Repeat terrestrial laser scanning for documenting and monitoring prescribed fire efficacy

Abstract Background Prescribed fire is an established means to reduce wildfire risk and accomplish silvicultural and cultural objectives via controlled consumption of fuels. Terrestrial laser scanning (TLS) has been demonstrated as a simple and efficient means to quantify fuel and vegetation conditions in terms of three-dimensional forest structure and occupied volume 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. We used a time-series approach to repeatedly scan 40 monitoring plots six times over a 4-year period (one pre-fire, one post-fire, and yearly scans over a 4-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 the percentage of all returns that were not classified as ground, as well as a volume-based, occlusion-adjusted percentage 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 difference vegetation index (NDVI) and normalized burn ratio (NBR). Results The TLS methods were most effective at characterizing changes in the low and tall shrub strata, which are commonly targeted during prescribed fire treatment. Post-fire change metrics were generally correlated with initial metric values and initial change in metric values, especially in the low and tall shrub strata where all Pearson and Spearman correlation coefficients were above 0.400 and as high as 0.955. We also document correlations among the TLS-based metrics and the initial change and rate of post-fire change in NDVI and NBR and the field- and visual-based composite burn index (CBI). Finally, we document that plot-level metrics can be aggregated to characterize entire burn units. Conclusions Multi-temporal TLS compliments NBR- and CBI-based assessment and improves the quantification of prescribed fire efficacy for reducing the abundance of fuels in the low and tall shrub strata.