Abstract
Cold damage leads to the rejection of millions of seedlings each year in northern forest nurseries. Nursery managers need a procedure to estimate the degree of hardening at a specific time in order to make decisions to avoid seedling loss. Estimates based on variables that are quick and easy to measure such as thermal time, a variable quantifying the cumulative effect over time of temperatures below a given threshold, and apical dry mass ratio (DM/FM) hold particular promise. In this study, two-year-old white spruce containerized seedlings were subjected to artificial freezing tests. The progression of DM/FM during the fall was determined and the thermal time was quantified by calculating hardening degree days (HDD) based on air temperature and a specific threshold for white spruce. Results establish that cold tolerance thresholds are associated with easily identifiable specific DM/FM and HDD values for nursery × species × seed source combinations.
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