A COMPARATIVE-STUDY OF PLASTICITY IN SEEDLING ROOTING DEPTH IN DRYING SOIL

1 Maximum depth of vertical root penetration into the soil (i.e. rooting depth) and shoot growth were measured for seedlings of 42 species of vascular plants in both unwatered and watered soil columns. The ratio of rooting depth in unwatered soil to watered soil was calculated to quantify plasticity in rooting depth. The objectives were to determine whether all species show equal plasticity in rooting depth and to measure the extent to which plasticity in seedling rooting depth influences the ability of seedlings to sustain growth in drying soil. 2 Seedlings were allowed to initiate growth in continuously moist sand during a 21-day pretreatment. Two watering treatments (unwatered vs. watered daily) were then applied for an additional 21 days. 3 Rooting depth was not equally plastic for all species. Rooting depth was significantly greater in unwatered soil than in watered soil for seven species. In contrast, five other species showed a significant decrease in rooting depth in unwatered soil. The remaining 30 species showed no significant effect of soil moisture treatment on rooting depth. 4 Species that showed the greatest plasticity in rooting depth also showed the greatest ability to sustain shoot growth in unwatered soil (Spearman rank correlation, r = 0.75, P < 0.0001). These results indicate that plasticity in rooting depth contributes to the ability of seedlings to sustain growth in drying soil. 5 Results of this study also suggest that selective pressures promoting plasticity in seedling rooting depth have been greater in species regenerating primarily from seeds than in species regenerating both vegetatively and from seeds.