Mangrove forest structure and composition along urban gradients in Puerto Rico

Abstract

Urban forests are repeatedly characterized as distinct in composition and structure in comparison with their non-urban counterparts. This holds true for mangroves, although previous studies lack quantified representations of urbanness as well as any inclusion of hydrology or water chemistry, which are important influences on mangrove forest structure, composition, and function. This study uses 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. In simple regression, urban metrics were the most powerful predictors of forest composition but not structure. Results show higher tree diversity but lower mangrove diversity in the most urban forests. Structural measurements, however, were best explained by flooding, surface water chemistry, and non-urban land cover metrics. Nitrogen concentrations best explained stem density and tree size, while flooding metrics best explained stand biomass and basal area, and surrounding vegetation cover best explained canopy cover and height metrics. In multiple regression, land cover and surface water chemistry were more important than flooding, with population density again being the most important variable in explaining mangrove forest diversity. Results show that urbanization is an important influence on mangrove composition and basal area, leading to higher tree diversity and lower basal area, consistent with patterns in terrestrial forests. But urban mangrove forests are also lower in mangrove diversity and tend to have representation only by Laguncularia racemosa. Nitrogen concentrations and surrounding vegetation cover, both of which are indirectly influenced by urbanization, were positively related to tree size and canopy cover and height, respectively. These tests suggest urbanization is an important influence on mangrove forest structure and composition, but that flooding and water chemistry must also be considered when managing these forests.

Mangrove leaf and root traits and their relation to urbanization

“…The effect of this metal on mangrove roots has been noted in another study(Da Souza et al 2014), which also notes the adaptive capacity of Laguncularia racemosa to mitigate the toxicity of varying anthropogenic contaminants, including metals. Indeed, this species showed the greatest number of morphological responses to metal content in the present study, and its dominance over the other species increases in urban areas, where it often forms monospecific stands(Branoff and Martinuzzi 2018). This suggests Laguncularia racemosa may be particularly capable of inhabiting urban landscapes, due to an ability to cope with toxic contaminants through morphological variations.…”

“…Green leaves and both live and dead roots were collected from twenty one-hectare mangrove forest stands across three watersheds in Puerto Rico (Figure 1 Table 1). Forest compositional and structural metrics at each site are described in detail in (Branoff and Martinuzzi 2018). All forests border a waterbody and extend from twenty to one-hundred meters inland.…”

“…Foliar δ 15 N increase with stand biomass, as taken from Branoff & Martinuzzi, 2018. This suggests variations in nitrogen stable isotopes are due not to source variability, but to overall site fertility and fractionation processes between the source and the mangroves.…”

Urbanization drives the acoustic diversity of mangrove anurans and avifauna in Puerto Rico

“…The urban mangroves of Puerto Rico, and the sites corresponding to the present study, are described in detail in Branoff and Martinuzzi (2018). Surface water chemistry and flooding dynamics are described in (Branoff, 2018).…”

“…Twenty one-hectare forested areas were selected among three watersheds, which were chosen because they harbored mangroves with the greatest range in urbanization (Figure 1). Urbanization was determined by sampling surrounding spatial datasets of urban land cover (impervious surfaces), vegetation cover, mangrove cover, road density, and population density, as described by (Branoff and Martinuzzi, 2018). The three watersheds are described as the Río Hondo to the Río Puerto Nuevo, referred to here as San Juan; the Río de la Plata, referred to here as Levittown; and the Río Inabón to the Río Loco, referred to here as Ponce.…”