Temporal and spatial changes in bryophyte diversity with urbanization in Philadelphia, PA

Background/Question/Methods

As an area is urbanized, habitat characteristics change and affect ecological niches, impacting species composition and ecological function. Tree mosses, with their epiphytic nature and high surface area to mass ratio, are highly susceptible to air pollution and are therefore expected to be sensitive to urbanization. In this study, we ask the following questions: How has the moss flora changed over the last 200 years in the Philadelphia region? and Does the current diversity and abundance of moss species change across and urban-suburban gradient within the city of Philadelphia? Philadelphia was chosen as the study area due to the existence of historic surveys of mosses from as early as the mid-18^th century. We hypothesized that moss richness has decreased over time into the early 20^th century due to rising air pollution and urbanization. We also expected that moss richness would increase from urban to suburban parks.

We compiled 5 moss floras and examined the change in moss species richness and composition over time. The floras we examined ranged from 1743 to 1933. In 2014, we surveyed 15 city parks in Philadelphia for tree moss species richness and abundance: five urban, five post-industrial, and five suburban sites were sampled. 

Results/Conclusions

The flora of the Philadelphia metropolitan region has changed considerably over time. There were 71 species found in the 1700s, 157 found in the 1800s, and 98 found in the 1900s. All current species except Dicranum montanum were found in at least one historical database. Eleven species of tree mosses and liverworts were found across Philadelphia. Five were present in all three site categories; one of these was a liverwort (Frullania eboracensis). Species richness was significantly higher in the suburban parks (average 6.2 species) than in the post-industrial (average 3.8 species) and city center (average 2.8 species) parks. Additionally, the average number of species found on individual trees was also highest in the suburban parks (average = 1.89/tree). The number of trees with moss presence did not differ across the urban-suburban gradient. Nonmetric multidimensional scaling separated the urban sites from the suburban sites in both moss and tree species. The suburban sites had higher beta-diversity of moss and tree species, indicating more variability among sites in the suburban region. We expect tree species diversity and size as well as air pollution and park management to be drivers of bryophyte diversity in cities.