Jill Kowal¹ and Jeffrey G. Duckett²

1- Royal Botanic Gardens Kew

2- Natural History Museum London

Corresponding AUthor: J.kowal@kew.org

Dataset DOI: 10.5061/dryad.sj3tx96md

Description of the data and file structure

Bryophytes, hitherto largely overlooked on street trees, may provide a novel bioindication of air quality at a local human scale, away from park buffer zones. We analysed epiphytic bryophyte communities on street trees across London England, comparing species composition with checkered air pollution, testing vehicle traffic densities typical of cities (low-medium-high) as proxies for NO₂.

Bryophyte distribution was strongly linked to traffic intensity, with higher traffic areas suppressing presence of 15 taxa. While tree host alone had minimal impact on bryophyte assemblages, traffic - tree host interactions were significant. Our findings highlight the dual role of bryophytes as enhancers of biodiversity and as key biological indicators of air quality. Integrating bryophyte monitoring into urban pollution policies offers a cost-effective, long-term strategy for tracking air quality recovery linked with human wellbeing in cities.

This dataset details recordings of bryophyte species' presence or absence on six common street trees (n = 5789) across London England in 2020 and 2021.

Data files and associated script

File: bryodf_DRYAD.xlsx

Description:  The detailed accounts, for each tree (rows; n = 5739) of the bryophyte species present (1) or absent (0) across sites (n = 137), with corresponding nitrogen concentrations and traffic flow for each site.

The .xlsx READ ME tab also contains the variable descriptions below.

Data (bryodf_DRYAD.xlsx) tabs

• 'spe' tab.

  Variables: rows by tree and columns for each bryophyte species (n = 44);

  Taxonomy of species follows Blockeel TL, Hodgetts NG, Pilkington SL, Pescott OL. 2021. A census catalogue of British and Irish bryophytes. British Bryological Society. Hobbs the Printers: Southampton, UK.

• 'combo' tab.

  Variables: species by site, NO₂ concentrations (ug/m3) (see: LAEI 2023, https://www.londonair.org.uk/london/asp/annualmaps.asp.), traffic level (low, medium, high), tree_spp (‘A’ = Acer, ‘Fe’** = Fraxinus, ‘P’ = Platanus x hispanica, ‘Pop’ = Populus, ‘Sa’ = Salix, ‘T’ = Tilia) 

• 'env' tab.

  Variables: reference data with site, traffic level and height (highest point on the tree (in meters) where bryophytes were recorded)

• 'traits' tab.

  Variables: phyla; form; family; perennial/annual; monoecious or dioecious; reproductive organs (bulbils, tubers, gemmae); ellenberg values e.g. elleN*

  *References:

  Callaghan, D.A. and Gadsdon, S., 2023. How basic bryophyte recording provides information on major changes in key conservation localities: a case study of Epping Forest, England, an internationally significant site. Journal of Bryology, 45(2), pp.159-171.

  Hill, M.O., Preston, C.D., Bosanquet, S.D.S. and Roy, D.B., 2007. BRYOATT: attributes of British and Irish mosses, liverworts and hornworts. Centre for Ecology and Hydrology.

  Simmel, J., Ahrens, M. and Poschlod, P., 2021. Ellenberg N values of bryophytes in Central Europe. Journal of Vegetation Science, 32(1), p.e12957.Code/software

Script code

The associated Rscripts (bryo_spp_by_tree_4_pub.R (a) and NMDS_2_8_Indicsp_pub.R (b)) provide detailed steps to (a) summarise diversity and bryophyte communities by tree species. Using R visualisation and statistical packages, the user can also standardise richness for different sampling regimes and determine adequacy of sampling coverage; and (b) construct a NMDS (Non-metric Multidimensional Scaling) to visualise the community by tree species (each dot represents a tree and its community), performs a Permanova analysis on the filtered data (excludes outliers) and produces a revised plot with convex hulls. This is followed by indicator species analyses related to traffic intensity. The later scripts require 'data_combined.csv' which is the csv formatted 'spe' tab from bryodf_DRYAD.xlsx.