Fluctuation in the number of needle sets and needle shed inPinus sylvestris

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Abstract
Number of needle sets and needle shed dynamics along the stem and branches of young Scots pines (Pinus sylvestris L.), growing on four dry upland sites and one moist upland site, were monitored annually for 10 years in five stands near the Arctic Circle in northern Finland. Standwise, number of needle sets varied in the summers of 1987–1997 between 3.9 and 6.2 needle sets along the branches and between 3.3 and 4.9 sets along the stem. The long‐term means for the branches were the same (5.4–5.5 sets) on all five sites. The pines on the dry sites had undergone an average needle loss of 2.3 needle sets in 1987. The long‐term mean annual needle shed was 1.0 needle sets. In the case of the 1987 needle loss, single pines on the dry site had lost 17–78% of their green needle sets, but they all recovered within two to three years to the normal needle‐retention level. In general, the number of needle sets in the summer correlated positively with the annual needle shed. Normally, a needle set remained attached as a whole (maximum of 10% loss) for four full years after its emergence, taking the following four years for the last needles to be gradually shed.