Root freezing tolerance and vitality of Norway spruce and Scots pine seedlings; influence of storage duration, storage temperature, and prestorage root freezing

Abstract

This study investigated the effect of different cold storage conditions on (i) root freezing tolerance of Norway spruce (Piceaabies (L.) Karst.) and Scots pine (Pinussylvestris L.) and (ii) the vitality of seedlings that suffered freezing injury to roots prior to storage. Container-grown seedlings, 1 year old, were stored from the end of October to April in three environments with different root temperatures: outdoor storage (−0.5 to 11.0 °C), cool storage (0.7 to 3.7 °C), or frozen storage (−5.0 to −3.8 °C). Root freezing tolerance was determined prior to storage in October and during storage in January and March. Maximum root freezing tolerance for both species occurred in January, when over 50% of spruce and pine seedlings survived 2 h exposure to −25 and −20 °C, respectively. At this time, roots of frozen-stored spruce were significantly more freezing tolerant than outdoor-stored seedlings, whereas storage environment had no significant effect on pine. Freezing tolerance in roots of both species decreased from January to March in all test environments but to a lesser extent at the subzero temperatures in the frozen storage. Root freezing to −10, −15, or −20 °C in late October before storage resulted in reduced poststorage survival of seedlings in April. Pine was more adversely affected (0–13% survival) than spruce (0–85% survival). Freezing of roots prior to storage caused the lowest survival with frozen storage.