The density and frond traits of the dimorphic fern Osmunda regalis were examined in 45 populations situated in central Italy, along a gradient of forest canopy cover and deer grazing. The survey involved 1704 ramets containing 6416 fronds, of which 5643 were sterile and 773 fertile. Within the ramets, the number of fertile fronds increased with the number of sterile fronds and with the mean height of ramets. Light seemed to promote fertility, as the number of fertile fronds decreased with increasing canopy cover. Fewer and smaller fronds were found in plots with high forest canopy cover. Our results showed that O. regalis performed best in conditions of full sunlight but can persist and recruit young sporophytes under closed canopy. As many marsh and lake environments have disappeared or been altered by human activity, more populations were located under high canopy cover in the forest than in open habitats. Deer grazing caused a significant decrease in fertile fronds, in total frond area and in ramet height, but the number of ramets and sterile fronds were unaffected. In conclusion, plant density and frond traits of O. regalis responded differently to the availability of light and to grazing.