Many aspects of forest structure are thought to contribute to the presence, abundance, and diversity of forest-floor bryophytes. To what extent easily measured characteristics of local environment (overstory structure or substrate availability) explain patterns of abundance and diversity remains unclear in most forest ecosystems. We explore these relationships in four mature, Douglas-fir dominated forests in southwestern Washington (U.S.A.). At each site, six 13-ha stands were sampled systematically (787 total plots) to capture variation in physical environment, forest overstory and understory vegetation, substrates, and forest-floor bryophytes. We compared bryophyte species composition, richness, and rank abundance among sites. We used multiple linear regression to model local cover and richness as functions of the physical environment, overstory and understory vegetation, and substrate availability. We compared the contributions of substrates to bryophyte abundance and richness and measured the strength and consistency of associations between individual species and particular substrates. Bryophyte composition differed significantly among sites, but patterns of richness and rank abundance were surprisingly similar, despite differences in stand structure and climate. Regression models explained only 18–23% of the variation in bryophyte cover or richness, likely due to weak relationships between vegetation structure and microclimate, disparity in the spatial scales of measurement, and temporal lags in bryophyte responses to structure. Most non-litter substrates (mineral soil, rock, fresh or decayed wood, stumps, shrub and tree bases) contributed minimally to available growing space, but disproportionately to species richness, particularly at the stand scale. Individual species were most often associated with decayed wood, although few species showed strong substrate specificity. In general, however, substrates contributed to ecological redundancy, with most species occupying multiple substrates and most substrates supporting a diversity of species.
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Vol. 115 • No. 2