Growth and nutrient dynamics of western hemlock with conventional or exponential greenhouse fertilization and planting in different fertility conditions

In many northern forests, low nutrient availability constrains growth of young trees. We tested the efficacy of exponential nutrient application to load western hemlock (Tsuga heterophylla (Raf.) Sarg.) seedlings with nutrients to enhance field performance. Seedlings were grown with conventional, constant-rate fertilization (100 mg N.L^-1) or with exponentially increasing fertilization at rates of 2% and 3% per day in a greenhouse. Growth and nutrient allocation were characterized. Seedlings from the three greenhouse treatments were then planted in an outdoor nursery experiment for 2 years with NPK fertilizer applied at 10, 25, or 100 mg N.L^-1, and in a 3-year field experiment with and without slow-release fertilizer. Seedling height, biomass, and nutrient concentration did not differ greatly among exponential or constant-rate fertilization treatments in the greenhouse. The influence of postplanting fertility, both in the nursery and in the field, outweighed the effect of greenhouse exponential nutrient application. In the outdoor nursery, seedlings from all greenhouse treatments showed increased growth with higher rates of fertilization. In the field, there were no differences in growth among seedlings from the three greenhouse treatments, but fertilization at planting increased growth. The capacity for nutrient loading in container-grown western hemlock was likely exceeded because large quantities of N were applied in all treatments.