To assess germination success in different microsites of a forested wetland environment, seeds of three common western New York wetland tree species, Acer x freemanii, Fraxinus pennsylvanica, and Ulmus americana, were sown into flats in the greenhouse with three substrates (mosses Hypnum imponens or Thuidium delicatulum or bare soil) and three hydrological conditions (wet, moist, or dry) in a factorial design. For the three species both treatment regimes and the interaction were highly significant, except for Acer, in which the substrate regime was not significant. Fraxinus germination had the highest tolerance for wet conditions and lowest for dry conditions followed by Acer and then Ulmus. Significant interactions showed that the effect of hydrological regime on germination is influenced by substrate type. Moss decreased germination under drier conditions and increased germination under wet conditions by lifting the seeds away from the soil and creating drier conditions than on bare soil. It is also possible that interspecific competition for moisture played a role in decreasing germination under dry conditions. By influencing the regeneration niche for three major tree species of swamps in the northeastern United States, the bryophyte layer plays an important role in determining community composition.

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