An examination of the complex ecological role of tall fescue in grassland restoration.

摘要

Tall fescue (Schedonorus arundinaceus (Schreb.), an exotic-invasive, cool-season grass has invaded millions of acres of grassland in the United States. Tall fescue's dominance can be attributed, in part, to a fungal endophyte (Epichloe coenophiala) that confers fitness benefits to the grass, and which lives symbiotically within the grass host. With invasion into native grasslands, tall fescue alters litter production, changes composition and structure of vegetation, and modifies fire behavior. This thesis presents results from two studies on the complex role of tall fescue in Iowa grasslands. In the first study, I investigated how grassland management with fire and grazing influences tall fescue cover and litter dynamics. From 2012-2014, three fire and grazing treatments were applied to pastures with a mixture of tall fescue and other native and exotic plants in the Grand River Grasslands of Iowa and Missouri. We hypothesized that the fire-grazing interaction (a.k.a. patch-burn grazing) would reduce tall fescue cover more than fire or grazing treatments applied independently. We also hypothesized that because tall fescue may impede fire movement across the landscape, litter depth and woody plant cover would be positively correlated with the presence of tall fescue following use of this management technique. Over the three years of the study, tall fescue cover was not reduced as a result of the treatments, and there was only a weak positive correlation between tall fescue and litter cover. No correlation was found between tall fescue and woody plant cover. Years-since-fire had the greatest effect on litter dynamics -- regardless of tall fescue abundance at the site - and the patch-burn grazing treatment had the most heterogeneous litter depths within each year. Our findings suggest that patch-burn grazing can benefit livestock producers and wildlife in fescue-invaded pastures, but it is not sufficient to reduce tall fescue cover. In the second experiment, we examined how invasive, exotic grasses like tall fescue may be contributing to the decline of grassland butterflies through alterations in forage quality. Alkaloids produced by the endophyte are known to be toxic to some foliar-feeding pest insects, but effects of the endophyte on non-pest insects such as butterflies are relatively unknown. We examined growth and survival parameters of tawny-edged skippers ( Polites themistocles) that were reared on endophyte-infected tall fescue (E+), endophyte-free tall fescue (E-), and Kentucky bluegrass (KBG). Results showed that the endophyte did not affect growth and survival of larvae compared to uninfected tall fescue, even though significant amounts of loline alkaloids (average 740 ppm) were measured in endophyte-infected plant material. Larvae feeding on KBG grew faster with greater survival rates than larvae on both tall fescue treatments. These results confirm that tall fescue invasion and dominance may be deteriorating the quality of grassland habitats for native pollinators; however, this effect does not appear to be linked to endophyte infection.