Mycorrhizas in Grasslands: Interactions of Ungulates, Fungi and Drought

Abstract

SUMMARYFumigated and non-fumigated plots were planted with several species of either mycorrhizal or non-mycorrhizal grasses. These plots were either inside or outside enclosures which allowed grazing by native North American ungulates and longhorn cattle. The morphology and growth of plants as well as the level of vesicular-arbuscular mycorrhizal (VAM) development were followed over one growing season during which growing season precipitation was 50 % of the normal level. Grazing was the most important treatment effect, greatly reducing plant size and relative growth rates. Mycorrhizal colonization had no effect upon plant growth rates or size. Even in grazed plots where nutrient content and soil moisture were higher, no mycorrhizal effect on plant growth was observed. Grazing did not influence VAM colonization, although grazed plants which were growing in non-fumigated soil had significantly lower colonization levels than any other treatment.

Root fungal symbionts interact with mammalian herbivory, soil nutrient availability and specific habitat conditions

“…Both the loss of foliar biomass to herbivores or reduced growth caused by plant-neighborhood competition may lead to carbon limitation. Findings of empirical studies of herbivore impacts on mycorrhizal symbiosis range from positive to negative depending on host plant species, study system and intensity of herbivory; however, most studies report negative effects (reviewed by Whitham 1994, 2002; see also Wallace 1987;Eom et al 2001;Gange et al 2002;Lugo et al 2003;Grigera and Oesterheld 2004;Kula et al 2005;Wearn and Gange 2007). A recent metaanalysis by Barto and Rillig (2010), however, challenged this view and argued that herbivory has only a minor impact on mycorrhizal colonization.…”

The Efficiency of Nutrient Acquisition over the Life of a Root

“…In the field, studies demonstrating improved P acquisition by mycorrhizas are often less compelling. This may occur only during relatively brief periods of a plant's life history, such as during flower or seedling establishment (Fitter 1986b;Wallace 1987). A notable exception is the bluebell (Hyacinthoides non-scripta), a vernal geophyte with a sparse, unbranched root system with relatively thick (0.5-1 mm) roots.…”

Mycorrhizal Colonization Patterns Under Contrasting Grazing and Topographic Conditions in the Flooding Pampa (Argentina)

“…Grassland ecosystem researchers agree on the need for more knowledge of mycorrhizal colonization patterns under field conditions (Bethlenfalvay et al 1985;Fitter 1985;Wallace 1987;Read 1991;Eom et al 2001). This is a consequence of numerous controversies between many glasshouse experiments and a few field -based experiments and observations (Bethlenfalvay 1992).…”

“…At the individual plant level, a variety of responses have been documented: Under intense defoliation regimes, colonization intensity decreased (Bethlenfalvay and Dakessian 1984;Bethlenfalvay et al 1985;Hetrick et al 1990), whereas, under milder regimes, it was either unaffected or increased (Wallace 1981(Wallace , 1987Wallace and Svejcar 1987;Eom et al 2001). At the community level, there are no studies directly assessing grazing effect on colonization.…”

“…under contrasting grazing and topographic conditions in the flooding pampa (Argentina). Mycorrhizal colonization of the community as a whole is, at the same time, a rangeland attribute and a biotic factor that contributes to regulation of its dynamics (Wallace 1981(Wallace , 1987McNaughton and Oesterheld 1990;Sanders and Fitter 1992). Because livestock grazing is the main use of rangelands, investigation of grazing effects on AM abundance should allow us to better predict and understand rangeland dynamics (Eom et al 2001).…”