Needle-retention and density patterns in Pinus sylvestris in the Rhone Valley of Switzerland: comparing results of the needle-trace method with visual defoliation assessments

We used the needle-trace method (NTM) to reveal the needle-retention patter ns of Scots pine (Pinus sylvestris L.) over the past 100 years. The average annual needle retention (ANR) on main stems has gradually decreased from f ive needle sets in the 1890s to four needle sets in the 1990s. Needle reten tion is significantly correlated with tree age and altitude, and the decrea se in needle retention may be at least in part due to the increasing ages o f sample trees. The average needle density varied plotwise between 7.2 and 10.5 short shoots/cm. In a comparison of ANR values and visually assessed f oliage percentage from the Swiss Forest Health Inventory between 1985 and 1 996, we found significant correlation between the mean annual values. While the direction of annual change was identical in two thirds of all years we found disagreement in 3 years. Both needle retention and foliage were lowe r in the early 1990s than in the late 1980s. With the help of the NTM we ca n show that there had been similar decreases in foliage usually connected w ith severe droughts during the last century.