Millions of seedlings are planted each year in restoration and reforestation projects. Planted seedlings will die if they fail to integrate into the soil water profile through root growth or if they fail to maintain hydraulic function under drought stress. This project aims to understand whether seedlings are more likely to be successful when planted with a large root volume or if root growth after planting was more important to growth and survival. We used nursery grown inland Douglas-fir (Pseudotsuga menziesii var. glauca), a species found on dry sites, but observed to have low survival when planted in dry locations. We excised portions of the root system to achieve starting root volumes that represented 100%, 66%, or 33% of the original container-grown volume; in the fourth treatment, seedlings were left with only a tap root. The seedlings grew in growth chambers for a six-week establishment period, then subjected to drought (growing medium dried to -1 MPa) or well-watered conditions for two months. We measured gas exchange throughout experiment, above- and belowground morphology at the end of establishment and end of the experiment, and hydraulic physiology (pressure-volume curves, hydraulic conductivity of root systems) at end of the experiment.
All the seedlings with 33% or more of their initial root volume survived the experiment; the tap root-only seedlings ceased gas exchange within days. The seedlings with 100% of the initial root system broke bud earlier, had greater shoot growth, and grew larger new needles than the seedlings with excised roots. Seedlings with root volumes of 100% and 66% had similar numbers of new root tips and new root volume and dry mass; there were fewer new root tips and less new root volume on seedlings with 33% of initial roots. The drought-stressed seedlings had a trend of greater root growth than the well-watered seedlings. However, the root system hydraulic conductivity per root volume was comparable across treatments. Likewise, pressure-volume curves and leaf conductivity did not differ. We observed that seedlings can adjust root growth to compensate for lower initial volumes, while maintaining hydraulic function. This suggests redundancy in nursery-grown root systems. Within the drought-stressed seedlings, the seedlings with smaller root volumes used less water per leaf area, suggesting that water use efficiency changes with initial root volume and subsequent root growth. This may result in seedlings that are better adjusted to the specific conditions of the outplanting site.