Mangrove leaf and root traits and their relation to urbanization

Abstract

Root and leaf traits are one means of understanding plant ecophysiological responses to environmental variation and disturbance. In mangroves, both chemical and morphological variations have been recorded in response to changes in inundation, salinity, and nutrient levels. Some have also been noted in urban environments, primarily in response to elevated nutrients and toxic substances. Yet these studies have not attempted to isolate the urban from the non-urban influences on both morphological and chemical traits. This study measured mangrove leaf and root chemical and morphological traits in herbarium samples and in field collected leaves and roots along a quantified urban gradient in three watersheds of Puerto Rico. It then correlated these traits with predictors of surrounding land cover, as well as with metrics of flooding and water chemistry. There were significant lines of evidence leading to an influence of urban sewage and roads on leaf and root traits. Leaf percent nitrogen increased with urbanization and with surface water nitrogen and phosphorus concentrations, but its isotopic content decreased with increasing phosphorus, leading to the hypothesis that both nitrogen and phosphorus are fueling an otherwise co-limited community of mangroves and nitrogen fixing microbes. The most urban site harbored some of the highest metal concentrations, and there was evidence that elevated concentrations primarily influence fine roots. Other morphological traits are more likely explained by both water chemistry and leaf chemistry and should be considered when interpreting the influence of urban landscapes on mangroves. Percent nitrogen in herbarium samples increased at the least urban site, but remained unchanged at the most urban site, reflecting the relative levels of urbanization at the time of the first samples and their subsequent changes. Most metals from herbarium samples decreased or remained unchanged, suggesting regulation and infrastructure have helped to reduce the release of trace metals to the estuaries. Understanding the influence of urbanization in the context of water chemistry and flooding dynamics will aid in the management of these systems as global urbanization continues.