remote sensing Article
Elevation and Distribution of Freshwater and Sewage Canals Regulate Canopy Structure and Differentiate Hurricane Damages to a Basin Mangrove Forest Qiong Gao and Mei Yu * Department of Environmental Sciences, University of Puerto Rico, Rio Piedras, San Juan, PR 00926, USA; q.gao@ites.upr.edu * Correspondence: meiyu@ites.upr.edu
Citation: Gao, Q.; Yu, M. Elevation and Distribution of Freshwater and Sewage Canals Regulate Canopy Structure and Differentiate Hurricane Damages to a Basin Mangrove Forest. Remote Sens. 2021, 13, 3387. https:// doi.org/10.3390/rs13173387 Academic Editors: Eileen H. Helmer, Patrick J. Comer, David Gwenzi, Wan Shafrina Wan Mohd Jaafar and Xiaolin Zhu Received: 4 July 2021 Accepted: 24 August 2021 Published: 26 August 2021
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Abstract: The coastal mangrove forest bears important ecosystem functions and services, including the protection of shorelines and coastal communities. While coastal mangroves often suffer severe damage during storms, understanding the vulnerability and resistance of mangroves to the damage at a landscape scale is crucial for coastal mangrove management and conservation. In September 2017, two consecutive major hurricanes caused tremendous damage to the coastal mangroves in the Caribbean. By utilizing LiDAR data taken before and after the hurricanes in a basin mangrove forest in Northeast Puerto Rico, we analyzed the spatial variation of a canopy structure before the hurricanes and hurricane-induced canopy height reduction and explored possible drivers by means of spatial regressions. Regarding the canopy structure, we found that the pre-hurricane canopy height of the mangrove forest decreased with elevation and distance to the freshwater/sewage canals within the forest, and these two drivers explained 82% of variations in the mangrove canopy height. The model, thus, implies that freshwater and nutrient inputs brought by the canals tend to promote the canopy height, and mangrove trees at lower elevation are especially more advantageous. Similarly, tree densities decreased with the canopy height but increased with the elevation and the distance to the canals. We also found that this mangrove forest suffered on average a 53% canopy height reduction, reflecting mostly heavy crown defoliation and the rupture of branches. The regression, which explains 88% of spatial variation in the canopy height reduction, showed that mangroves with a higher canopy or lower density, or growing in lower elevation, or being closer to the canals suffered more damage. Our findings indicate that delivered freshwater/sewage by means of humanmade canals has a strong impact on the canopy structure as well as its resistance to tropical storms. Freshwater and sewage tend to release the salinity stress and nutrient deficit and, thus, to promote the mangrove canopy height. However, the addition of freshwater and nutrients might also increase the risk of mangrove damage during the storms probably because of an altered allometry of assimilates. Keywords: coastal mangroves; anthropogenic activities; canopy structure; tropical cyclones; Puerto Rico; LiDAR
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Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).
1. Introduction Mangrove forests were estimated to occupy 137,760 km2 of the global coast [1], and are homes of many fauna from invertebrate to vertebrate [2]. Mangrove forests grow in a salty and anaerobic environment with a great carbon sequestration capability due to a high carbon assimilation but low ecosystem respiration, and the extremely high water and nutrient use efficiencies of the mangrove trees [3]. The capability of fixing great amount of nutrients makes mangrove the best filter of toxicants in the coastal water [4,5]. Meanwhile, mangroves tend to stabilize the coastline because of their high capacity of vertical accretion to offset the coastal retrieve due to sea level rise [6]. Tropical and subtropical coasts are often the places of historical agriculture, tourism, and urban development, which compete with mangrove forests for land and other re-
Remote Sens. 2021, 13, 3387. https://doi.org/10.3390/rs13173387
https://www.mdpi.com/journal/remotesensing