The impact of precision of tree position measurements and different plot designs on the estimates of tree level production and diversity parameters

Sample plots are basic units of statistical forest inventories. The choice of their shape and size, and sampling methods have changed over time due to economic constraints, efficiency and changes in human demands on data about forests. In the presented study we analysed the impact of three different sampling units: fixed-area plots, fixed-different-area plots, and nested concentric plots, on the estimates of tree level production and diversity parameters. These sampling units were measured during the regional inventory at the University Forest Enterprise of Technical University Zvolen, Slovakia, which was repeated four times (1986, 1992, 1998, 2012). Within each inventory plot, all positions of trees were repeatedly and independently measured three times (1986, 1998, 2012) by different operators using different tools. From these data we quantified the error of tree position resulting from human and technological factors and analysed its impact on the estimates of tree level diversity and production parameters. The selected parameters were: number of trees, stand basal area, standing volume per hectare, number of tree species and number of vertical tree layers. The results indicate that the plot design primarily affects ecological characteristics of forests. Fixed-area plots seem to be the most suitable sampling unit from the point of multi-criteria evaluation of forest status and forest change.