PARENT SESSION
Oral Session 134: Herbivory V: Algal; Mammal.
Friday, August 6, 8:30 AM to 12:00 PM, Meeting Room C 120.
Resource competition or apparent competition: Direct and indirect impacts of hay-scented fern cover on tree seedling establishment.
Royo, Alejandro*,1, 2, Carson, Walter1, 1 University of Pittsburgh, Pittsburgh, PA, 152602 USDA Northeastern Experimental Station, Irvine, PA, 16329
ABSTRACT- An extension of the classic apparent competition model posits that by providing a favorable microhabitat for herbivores, one plant species can indirectly reduce densities of a second species by facilitating increased predation. In forest understories dominated by a dense herbaceous layer, this model provides a compelling alternative to competition for explaining understory layer interference on tree seedling recruitment. We tested this model in northern hardwood forests with understories dominated by hay-scented fern and hypothesized that presence of a fern canopy elevates rodent activity thereby decreasing seedling establishment. Specifically, we predicted that seedling establishment would be lowest in areas subjected to direct competitive and indirect apparent competitive effects of fern. Conversely, establishment would be highest in areas devoid of both the direct and indirect impacts of fern cover. We tested these predictions through factorial manipulations of rodent predation using exclosures and fern cover presence/absence at three sites in Pennsylvania. Trapping data confirmed that fences effectively excluded small mammals and documented elevated mammalian activity beneath fern cover. Seed removal trials revealed clear preferences by rodents toward larger-seeded species, but only in the absence of fern cover. As predicted, seedling densities were greatest in fenced plots without fern and lowest in unfenced, fern-covered plots. Excluding small mammals significantly increased establishment of the large-seeded black cherry by 55%; however, we detected no such effect for the relatively small-seeded red maple. This is one of a few tests of the apparent competition model in plant communities and our results demonstrate that microhabitat influenced predation strongly limits seedling establishment. Thus, elucidating the role of small mammals is critical since it could force us to reevaluate our understanding of the factors controlling forest development.
Key words: Hay-scented Fern, Interference, Apparent Competition, Herbivory