The Structure and Composition of Puerto Rico's Urban Mangroves.

Benjamin L Branoff,Sebastián Martinuzzi

doi:10.3390/f11101119

Abstract

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.

Highlights

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 few decades [1,2,3,4]
Fifty-one percent of stems were represented by Laguncularia racemosa, 29% were Rhizophora mangle, 9% were Avicennia germinans, 7.5% were
Results from three separate statistical methods (i.e., Non-metric multidimensional scaling, simple regression, and multiple regression) suggest an indirect influence of urban land use on mangrove forest composition and structure in Puerto Rico, and that this influence is shared between flooding dynamics and water chemistry

Summary

Introduction

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 few decades [1,2,3,4]. The conversion of forests to developed lands usually follows economic transitions that favor industrialized societies and continuing transitions may in some cases support regrowth, albeit with novel forests [5,6]. Forests in urban and surrounding lands are often represented by unique anthropogenic influences and ecological traits [7,8,9]. Characterizations of urban forests often consist of lower stem densities and larger individuals, with stands exhibiting increased edge openness and regeneration failure [8]. These forests are further generalized as having higher floral diversity, usually due to non-native species from residential gardens and municipal landscaping. The function of these systems is novel, as evidenced by Forests 2020, 11, 1119; doi:10.3390/f11101119 www.mdpi.com/journal/forests

Methods

Results

Discussion

Conclusion