Abstract:
Chapter 1 Growth and Survival of Root-Wrenched Douglas-fir Seedlings Root wrenching of seedlings (severing the roots 15 cm below the soil surface) was investigated as a nursery practice to improve growth and survival of field-planted Douglas-fir from six local seed sources
in the Pacific Northwest. At lifting, shoots of wrenched seedlings were shorter, lighter, and had smaller diameters than those of
unwrenched seedlings. Among seedlings from four of the seed sources, wrenching resulted in significantly lighter taproots but did not
significantly affect lateral root and total root weights. There were no significant differences between these measurements among wrenched and unwrenched seedlings from the other two sources. Mainly because of lighter shoots, shoot-root ratios were smaller for wrenched than for unwrenched seedlings. In no case did root wrenching improve field
height growth or survival after one year, and among four of the sources shoot growth was significantly less than that of unwrenched seedlings. Chapter 2 Water Relations of Root-wrenched Douglas-fir Seedlings Root wrenching was investigated as a nursery practice to precondition Douglas-fir seedlings to droughty field conditions. Wrenching shocked the seedlings while in the nursery, lowering plant water
potential and transpiration rate. After planting, however, wrenched and unwrenched seedlings transpired at equal rates when exposed to stress with osmotic solutions of Polyethylene Glycol 1000 and under field conditions. Throughout exposure in pots to a drought simulating the Pacific Northwest summer drought, wrenched and unwrenched
seedlings did not differ in plant water potential, leaf relative water content, or seedling condition. However, all wrenched seedlings of four seed sources reflushed in the middle of the drought and had
significantly fewer active roots than unwrenched seedlings. Among the four seed sources used, differences in ability to withstand drought were apparent.
