Post-variable density treatment monitoring in dry-site mixed conifer stands in unmanned aircraft systems.

Abstract

Variable density thinning (VDT) post-treatment monitoring is challenging and potentially costly because resulting stand structure is not well characterized by small area plots used in forestry. Unmanned aircraft systems (UAS) offer a potential solution for efficient VDT monitoring via rapid survey and subsequent generation of a stem map. In this study we used a UAS to survey and estimate spatial dispersion and stand structure (e.g., basal area, trees per acre, and quadratic mean diameter) in untreated and VDTtreated stands. Results showed evidence of increased clustering in the treated stand at intertree distances between 17 and 25 feet while the untreated stand exhibited a pattern of random dispersion. UAS-derived stand structure estimates differed in comparison to stand exam estimates with UAS underestimating basal area and trees per acre and overestimating quadratic mean diameter. However, comparison between UAS estimates in the treated and untreated stands revealed expected trends of decreased density and increased diameter. UAS survey and data processing time was less than one-fifth of the time required for common stand exams. Given the increased time efficiency, the biased UAS estimates are likely an acceptable tradeoff for VDT monitoring. Although this study demonstrates the utility of UAS for post-treatment monitoring, additional testing through research and management collaboration is required to refine the method and quantify error.