Designing circular fixed-area plots in large-scale forest inventories: effect of horizontal distance measurement uncertainty and tree position pattern

Plot design is one of the key elements that must be defined in forest inventories. This is particularly challenging in large-scale inventories, as both forest variability and measurement uncertainty generally increase with scale. Nonetheless, plot design is usually based exclusively on targeting (i) an average minimum number of trees to reduce the random measurement errors, and (ii) an average maximum number of trees to optimize the efficiency of fieldwork, while ignoring the uncertainty in tree position measurement. The present study focused on the effect of horizontal distance measurement errors on stand level estimates in large-scale forest inventories including circular fixed-area plots. The error was characterized in forests with non-regular (natural stands) and regular (plantations) patterns of tree positions. The effect on stand volume, stand basal area, and stand density estimates was simulated using Monte Carlo techniques. Different horizontal distance measurement uncertainty was observed in natural stands and plantations. However, similar effects were observed in the three stand variables estimates for both tree spacing patterns, with stabilization of errors for radii between 12 and 20 m. Doubling or halving the error uncertainty yielded similar results. The proposed method can help with selecting plot size in forest inventories based on circular fixed-area plots.