remote sensing Article
Canopy Density and Roughness Differentiate Resistance of a Tropical Dry Forest to Major Hurricane Damage Qiong Gao and Mei Yu * Department of Environmental Sciences, University of Puerto Rico, Rio Piedras, San Juan, PR 00936, USA; q.gao@ites.upr.edu * Correspondence: meiyu@ites.upr.edu; Tel.: +1-787-764-0000
Citation: Gao, Q.; Yu, M. Canopy Density and Roughness Differentiate Resistance of a Tropical Dry Forest to Major Hurricane Damage. Remote Sens. 2021, 13, 2262. https://doi.org/ 10.3390/rs13122262
Abstract: Tropical dry forest is vulnerable to increased climate variability with more frequent and severe storms. Studies of hurricane impact on tropical dry forest often focused on individual tree traits. How trees in tropical dry forests work together to combat wind damage is still unclear. To address this, we integrated ground-observed ecosystem structure from National Ecological Observation Network (NEON) with airborne-LiDAR images and analyzed resistance in forest structure of Guánica dry forest in Puerto Rico to major hurricanes in 2017 at the forest-stand level. Using each plot instead of the individual tree as the base unit, we regressed mean changes in stem height and fractions of lost or damaged stems at 15 plots on mean stem diameter, mean and standard deviation of stem height, stem density, and topography. Meanwhile, using the LiDAR-derived canopy heights, we compared the changes in canopy height before and after the hurricanes and regressed spatially the canopy height change on prior-hurricane tree cover, canopy height, and rugosity. We found that the damage was small in places with high stem density or high tree cover. Ground-observed damage in terms of height reduction significantly increased with the standard deviation of stem height, an index of roughness, but decreased with the mean stem diameter of the plots. LiDAR-detected damage in terms of reduction in canopy height was also found to decrease with tree cover and mean canopy height when the canopy height was small or moderate but increase with the rugosity. The fraction of lost stems significantly decreased with the stem density, and the fraction of damaged stems significantly increased with the roughness and the plot elevation. The collective parameters of forest stand quantified from ground-observation and LiDAR, such as stem density, tree cover, and canopy roughness or rugosity, highlighted mutual supports of trees and played important roles in resisting damages to the tropical dry forest during major hurricanes.
Academic Editor: Eileen H. Helmer Received: 4 May 2021
Keywords: major hurricane; tropical dry forest; canopy density; surface roughness; canopy height model; LiDAR; Caribbean
Accepted: 7 June 2021 Published: 9 June 2021
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1. Introduction Tropical and subtropical dry forest accounts for 42% of the global tropical and subtropical forest cover [1] and spans climate regions with annual rainfall ranging from 250 to 2000 mm, annual mean bio-temperature greater than 17 ◦ C, and the annual ratio of potential evapotranspiration to precipitation greater than 1 [2]. The C stock of tropical dry forest accounts for 8–9% of the global estimate, yet the net primary production was 620 g C m−2 , which is only lower than that of the tropical wet forest [3,4]. There are always 2–6 months of dry season, and the water-limited nature makes tropical dry forests especially sensitive to climate variability and extreme climate events such as drought and storms [5–8]. The structure of the tropical dry forest is substantially different from that of the tropical wet and moist forests. The dry forest has, in general, lower tree richness than wet forest, and the low diversity is especially found in island forests [1]. According to the negative exponential relationship between root to shoot ratio and annual rainfall [9], the tropical dry forest has a larger root:shoot ratio (0.57–0.71) than moist and wet forests (0.17–0.35) to
Remote Sens. 2021, 13, 2262. https://doi.org/10.3390/rs13122262
https://www.mdpi.com/journal/remotesensing