Mosses facilitate above- and belowground vascular plant growth in rich fens

Mineral-rich fens support high plant species richness, but the ecological mechanisms supporting their diverse flora are poorly understood.  Mosses within fens influence their physical and chemical environment by covering the peat surface and providing a substrate for plant rooting.  We conducted a field study and a moss removal experiment at four rich fen sites to investigate the role of the moss layer in vascular plant growth, rooting patterns, and nutrient availability.  In the field study, forb seedling density and root biomass were greater where mosses were present than where they naturally did not occur.  By the second year of the moss removal experiment, seedling density, biomass, root length, and root branching were significantly greater by factors of 2.0-2.5 where the moss layer was intact.  Greater root branching may be explained by greater phosphorus availability underneath the moss layer.  Phosphorus supply rate to resin membranes was consistently greater under intact moss (0.13-0.31 μg PO₄^3- 10 cm^-2 month^-1) than when moss was removed (0.09-0.19 μg PO₄^3- 10 cm^-2 month^-1).  Greater root growth also may be related to increased aeration of the rooting environment by the moss layer.  Mosses increased aerated substrate for roots by elevating the surface 4-5 cm farther above the water table relative to moss removal plots.  Mosses may affect plant species composition and diversity in mineral-rich fens by physically and chemically facilitating the establishment and growth of shallowly rooted forbs.