Mount St. Helens Ash: Recreating Its Effects on the Steppe Environment and Ecophysiology

The 18 May 1980 ash fall from Mount St. Helens was experimentally reproduced in May 1982 by applying silt—sized ash to a stand of the Artemisia tridentata/Agropyron spicatum association in south—central Washington. Compared to the adjacent control site, ash caused an immediate increase in albedo from 13% to 28%, while other parameters of the energy budget were simultaneously lowered: net radiation by °20%, soil surface temperatures by as much as 10°C, and soil heat flux by as much as 50%. The ash's mulching action initially increased water availability and delayed leaf abscission in Artemisia tridentata (Big sagebrush) by 2 wk in summer 1982. But after summer 1982 water availability declined, while water use increased, illustrating the diverse effects of the ash. Increased reflection from the ash—covered surface increased the radiation load on plant canopies. In turn, air temperature at 0.5 m increased, latent heat flux often doubled in summer, and xylem pressure potentials decreased. Available water at the —1 m soil depth eventually decreased as much as 40%. This decrease was the result of the increase in latent heat flux and the decline in infiltration through the stratified layer created by the ash cap. The sudden alteration of the soil surface by an ash layer revealed the close coupling of the water use of A. tridentata with available soil water. In summer in both 1982 and 1983, predawn xylem pressure potential declined as soil water potentials approached —4.5 MPa on both sites. Total shrub leaf area index routinely decreased 75% as large (> 2 cm long) spring—emergent leaves were shed. Highest leaf conductance (0.3 to 0.5 s/cm) and highest transpiration occurred among the remaining smaller (<2 cm long) leaves after leaf abscission in spring. Water conservation through leaf area reduction followed by high transpiration rates apparently allows the development of inflorescences during the dry, hot summer, culminating in autumn flowering and seed maturation. Lupinus sulphureus on the ash—covered surface exhibited no difference from conspecifics on the control site in xylem pressure potentials, but did have higher leaf conductances during the 1st yr following the ash fall; no differences were observed in the 2nd yr. Lupinus sulphureus aestivated °2 wk earlier on control sites the 1st yr following ash deposition. The responses to ash that we measured were probably the maximum expressed by Big sagebrush; the thin layer of ash to the east and the coarse—textured ash to the west did not significantly alter the movement of water. Short—term increases in A. tridentata in the Pacific Northwest °6700 yr BP may have been caused by similar decreases in soil temperature and increases in aridity with the deposition of Mazama ash. In addition to allowing assessment of the effects of the 18 May 1980 ash fall on arid steppe, application of ash provided an unexpected level of precision in detecting the often subtle effects that occur when some microenvironmental parameters change while the overall macroclimate remains the same.