Efficiency of C-nn distance sampling method in comparison to sampling with fixed area plot for estimation of forest stem basal area in Kheyroud forests of northern Iran

Plotless or distance sampling methods, also known as k- tree sampling, are practical field sampling methods for forest inventories and ecological surveys. In this research, in order to test the efficiency of Continues Nearest Neighbor (C-nn) distance sampling method a 220 ha forest area was selected in Kheyroud forests of northern Iran. Then field sampling was performed using: 1- circular sample plots of 1000 m2 which is conventional in these forests and 2- C-nn samples with k = 15 trees, both based on a 150 m×200 m systematic grid. In total, we measured 77 circular sample plots and 77 C-nn samples in the study area. Results showed that there was no significant difference between estimation of forest stem basal area using the two methods and even among different values of k (6 to 15). However, sampling precision decreased significantly by decreasing the amount of k from 7 to 6 trees. Furthermore, time cost in the C-nn method was significantly lower than the other one. Therefore, regarding to the ease of implementation and the statistical precision of the C-nn sampling method, we concluded when sampling using plotless methods is needed to apply in these kind of forest, the C-nn sampling method can be implemented with k =7 trees based on cost and precision of inventory.