Freeze injury to roots of southern pine seedlings in the USA

Abstract

Roots of pine seedlings can be injured by freezing temperatures and the degree of injury is affected by genotype and stage of acclimation. Local sources of Pinus echinata and P. virginiana that were acclimated by cold temperatures were relatively freeze tolerant when grown in nurseries located where average minimum temperatures ranged from –18 to –23°C. However, seedlings of P. taeda, P. elliottii and P. palustris that were grown in warmer climates were injured by less severe freezes (i.e. –4 to –18°C). Some fast-growing half-sib families from moderate Coastal Plain regions were more susceptible to freezing than Piedmont genotypes located in cooler climates. When seedlings are acclimated to cold weather, the temperature required to cause injury is lower than when seedlings have been deacclimated due to warm weather. For example, in 2004, unusually high temperatures in the month following the winter solstice deacclimated pine seedlings and this resulted in root injury from a –8°C freeze. Shoots did not show symptoms of injury (since they were not actively flushing) and therefore root injury was often overlooked. Many freeze-injured seedlings died within two months of the freeze event. Since freeze injury symptoms to roots were overlooked, foresters offered various reasons (other than the freeze) for the poor seedling performance.