Needle traces as indicators of growing conditions in Scots pine (Pinus sylvestris L.)

Needle retention (number of needle sets) and needle density (number of needle pairs per centimeter of shoot) were surveyed on Scots pines in five forest regions of Slovakia. The Needle Trace Method (NTM) was used to determine needle retention and needle density along the main stem retrospectively for the last four decades. In all forest regions, the values of these indicators varied from year to year. However, in Záhorská lowland, Vtáčnik, Krupinská plain, and High Tatras, the trends of both observed indicators were constant over the time series. The situation was different in the Levočské hills, where the needle retention displayed a decreasing trend and needle density an increasing trend. These trends probably reflected a long-term stress of air pollution on pines in this forest stand.

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 patterns of Scots pine (Pinus sylvestris L.) over the past 100 years. The average annual needle retention (ANR) on main stems has gradually decreased from five needle sets in the 1890s to four needle sets in the 1990s. Needle retention is significantly correlated with tree age and altitude, and the decrease in needle retention may be at least in part due to the increasing ages of 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 foliage percentage from the Swiss Forest Health Inventory between 1985 and 1996, 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 lower in the early 1990s than in the late 1980s. With the help of the NTM we can show that there had been similar decreases in foliage usually connected with severe droughts during the last century.