Abstract
In the pine savannas of the southeastern United States, prescribed fire is commonly used to manipulate understory structure and composition. Understory characteristics have traditionally been monitored with field sampling; however, remote sensing could provide rapid, spatially explicit monitoring of understory dynamics. We contrasted pre- vs. post-fire understory characteristics collected with fixed area plots with estimates from high-density LiDAR point clouds collected using the unmanned aerial vehicle (UAV)-borne GatorEye system. Measuring within 1 × 1 m field plots (n = 20), we found average understory height ranged from 0.17–1.26 m and biomass from 0.26–4.86 Mg C ha−1 before the fire (May 2018), and five months after the fire (November 2018), height ranged from 0.11–1.09 m and biomass from 0.04–3.03 Mg C ha−1. Understory heights estimated with LiDAR were significantly correlated with plot height measurements (R2 = 0.576, p ≤ 0.001). Understory biomass was correlated with in situ heights (R2 = 0.579, p ≤ 0.001) and LiDAR heights (R2 = 0.507, p ≤ 0.001). The biomass estimates made with either height measurement did not differ for the measurement plots (p = 0.263). However, for the larger research area, the understory biomass estimated with the LiDAR indicated a smaller difference after the burn (~12.7% biomass reduction) than observed with in situ measurements (~16% biomass reduction). The two approaches likely differed because the research area’s spatial variability was not captured by the in-situ measurements (0.2% of the research area measured) versus the wall-to-wall coverage provided by LiDAR. The additional benefit of having spatially explicit measurements with LiDAR, and its ease of use, make it a promising tool for land managers wanting greater spatial and temporal resolution in tracking understory biomass and its response to prescribed fire.
Highlights
In the southeastern United States, the forested landscape was once dominated by longleaf pine (Pinus palustris Mill.) stands, mixed stands of longleaf pine, and slash pine (Pinus elliottii Engelm.), or mixed stands of longleaf with other species [1]
The prescribed fire occurred in June 2018, and understory biomass was remeasured ~one week (0 months) following the fire, which inthe understory biomass was remeasured ~one week (0 months) following the fire, which dicated that only ~23% of living understory biomass remained (0.79 ± 0.59 Mg ha−1) (Figure indicated that only ~23% of living understory biomass remained (0.79 ± 0.59 Mg ha−1 )
We found that understory structure, height and biomass, recovered quickly after the fire in these flatwood pine savannas
Summary
In the southeastern United States, the forested landscape was once dominated by longleaf pine (Pinus palustris Mill.) stands, mixed stands of longleaf pine, and slash pine (Pinus elliottii Engelm.), or mixed stands of longleaf with other species [1] Mature forests featuring these species associations once covered 37 million ha [1,2], but longleaf pine in particular declined by nearly 98% due to logging, fire suppression, conversion to agricultural and other land uses, and replacement by other types of forests [3,4,5,6,7]. As seedlings, both longleaf pine and slash pine are intolerant to competition for light, making their natural regeneration dependent on some repression of competing vegetation with fire [9]
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