Demystifying field application of Critical Height Sampling in estimating stand volume

Abstract

Critical Height Sampling (CHS) estimates stand volume free from any model and tree form assumptions. Despite its introduction more than four decades ago, CHS has not been widely applied in the field due to perceived challenges in measurement. The objectives of this study were to compare estimated stand volume between CHS and sampling methods that used volume or taper models, the equivalence of the sampling methods, and their relative efficiency. We established 65 field plots in planted forests of two coniferous tree species. We estimated stand volume for a range of Basal Area Factors (BAFs). Results showed that CHS produced the most similar mean stand volume across BAFs and tree species with maximum differences between BAFs of 5–18 ​m³·ha⁻¹. Horizontal Point Sampling (HPS) using volume models produced very large variability in mean stand volume across BAFs with the differences up to 126 ​m³·ha⁻¹. However, CHS was less precise and less efficient than HPS. Furthermore, none of the sampling methods were statistically interchangeable with CHS at an allowable tolerance of ≤55 ​m³·ha⁻¹. About 72% of critical height measurements were below crown base indicating that critical height was more accessible to measurement than expected. Our study suggests that the consistency in the mean estimates of CHS is a major advantage when planning a forest inventory. When checking against CHS, results hint that HPS estimates might contain potential model bias. These strengths of CHS could outweigh its lower precision. Our study also implies serious implications in financial terms when choosing a sampling method. Lastly, CHS could potentially benefit forest management as an alternate option of estimating stand volume when volume or taper models are lacking or are not reliable.