Frost hardiness of Thujaplicata and Pseudotsugamenziesii seedlings when nutrient supply varies with season

Seedlings of coastal Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) and western redcedar (Thujaplicata Donn ex D. Don) were subject to three nutrient treatments varying the rate of application of nitrogen (N) and phosphorus (P) over the growing season. A control treatment provided seedlings with a constant supply of N and P. A high–low treatment reduced the rate of N and P supply after mid-July. A low–high treatment initially supplied seedlings with low amounts of N and P, and then increased the rate of supply after mid-July. Foliar nutrient concentration of seedlings of each species from each nutrient treatment was measured in September and the following April. Cold tolerance of seedlings was assessed five times over the winter season by freeze-induced electrolyte leakage and twice by differential scanning calorimetry (DSC). Nutrient treatment did not significantly affect foliar nutrient concentration in either species. Height of Douglas-fir seedlings was also unaffected by nutrient treatment, but western redcedar seedlings in the control treatment were 6.3 cm taller than those in the high–low treatment. Interpolated damage at −13 °C from freeze-induced electrolyte leakage indicated that Douglas-fir seedlings from the control treatment had 19.8% less damage than high–low seedlings in October, 18% less damage in November, and 1.6% less damage in December. The same trend was evident, although not always significant, in western redcedar. Differential scanning calorimetry analyses of Douglas-fir in October and November showed freezing and melting curve onset and peak temperatures to be, on average, 2.3 °C and 0.4 °C warmer, respectively, in control seedlings than in high–low plants.