Derivation of locally valid estimators of the aboveground biomass of Norway spruce

The carbon (C) stocks of forests are usually calculated from inventory-based estimates of the stem volume, tree-species-specific wood densities, biomass expansion factors (BEF) and functions for the mass of branches and needles/leaves, and the C concentration. However, reported BEFs in the literature are inconsistent, and especially the reliability of the C stock estimates for young stands is unknown. We describe an efficient method for deriving locally valid functions in order to estimate the aboveground biomass of young Norway spruce (Picea abies Karst.) stands. Data were collected from two adjacent stands, representing the treatment Control and Fertilized of an amelioration experiment. The stem volume was derived from Mass Tables as a function of diameter and height and was converted to stem mass with a species-specific conversion factor. Subsamples of branches from different positions within the canopy were selected by probability proportional to size (PPS) sampling. Needles and branches were weighed separately. The obtained expansion functions for branch and needle biomass have dbh as the only input variable and are accurate (0.88<R²<0.96). Earlier published allometric functions carried a considerable bias, especially when applied for young stands. We found that differences in soil fertility do not alter the ratio between the masses of stems, branches, and needles. Regionally derived biomass expansion functions can be used for sites with different soil fertility, can greatly improve the estimate of the aboveground biomass, and can be generated with a modest effort of field and laboratory work.