Mechanisms of interference between hay-scented fern and black cherry

Hay-scented fern (Dennstaedtia punctilobula Michx.) interferes with establishment of black cherry (Prunus serotina Ehrh.) in the Allegheny hardwood forest of Pennsylvania. In stands where fern cover is dense, black cherry seeds germinate, but seedlings do not become established. Allelopathy was eliminated as the cause of interference in previous work: the present studies evaluated the resources of soil water, soil phosphorus, soil nitrogen, and light. The impact of hay-scented fern on the level of each resource, the availability of each resource to black cherry seedlings, and the growth response of black cherry seedlings to changes in resource availability were evaluated. Hay-scented fern had little effect on soil moisture levels, nor did it affect plant availability of soil water. The level of soil phosphorus was not reduced by the presence of hay-scented fern nor was phosphorus availability to black cherry seedlings less when they grew with fern. Both the speed and degree of mycorrhizal infection and the foliar phosphorus concentrations were similar in seedlings growing with or without fern cover. Hay-scented fern had no effect on rates of ammonium- or nitrate-nitrogen production nor was the soil concentration of ammonium affected. The presence of hay-scented fern significantly reduced the soil nitrate-nitrogen concentration in the first year after a shelterwood seed cut, but not in the second. Overstory tree roots were more important than hay-scented fern in removing soil nitrate. Availability of soil nitrogen was unaffected by the presence of hay-scented fern or overstory tree roots. Black cherry seedlings grew more where ferns were absent, regardless of whether overstory tree roots were present or absent. Competitive reduction in light was the key mechanism of fern interference with black cherry. Hay-scented fern caused dramatic reductions in both the photon flux density and the ratio of red to far-red light beneath it. Black cherry seedlings survived and grew poorly in the presence of fern foliage shade. Survival and growth similar to that above fern could be restored by restraining fern foliage, allowing light to reach black cherry seedlings from above.