Frequency Versus Individual, Clumps, and Openings Prescription Marking: Comparison in Ponderosa Pine Restoration Treatments Using Uncrewed Aerial Systems

Abstract Management of western United States dry-conifer forests has experienced a design shift from spacing-based treatments to maximize forest growth and minimize fire hazard towards balancing fuels reduction while restoring historical forest spatial patterns. However, successfully implementing spatially heterogeneous silvicultural treatments capable of mimicking a landscape’s historical range of variability (HRV) has proven challenging. Early individual tree marking (ITM) strategies aiming to create heterogeneous forest structure often had unclear or confusing instructions. This study evaluates the ability of five ITM strategies to recreate HRV forest structure in Black Hills, South Dakota ponderosa pine forests. We started by first comparing uncrewed aerial system (UAS) derived forest inventory data against field plots and then assessing the UAS-derived tree spatial patterns of each ITM strategy against HRV. The UAS-detected trees were comparable to field plots providing an F-score averaging 0.943. The five ITM strategies created tree group size distributions that captured the range of individual trees and tree-clump-sizes but varied in their approximation of the HRV distribution. The individuals, clumps, and openings strategy and the two lowest cut-to-leave tree frequency strategies consistently matched four of five HRV group size metrics, while the other strategies typically only matched two or three metrics. Additionally, the frequency cut-to-leave tree strategy with 1:2 small-tree and 1:1 large-tree more often achieved all five HRV group size metrics compared to all other ITM strategies. Given the relatively even-aged starting forest structure from the stand’s past management history, the lowest frequency cut-to-leave tree strategy provided the easiest to implement and most effective means of reproducing the HRV forest structure.