Impact of early root competition on fitness components of four semiarid species.

Plant demographic and root exclusion approaches were used to examine the influence of roots of adult Artemisia tridentata, Agropyron desertorum, and Agropyron spicatum individuals on seedling survival of four C3 semiarid species, three perennials, Ar. tridentata, Ag. desertorum, Ag. spicatum, and an annual, Bromus tectorum. Furthermore, height of Ar. tridentata seedlings and seed production of B. tectorum were assessed. The probability of a seedling being alive significantly depended on the seedling species, the neighboring adult species, and on the depth to which root competition was excluded. As seedlings, survival of Agropyron species did not differ, whereas survival of Ar. tridentata seedlings was higher than Ag. desertorum and was similar to Ag. spicatum. Bromus tectorum maintained significantly higher survival rates than perennial seedlings. Established individuals of Ar. tridentata reduced seedling survival more than established individuals of either Agropyron species. Seedling survival significantly increased with greater depth of root exclusion for the perennials but did not significantly affect seedling survival of B. tectorum. Height of Ar. tridentata seedlings and seed production of B. tectorum significantly increased with depth of root exclusion. Seed production of B. tectorum was highest when competing with Ag. desertorum and was lowest with Ar. tridentata. Root competition decreased the seed population of B. tectorum in the next generation even though it had no impact on survival. Competition in the upper soil horizon occurs between seedlings and established adults early in the growing season and potentially restricts root growth of seedlings. In arid and semiarid ecosystems, soil moisture is depleted from the upper horizons first, resulting in the death of seedlings that do not have access to moisture.