Rooting dynamics of Medicago sativa seedlings growing in association with Bothriochloa caucasica.

Medicago sativa was seeded into an established sward of Bothriochloa caucasica. Two frequencies of sward defoliation were compared to a nondefoliated control. Although 84 to 92% of the root mass at 0-20 cm belonged to B. caucasica during the spring and summer samplings, defoliation generally reduced the root mass of B. caucasica. During the summer, aboverground mass of B. caucasica ranged from 735 to 1220 g m-2 and 295 to 410 g m-2 for control and defoliation treatments, respectively. Defoliation apparently reduced the competitive effects of B. caucasica on M. sativa, as taproot and lateral root mass of M. sativa was much higher in defoliated than in control plots by the end of the growing season. Predawn xylem potential of M. sativa was 0.4 to 0.8 MPa more negative in control than in defoliated plots from mid-summer to fall. Thus, we suggest that the transpirational demand of undefoliated B. caucasica reduced soil moisture available to M. sativa, thereby reducing its growth and survival during late summer and fall, compared to defoliated treatments.