Repeat terrestrial laser scanning for documenting and monitoring prescribed fire efficacy

Background Prescribed fire is an established means to reduce wildfire risk and accomplish silvicultural and cultural objectives via controlled consumption of fuels and, most commonly, subcanopy fuels. Terrestrial laser scanning (TLS) has been demonstrated as a simple and efficient means to quantify fuel and ecological conditions in terms of three-dimensional forest structure at the plot-scale; however, there is currently a lack of studies that explore a repeat time series of TLS measurements for documenting fire treatment efficacy over time and to inform when a subsequent prescribed burn should be applied. We used a time-series approach to repeatedly scan 43 monitoring plots six times over a four-year period (one pre-fire, one post-fire, and yearly scans over a four-year period) within a burn unit in the New Jersey Pine Barrens, USA. Our TLS analysis evaluates metrics to quantify first-order effects on forest structure and subsequent structural changes and regrowth. Metrics include (1) the percentage of all returns within a 10 m plot radius that were not classified as ground and (2) an occlusion-adjusted percent of non-occluded voxels with returns at the plot- and three strata-levels: canopy, tall shrub, and low shrub. We also made comparisons to other commonly used burn severity assessment methods, including the visual field-based composite burn index (CBI), as well as the Sentinel-2 Multispectral Instrument (MSI)-derived normalized burn ratio (NBR) and difference normalized burn ratio (dNBR). Results We document that the time series of TLS metrics for the tall shrub and low shrub strata were very effective in characterizing post-fire dynamics of subcanopy fuels and can complement other commonly used burn severity assessment methods. Conclusions We argue that TLS-based, occlusion-adjusted methods should be considered as a repeatable, defendable, and low bias means for land managers to document prescribed fire efficacy and to inform when to re-apply treatment.