Establishment of Conifers from Seed on Tephra Deposited by the 1980 Eruptions of Mount St. Helens, Washington

Abstract

The establishment and growth of six coniferous species, Abies amabilis Dougl. ex Forbes (Pacific silver fir), A. procera Rehd. (noble fir), Pinus monticola ex D. Don (western white pine), P contorta ex Loud. (lodgepole pine), Pseudotsuga menzieszz (Mirb.) Franco (Douglas-fir) and Tsuga heterophylla (Raf.) Sarg. (western hemlock) were related to: (1) depth of tephra deposit, (2) condition of tephra surface crust, and (3) availability of shade on seedbed. In October 1980, seeds were planted on four sites 20 km NE of Mount St. Helens, Washington, in a split-split plot design that incorporated plantings both on tephra deposited 18 May 1980 and on pre-eruption soil. All six species established from seed on tephra deposits less than 20 cm deep. Cultivation of the tephra surface crust yielded more seedlings than either total tephra removal or untreated tephra. Seedlings grown on tephra were generally shorter than those grown on underlying soil, regardless of surface treatment. The principal factors affecting establishment in this study were: (1) loss of seed prior to germination, (2) failure of germinating seeds to penetrate the surface crust of the tephra, and (3) the occurrence of extended drought and/or extreme surface temperatures.