Nutrient deficits increase frost hardiness in Sitka spruce (Picea sitchensis) needles

Abstract Frost hardiness of Sitka spruce ( Picea sitchensis (Bong.) Carr) trees growing in a nutrient deficiency trial was monitored between September and May by freezing detached shoots in a purpose built cabinet. Hardiness was assessed visually, as the temperature causing 50% shoot death (LT 50 ), and from measurements of electrolyte leakage. Trees, 14–20 years old, growing on a raised upland peat bog were given fertilizer applications designed to exacerbate macronutrient (N, P, K) deficiency. Depriving these trees of N, P and K resulted in N, P and K concentrations of 1-year-old needles of 1.01, 0.09 and 0.19% dry weight, respectively, whereas fertilized control trees whose nutrient concentrations were considered normal, resulted in concentrations of 1.43, 0.22 and 0.82% dry weight, respectively. The timing of autumn hardiness acquisition and the level of mid-winter hardiness were strongly influenced by nutrient deprivation. One-year-old shoots deprived of K were hardy to −20°C 5 weeks before shoots from the full nutrient treatment, and in mid January −K trees were 5 to 10°C harder than trees supplied with all three macronutrients. Likewise, prior to budburst in spring, −K trees were approximately 10°C harder than control trees. Similar, but smaller, effects were induced by deficiency of N and P. Effects of nutrient deprivation were greatest during the hardening phase and smallest over the winter period. Effects of needle age were detectable in autumn when 2-year-old needles were hardy to colder temperatures than 1-year-old needles. The observation that N, P or K deprivation increases frost hardiness in Sitka spruce is important in view of the concern over rising levels of N deposition.