Frost hardening and resistance in three Aleppo pine (Pinus halepensis Mill.) provenances

The paper investigates frost hardening and resistance in two-year-old seedlings of three Aleppo pine provenances (Litorale tarantino, Eubea, Kassandra). Starting in early autumn, field-raised seedlings were sampled fortnightly and subjected to temperatures of +4, -6, -12, and -17 degreesC. Electrolyte release was measured before and after boiling of the samples, separated according to organ type. Conductivity values were converted into damage index I-t. The findings were then correlated to mean climate data (maximum, minimum, and mean temperature, theoretical sunshine duration) recorded over the 2-12 days prior to sampling. Acquired frost resistance increased gradually from the first sampling till January, as the day length and air temperature decreased. Subsequent hardening-dehardening fluctuations indicated that frost resistance in Aleppo pine is strongly dependent on microclimate conditions. Provenance Kassandra displayed the greatest frost resistance. The differences between the 3 provenances were not marked. Aleppo pine is considered a thermophilic species. From our data, however, it appears capable of tolerating winter temperatures as low as -12 degreesC. Damage index at -12 degreesC (I-12) showed the greatest fluctuations from November to March and correlated best with the climate parameters. The most significant correlation coefficient was provided by the mean temperature of the 10 days prior to sampling.