Seasonal Timing of Root Growth in Favorable Microsites

The heterogeneous nature of soil is well known. Over short distances, a soil may vary considerably in nutrient and water availability, physical impedance, toxic ion concentration, and other factors that affect plant growth and function. Proliferation of roots in small volumes of soil with favorable chemical and physical characteristics has been shown (Fitter and Hay 1981, St. John et al. 1983, Wang et al. 1986). Such responses are generally considered to be mechanisms by which plants more efficiently exploit the soil environment (e.g., St. John et al. 1983). In this field study, we compare rates and spatial patterns of root growth in favorable microsites by two Agropyron species and a common shrub codominant, Artemisia tridentata ssp. vaseyana (Rydb.) Beetle. These two Agropyron tussock grasses differ strikingly in their ability to compete with A. tridentata (Eissenstat and Caldwell 1988). The grass of greater competitive ability, Agropyron desertorum (Fisch. ex Link) Schult., was introduced from Eurasia and has been widely planted in the Great Basin steppe of North America. Agropyron spicatum (Pursh) Scribn. and Smith and A. tridentata are native. (A recent revision of the perennial North American Triticeae Barkworth and Dewey 1985 recommends that the name Agropyron spicatum be changed to Pseudoroegneria spicata Pursh A. Löve subsp. spicata.)