The effects of truncating the angle count sampling method on the Austrian National Forest Inventory

Abstract

Key message: Truncating the angle count sampling plots leads to a higher stock estimate because fewer trees are missed. It also causes a higher uncertainty of the estimates. Context: The Austrian National Forest Inventory changed its plot design to angle count sampling plots that are limited to circles of 300 m2. A fixed plot size is beneficial for multistage inventory systems. Aims: This study aims to quantify the ensuing effects on the national forest inventory on the plot and the national level regarding stock estimates and their uncertainty along with effects on as well as of upper diameter models. Methods: Fully surveyed large-scale plots with simulated angle count samples were used to gauge the increase of variability. The full NFI 2007/09 data set was employed to simulate truncations at different distances as well as to parameterise eight alternatives for upper diameter models and to analyse the effects on the stock estimates. Results: There is an increase in the stock estimate because the probability of missing sample trees is reduced. The change in upper diameter models had little effect. Conclusion: The truncation removes a source of bias, facilitates field work, and is a favourable way forward.