Growth and moisture relations of western hemlock seedlings as affected by root or shoot disturbance

Graduation date: 1979The need for increased efficiency in regeneration of western hemlock (Tsuga heterophylla (Raf.) Sarg.) requires greater understanding of seedling physiology. Experiments examined western hemlock seedling growth responses to in-bed root pruning, and pruning of shoots or roots prior to transplanting. Measurement of plant\ud moisture stress (PMS) after physical root disturbance, and measurement of cytokinin activity in xylem sap were also taken.\ud Seedlings that were root pruned in the nursery bed initially had decreased root dry weights and increased shoot-to-root ratios. In time, both July and September pruned seedlings tended to match control seedling shoot-to-root ratios. The rate of recovery for the July pruned stock was faster than the September pruned seedlings. In pruning experiments conducted in both the field and growth chamber, the shoot-to-root ratio was generally uniform among control, root pruned, and top pruned seedlings by the end of the test period. Significant differences did exist between growth environments. Highest shoot-to-root ratios were recorded in the site where moisture stress was\ud probably lowest. Root pruning and root abrasion generally increased stress level in western hemlock and Douglas-fir seedlings compared to controls. However, abraded seedlings had significantly lower PMS levels than controls on the first morning following planting. The hemlock seedlings had generally higher PMS levels than the Douglas-fir. Cytokinin activity in western hemlock xylem sap was negligible from August to early April. A sharp increase in zeatin-like and zeatin riboside-like substances was found in mid April. These findings suggest that the balance of shoot and root growth is environmentally directed. Moisture stress may play a role in interfacing environmental stimuli and internal metabolism. Acknowledging the role of environmental factors in determining growth patterns, it is suggested that seedlings should be grown for specific cultural conditions