Sampling designs for bark stripping caused by red deer (Cervus elaphus L.): Simulation studies based on nine fully censused stands

Abstract

Precise assessment of bark stripping damage is of high economic importance, since bark stripping makes wood unusable for saw timber and it is the basis for compensation payments for game damage. Bark stripping is clustered and more frequently observed on small trees, so that standard forest inventories optimized for volume or volume increment are unlikely to be optimal. In this study we analysed different sampling designs (random sampling, systematic sampling), sampling intensities, tree selection methods (fixed radius plot, angle count sampling) and number of plots and plot sizes (plot radius: 2-20 m; basal area factor: 1-6m²/ha) for bark stripping assessment. The analysis is based on simulation studies in 9 fully censused stands (9026 trees). Simulations were done for actually assessed damage and randomly distributed damage and each scenario was repeated100 times with different random points or different random grid locations. Systematic sampling was considerably more precise than random sampling. Sampling intensities to attain a standard error of 10 % ranged between 12 and 18 % dependent on the plot size. For a given sampling intensity, precision increases with decreasing plot size or increasing basal area factor. This implies, however, that numerous plots need to be measured, which is unlikely to be the cheapest alternative, when travel costs are high. Differences between tree selection by fixed radius plots or angle count sampling were minor. For sampling bark stripping we recommend fixed radius plots with a radius of 4-6 m and a sampling intensity of 10-15 %.