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The study was conducted at three different locations (three sub-compartments of different management units) within the forest management area of the Republic of Croatia (Figure 1) with the aim of determining the suitability of using allometric equations for calculation of the common beech biomass in different stand conditions, constructed on the basis of input data collected directly by in situ destructive method.
Two locations were situated in high forests (stand A in regular managed beech forest and stand C in selective managed fir-beech forest) and one location, stand C was a coppice forest (Tables 1 and 2). Durring the investigation, a preparatory felling was conducted in the stand A, a tninning was conducted in the stand B, and a selection cut was conducted in the stand C.
At each site a number of trees was cut down and measured; 15 at the felling site A, 14 at the felling site B and 17 model trees at the felling site C. In doing so, attention was given to the representativeness of the sample (dbh) given the distribution of marked trees. For each cut tree dbh and height (length) were measured. Volume of wood >7 cm was determined by the sectioning method. Branches with a diameter of 3 cm to 7 cm with bark was measured (sectioned) and for the rest of the brushwood, thinner than 3 cm, fresh mass was determined. In felling sites A and B research was conducted in the dormant season, and in the felling site C research was conducted during the growing season. Therefore, the amount of brushwood thinner diameter than 3 cm biomass included foliar biomass.
Modeling of three components of biomass, and total aboveground biomass was carried out according to equations 1, 2 and 3, Equation 1 uses dbh as input with two coefficients (a i b), in Equation 2 an additional independent variable (tree height) was included in order to improve the model and it contains three coefficients (a, b and c). When planning harvesting operations, under the felling plan based on dbh of the marked trees (which are directly measured) with the help of prescribed tariffs planned gross volume of a tree in a specified dbh class is calculated. For this reason (availability of data) in equation 3 volume of tree from tariffs is included as the independent variable with two coefficients a and b. For the evaluation of the models two parameterswere used, the coefficient of determination (R2) and root mean square error (RMSE).
Based on these parameters the best model for the calculation of all three abovegaround biomass components and for the total aboveground biomass is model 2, the exponential equation with two independent variables (d, h), and three coefficients (Table 4). In 11 of the 12 cases this model gives the best results. After the model 2, from the other two models tested, the best model has proven to be the model 1 (in 9 cases). This is somewhat unexpected because the remaining model 3, which uses the volume of tree calculated on the basis of local tariffs prescribed by the management plan as an independent parameter, already includes information on