The effect of late summer fertilization on the frost hardening of second-year Scots pine seedlings

In this study the effect of summer fertilization on the initiation of frost hardening of containerized second-year Scots pine (Pinus sylvestris L.) seedlings is studied. During the second growing season three different fertilization programs (water soluble NPK with micronutrients) determined by electrical conductivity of peat water extract (0.2, 0.5 and 1.2 mS cm^-1) were initiated. The growth and nutrient concentrations of needles were monitored during the fertilization period. The frost hardiness of seedlings was assessed on four separate occasions at two week intervals from August 7 to September 18. This assessment was based on artificial freezing tests and visual damage scoring of tissue browning on current-year needles. Clear differences in foliar N, P and K concentrations were observed between the fertilization treatments. Fertilization prolonged the growing period of needles and increased root collar diameter. In all the tests, the highest fertilization level resulted in the highest level of frost hardiness. The difference between the fertilization treatments ranged from 1 °C to 2.2 °C. Frost hardiness increased with an increase in foliar nitrogen concentration and slightly less consistently with increases in foliar phosphorus and potassium concentrations.