Smaller‐seeded tree species often require mineral soil and coarse woody debris (CWD) to establish. Limited data suggest that species‐based variation in tree seedling–CWD relations exists; however, the generalizability of this pattern and its mechanistic basis remain unknown. We investigated interspecific substrate–seedling relations and the potential that differences in mycorrhizal status and substrate nutrition underlie these patterns with a potted plant experiment. Eight temperate tree species were grown in a lath shade house on six species of CWD and mineral soil that had either been sterilized with gamma irradiation or left untreated to separate the effects of mycorrhizae from substrate nutrition. We assessed the impact of substrate identity on height growth and investigated possible mechanisms via relating growth to substrate inorganic N concentration, seedling foliar N (%), and mycorrhizal colonization. Seedling height varied across substrates consistently among species (i.e., substrate effects were strong, but seedling species x substrate interactions were weak). Seedlings were generally tallest on mineral soil followed by Betula papyrifera and Thuja occidentalis CWD, and shortest on B. alleghaniensis or Acer saccharum CWD. Sterilization main effects on height were not significant; however, the response of species to sterilization varied, as Acer rubrum and T. occidentalis seedlings were significantly shorter on sterilized substrate. For all seedling species, growth correlated positively with substrate [N] and foliar N. Among the four species examined for mycorrhizae, root colonization density was highest on mineral soil and lowest on A. saccharum and B. alleghaniensis CWD). For these species, mean colonization density was more strongly associated with mean seedling height than substrate inorganic [N]. Beneficial mycorrhizal effects were suggested by positive height–mycorrhizal root density relationships for B. alleghaniensis, A. rubrum, and T. occidentalis. Collectively, for the group of northern hardwood tree species examined in this study, these results demonstrate that in a controlled environment, most species grow best on mineral soil, B. alleghaniensis and Picea glauca attained maximum growth on CWD, species‐specific growth responses occur on CWD, and that substrate nutrient availability and mycorrhizal fungi contribute to the variation observed in seedling growth response across substrates.