Urbanization impacts on epiphytic nitrogen and metal cycling in Acer macrophyllum stands in Western Washington, USA

Abstract

Bryophytes have been used extensively in Europe to assess the impacts of air pollution and atmospheric deposition; however there have been no studies conducted in Washington State to examine how pollution associated with urbanization is impacting canopy and forest floor bryophyte communities. This study investigated in N2 fixation rates and metal concentrations in bryophytes across Acer macrophyllum Pursh stands in western Washington. We used an intensive, vertically stratified sampling approach in A. macrophyllum canopies in the Hoh Rainforest on the Olympic Peninsula, WA and in Seattle, WA to collect 214 samples of Isothecium stoloniferum (Brid.). An extensive, ground-based sampling approach was used across an urban-to-wildland gradient to collect 59 Kindbergia praelonga (Hedw.) Ochyra samples. Intensive samples were collected four times (April, July, and October of 2016; and in January 2017) and extensive samples three times (April, July, and October 2016) to assess seasonal differences in N2 fixation rates and metal concentrations across sampling locations. Nitrogen fixation rates in canopy I. stoloniferum samples was found to be dependent on sampling season and location. Average N2 fixation rates reached peak levels in spring 2016 for both peninsula and urban samples (588.34 and 179.02 µmol m-2 day-1, respectively) and were lowest in fall 2016 (18.47 and 5.29 µmol m-2 day-1). In total, I. stoloniferum in canopies of A. macrophyllum was found to yield 1.13 kg N ha-1 yr-1 in the Hoh Rainforest and only 0.009 kg N ha-1 yr-1 in the Seattle urban area. For the heavy metal analysis, elevated concentrations of Cd, Cr, Cu, Fe, K, Mg, Mn, Ni, Pb, Sr, Ti, and Zn were found in both intensive and extensive bryophyte samples collected in Seattle across all collection seasons. The elevated concentrations of heavy metals observed in I. stoloniferum samples collected from Seattle may be related to the low the N2 fixation rates observed in the same samples. The results from this study highlight that canopy and ground floor bryophytes has the potential to be used as a low-cost screening tool to evaluate N and heavy metal pollution in urban and rural areas.