(1) One- to six-year-old natural seedlings of Abies amabilis, Tsuga heterophylla and T. mertensiana were collected from 15-cm-deep volcanic tephra deposited beneath a surviving tree canopy during the 1980 eruption of Mount St Helens, Washington, U.S.A. Seedling dimensions and architecture were measured and dry weights of leaves, stem and roots determined as loss-on-ignition. (2) Abies seedlings were heavier than Tsuga initially, with deeper and more-complex roots; however, biomasses of Abies and Tsuga seedlings became more similar with time. Height above the cotyledons was similar in older seedlings of the two genera, but branching began earlier in Tsuga than in Abies. (3) Tsuga had a higher relative growth rate than Abies, with both greater leaf-biomass ratio and greater assimilation efficiency. Leaf allocation of Abies exceeded that of Tsuga at first, but eventually became less than that of Tsuga. (4) Most measures of size decreased for Tsuga as canopy cover increased; for Abies, only the amount of root branching declined. As litter increased, root dimensions of both genera declined, but biomass of Tsuga increased whereas that of Abies decreased. Abies seemed less affected by both shade and limited soil moisture than Tsuga. (5) Seedling growth rate and allocation appear to be important in determining the types of micro-environments in which the two genera can establish, and thus may affect long-term stand dynamics. Root characteristics differentiated taxa more clearly than those of shoots.