Frost hardening of Scots pine seedlings in relation to the climatic year-to-year variation in air temperature

The study addressed the effect of year-to-year variation in air temperature on the early needle frost hardening of first-year Scots pine (Pinus sylvestris L.) seedlings in autumn. To this end a novel method, combining biometeorological and plant ecophysiologal research, was introduced. In the biometeorological part, climatic year-to-year variation in air temperature during August and September was examined by analysing a 51-year set of daily air temperature data from central Finland. The cumulative occurrence of low air temperatures in August and September was quantified by calculating a cold sum for that period each year, and the climatic year-to-year variation of the cold sum accumulation was subsequently determined. Similar cold sum calculations were carried out for different air temperature treatments in hypothetical growth chamber experiments. By comparing the results of these two sets of calculations, experimental designs were defined for air temperature treatments covering the climatic year-to-year variation of cold sum accumulation in August and September. In the ecophysiological part of the study, the effects of low air temperatures on the early needle frost hardening of Scots pine seedlings were studied experimentally over two years in central Finland, using the air temperature treatments defined in the first part of the study. The hardening treatments were implemented in growth chambers under conditions simulating natural autumn conditions: gradually increasing night length combined with decreasing and diurnally fluctuating air temperature. Out of the eight air temperature treatments applied, only one had a clear effect of accelerating the hardening. Comparisons of the cold sum accumulations in the experimental treatments with those in natural conditions suggest that low air temperatures do not accelerate the early frost hardening of Scots pine seedlings in most years; further experimental studies are needed in order to determine the frequency of the years when they do. The results of the cold sum comparisons in the present study will help to identify the experimental designs needed in forthcoming studies.