The Effect of Tephra Deposition and Planting Treatment on Soil Oxygen Levels and Water Relations of Newly Planted Seedlings

Abstract Seedlings planted in late June 1980 near Mount St. Helens, following the May 18 and 25 eruptions, continuously lost vigor. Studies on seedlings planted in April 1981 were conducted in 1981-82 on sites covered with 10 to 35 cm of tephra. Soil oxygen levels measured at 10 and 20 cm depths below the tephra surface decreased with increasing tephra thickness, tephra moisture, and sampling depth in the tephra. In addition, the method of planting appeared to affect soil oxygen levels. Holes dug into the tephra for planting seedlings in the mineral soil allowed water and fine particles to collect and possibly interfere with soil gas exchange. However, oxygen levels were never less than 11 percent and generally about 14 percent. Levels were never low enough to result in a root oxygen stress. Measurement of the oxygen diffusion coefficient in tephra and the physical characterization of the tephra failed to demonstrate the existence of any unusual features that might lead to the development of an oxygen stress. A controlled environment study was conducted to observe seedlings under conditions of tephra coverage of the soil. The water relations of these seedlings indicated no response due to a root oxygen stress. The lack of oxygen stress in the field corresponded with the maintenance of high vigor in seedlings planted in 1981. Forest Sci. 31:109-116.