Abstract:
Information concerning the factors controlling seedling establishment on tephra deposits (airfall volcanic ejecta) is limited. The establishment and growth of six coniferous species, Abies amabills (Dougl.) Forbes (Pacific silver fir), A. procera Rehd. (noble fir), Pinus monticola Dougl. (western white pine), P. Contorta Dougl. (lodgepole pine), Pseudotsuga menziesii (Mirb.) Franco (Douglas-fir) and Tsuga heterophylla (Raf.) Sarg. (western hemlock) are assessed as a function of: 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 northeast of Mount St. Helens, Washington in a split-split plot design which incorporated plantings on both tephra deposited May 18 and 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 established seedlings than either total removal of tephra deposit or untreated tephra. Where differences between species were significant, Pseudotsuga menziesii was tallest and Tsuga heterophylla the shortest of the species tested. 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 tephra surface crust, and 3) the occurrence of extended drought and/or extreme surface temperatures.
