Moss functional trait ecology: Trends, gaps, and biases in the current literature

Functional traits are critical tools in plant ecology for capturing organism-environment interactions based on trade-offs as well as making links between organismal and ecosystem processes. While broad frameworks for functional traits have been developed for vascular plants, we lack the same for bryophytes, despite an escalation in the number of bryophyte functional trait studies conducted in the last 45 years and an increased recognition of the ecological roles bryophytes play across ecosystems. In this review, we compiled data from 282 published articles (10005 records) focusing on functional traits measured in mosses, and sought to (a) examine trends in types of traits measured, (b) capture taxonomic and geographic breadth of trait coverage, (c) reveal biases in coverage in the current literature, and (d) develop a bryophyte-function index (BFI) to describe completeness of current trait coverage and identify global gaps to focus research efforts. The most commonly measured response traits (those related to growth/reproduction in individual organisms) and effect traits (those that directly affect community/ecosystem scale processes) fell into the categories of morphology (e.g. leaf area, shoot height) and nutrient storage/cycling, and our BFI revealed that these data were most commonly collected from temperate and boreal regions of Europe, North America and east Asia. However, fewer than 10% of known moss species have available functional trait information. Our synthesis revealed that there is a need for research on traits related to ontogeny, sex, and intraspecific plasticity, and on co-measurement of traits related to water-relations and bryophyte-mediated soil processes. 

Methods
We performed two searches in the Web of Science Core Collection on March 13, 2023 to compile a list of all published research on terrestrial moss functional traits. Search terms were selected based on our preliminary literature review and general expertise on mosses and functional trait ecology. We did not use an a priori list of moss functional traits, and, rather, allowed the literature to inform the diversity of traits analyzed in this review. Our first search identified articles with the terms moss or bryophyte and trait or morphology ((moss* OR bryophyte*) AND (trait* OR morpholog*)) and the second search identified articles with the terms moss or bryophyte AND function AND ecosystem ((moss* OR bryophyte*) AND function* AND ecosystem*). Combined, these searches identified 9989 research articles that met our criteria. Article titles and abstracts were subsequently scanned in two stages to exclude duplicates and articles without trait data connected to a specific moss genus (versus moss or biocrust community for example; Appendix S3). Non-English language articles were included when an English-language abstract was available or the article aligned with one of our language proficiencies (Spanish, French, Portuguese, German). This narrowed the number of relevant articles on moss functional traits to 282 with the earliest articles dating back to 1979. Functional trait concepts predate 1979, and so our lack of earlier records likely reflects our choice of search terms, however, our database covers the current rapid expansion of functional trait ecology.  We acknowledge that our strategy may have missed some articles from smaller journals outside of the Web of Science Core Collection but are certain that we captured the majority of research published on this topic with our chosen methods.