Effects of nitrogen supply and wood species on Tsuga canadensis and Betula alleghaniensis seedling growth on decaying wood

Eastern hemlock (Tsuga canadensis (L.) Carrière) and yellow birch (Betula alleghaniensis Britt.) in primary Michigan forests depend on decaying wood for seedling-establishment sites, but seedling densities vary across wood species (hemlock, yellow birch, and sugar maple (Acer saccharum Marsh.)). We collected seedlings and wood from a natural field experiment and conducted a companion greenhouse experiment to determine whether seedling mass and nitrogen (N) content varied with wood species and whether they were related to wood inorganic N supply. Yellow birch seedlings were largest on hemlock wood in the field (P = 0.003) and greenhouse (but P > 0.05), while hemlock seedling mass did not vary across wood species. N concentration and N mineralization rate varied by species (N concentration: hemlock < yellow birch < maple; N mineralization rate: hemlock > yellow birch ≈ maple), but neither seedling mass nor N content was significantly correlated with wood inorganic N supply. In the greenhouse, yellow birch seedlings responded to fertilization with N when growing on hemlock and maple but not yellow birch wood and appear to be limited by phosphorus when growing on yellow birch wood. We conclude that yellow birch seedling growth varies with wood species, and is limited by both N and phosphorus, while hemlock seedlings are unresponsive to variation in wood species during the first two growing seasons.