PHENOLOGICAL VARIATION AMONG PLUS-TREE CLONES OF PINUS-SYLVESTRIS (L)IN NORTHERN SWEDEN

Eighteen plustree clones of Pinus sylvestris (L.) from latitudes 67 de grees N, 65 degrees N and 62 degrees N growing in a Swedish coastal cl one archive on latitude 63 degrees 40'N were tested for cold tolerance on 20 occasions from August 1990 till May 1992 using artificial freez e testing of detached current-year needles. Freezing injury was determ ined by visual scoring of the proportion of discoloured needle tissue. Clonal variation in needle cold tolerance was significant on all test occasions except for 2 freezing tests in the summer. The southernmost population had a slower autumn cold acclimation than the populations from 65 degrees N and 67 degrees N. In the winter and spring no effect of the latitude of origin on needle cold tolerance was observed. Fluc tuations in winter cold hardiness, probably induced by changing weathe r conditions, were considerable. Winter cold hardiness was uncorrelate d with autumn cold hardiness. Rapid deacclimation occurred in April. E arly cold acclimation in the autumn was related to poor height growth. The shoot elongation rhythm was uncorrelated with needle cold toleran ce at all times of the year. The attachment of current-year needles to the terminal shoot in late summer was positively correlated with need le cold tolerance in early autumn. The needle dry matter proportion in the autumn was positively correlated with needle cold tolerance in ea rly winter. Progeny cold hardiness in early autumn was positively corr elated with needle cold tolerance of the mother in early autumn but un correlated with needle cold tolerance at other times of the year. Poss ible implications of selection far phenological characteristics for ve getative propagation and breeding of Scots pine on northern latitudes are discussed.