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The Structure and Composition of Puerto Rico’s Urban Mangroves
Citation:
branoff, B. AND S. Martinuzzi. The Structure and Composition of Puerto Rico’s Urban Mangroves. Forests. MDPI, Basel, Switzerland, 11(10):1119, (2020). https://doi.org/10.3390/f11101119
Impact/Purpose:
Urbanization has been an important contributor to forest disturbance at the turn of the twentieth century, with trends indicating an increasingly important role in tropical coastal areas over the next few decades. To date, research has repeatedly indicated that urban forests are characterized as distinct in composition and structure in comparison with their non-urban counterparts. This distinction, in combination with more people in densely populated urban areas, creates novel scenarios for the provisioning of ecosystem services in urban social-ecological forests. This is especially true for mangroves, whose ecosystem services are highly valued due both to the unique capability of these forests to provide them as well as to the increasing number of dependents on these services in densely populated areas. Thus, a greater understanding of how urbanization influences mangrove forest structure and composition will lead to more effective management of these systems towards optimal provisioning of ecosystem services. This study uses (Light Detecting and Ranging) LiDAR and ground-based measurements of mangroves within well quantified urban gradients in Puerto Rico to test for the relative importance of urbanization alongside flooding metrics and surface water chemistry in explaining observed patterns of forest structure and composition. Three separate statistical tests suggest a significant but limited influence of urbanization on forest composition and structure. This influence was most marked in composition, with the most urban forests containing higher overall tree diversity but lower mangrove diversity. In structural metrics, water chemistry was most influential, with higher nitrogen concentrations associated with larger trees, higher canopies, and lower stem densities. Still, multiple regression suggested that while a given forest metric may be strongly linked to either land cover, water quality, or flooding, all three are likely important and should be considered when characterizing these forests. With little known on how urbanization specifically influences mangrove forests, this study provides important groundwork that will guide present management of increasingly urbanized mangrove shorelines, as well as future studies that seek to further understand the impact of urban land use on mangrove structure, function, and the provisioning of ecosystem services. Further, because these measurements were taken just before the 2017 hurricane season and its associated disturbance of Puerto Rico's forests, this study provides a pre-disturbance baseline by which to measure the ongoing response. We anticipate high interest in this research by local ecosystem managers seeking to understand how ongoing or future urbanization may impact local mangrove ecosystems, as well as by the global scientific community seeking to further understand the dynamic complexity of urban social-ecological systems.
Description:
This study characterizes the structure and composition of mangrove forests across urban gradients in Puerto Rico. It then uses a suite of hydrologic, water chemistry, and land cover variables to test for the relative importance of urban intensity alongside flooding and water chemistry in explaining observed variability in forest structure and composition. Three separate statistical tests suggest a significant but limited influence of urbanness on forest composition and structure. In the most urban sites, the diameters of the largest trees were 27% larger, but all structural measurements were best explained by surface water chemistry, primarily nitrogen concentrations. Concentrations of ammonium and total Kjeldahl nitrogen best explained stem density, tree girth and canopy height. The most urban forests also contained 5.0 more species per hectare, on average, than the least urban forests, and simple regression suggests that urban metrics were the most powerful predictors of forest composition. The most urban forests were more dominated by Laguncularia racemosa, while both Avicennia germinans and Rhizophora mangle were found to be less abundant in the most urban sites, a trend that may be linked to the influence of precipitation and tidal connectivity on porewater salinity across the urban gradient. In multiple regression, no statistical difference was detected in the importance of surrounding land cover, flooding, or water quality in explaining the variance in either composition or structural metrics. This suggests that while a given forest metric may be strongly linked to either land cover, water quality, or flooding, all three are likely important and should be considered when characterizing these forests. With more human dependents in urban areas, the provisioning of important ecosystem services may be influenced by land use variables in addition to the more commonly measured metrics of water chemistry and flooding.
