Fig. 2. Change in viable seed densities of (A) giant foxtail and (B) velvetleaf between November 2002 and April 2006 in each rotation system, in the top 20 cm of soil. Means and their standard errors are shown. Negative values indicate a decline in mean seed densities over time. Asterisks indicate a significant difference (P < 0.05, two-tailed test) between the value shown and 0. Within a species, values represented by columns not underwritten with the same lowercase letter are significantly different (P < 0.05).