increasing site fertility (Skre and Oechel 1979, Makipaa 1994). Mosses that grow on the forest floor are sensitive to disturbances in environmental conditions, especially changes in nutrient, light and moisture availability.