The relation between growth cessation and frost hardening in Scots pines of different origins

The cessation of shoot elongation, diameter growth and needle elongation we re compared with the initiation of frost hardening of the stems and needles in an 8-year-old provenance trial of Scots pine (Pinus sylvestris L.) esta blished in central Finland. The saplings were of six different origins rang ing from Estonia to northern Finland, forming a latitudinal gradient of ca. 10 degrees N. The frost hardiness of the stems of current-year shoots was assessed by electrical impedance analysis and that of current-year needles by electrolyte leakage and visual scoring of damage. Artificial freezing te sts were used in the assessments. The pattern of growth cessation (shoot an d needle elongation, diameter growth) tended to follow the latitude of orig in, i.e. growth ceased in the northernmost provenance first and in the sout hernmost one last. Both stems and needles of the northern provenances harde ned earlier than the southern ones, but the differences in hardiness disapp eared as hardening progressed. Growth cessation and initial hardening to -1 5 degrees C were clearly correlated at the provenance level, indicating tha t growth must cease prior to hardening, and that earlier cessation of growt h predicts earlier frost hardening of stems and needles. No differences in frost hardiness of stems were found at the provenance level at the end of t he growing period in August. At that time, the frost hardiness of needles o f the northernmost provenance was higher than that of other origins. Within the provenance, the stems were less hardy than the needles.