Establishing bridging functions for harmonizing growing stock estimates: examples from European National Forest Inventories

Estimates of growing stock in European countries vary mainly by using different thresholds for dbh of sample trees, as well as by the inclusion or exclusion of stump and stem top volume. European national forest inventories use dbh thresholds ranging from 0 to 12 cm in estimating the volume of growing stock. COST Action E43 has agreed to a reference definition for growing stock with a dbh threshold of 0 cm. With use of national volume distributions by dbh classes, models for estimating the proportions of growing stock between the national threshold and the 0-cm threshold were constructed. Models for characterizing growing stock distributions were tested, and their predictive abilities were investigated. Similar comparisons were made with respect to the volume of stumps and stem tops. Examples of estimation methods and the resulting percentages of these tree elements of total growing stock are presented.